atomic spectrometry update. atomic mass spectrometry€¦ · applications of atomic ms are not...

Atomic spectrometry update. Atomic mass spectrometry

Jeffrey R. Bacon,a Joanna C. Greenwood,b Luc Van Vaeckc and John G. Williamsd

aThe Macaulay Institute, Craigiebuckler, Aberdeen, UK AB15 8QHbSmartWater Europe Ltd., PO Box 115, Wellington, Telford, Shropshire, UK TF1 5WEcMicro- and Trace Analysis Centre, Department of Chemistry, University of Antwerp,

Universiteitsplein 1, B-2610 Wilrijk, BelgiumdNu Instruments Ltd., Unit 74, Clywedog Road South, Wrexham Industrial Estate,

Wrexham, UK LL13 9XS

Received 7th June 2004

First published as an Advance Article on the web 19th July 2004

1. Accelerator mass spectrometry (AMS)1.1. Reviews1.2. Instrumentation1.3. Developments in radiocarbon analysis1.4. Developments in the measurement of elements other

than carbon2. Glow discharge mass spectrometry (GDMS)2.1. Instrumentation2.2. Fundamental studies2.3. Analytical methodology2.4. Quantification2.5. Depth profiling3. Inductively coupled plasma mass spectrometry (ICP-MS)3.1. Fundamental studies and instrumentation3.2. Interferences3.3. Sample introduction3.3.1. Laser ablation3.3.2. Thermal and chemical vaporization3.3.3. Nebulization3.3.4. Flow injection3.4. Separation and speciation3.5. Analytical methodology3.6. Isotope ratio measurement4. Laser ionization mass spectrometry (LIMS)4.1. Instrumentation4.2. Fundamental studies4.3. Analytical methodology5. Resonance ionization mass spectrometry (RIMS)5.1 Reviews5.2 Instrumentation5.3 Analytical methodology6. Secondary ion mass spectrometry (SIMS)6.1. Instrumentation6.2. Fundamental studies6.3. Analytical methodology6.4. Quantitative analysis6.5. Single and multi-dimensional analysis6.5.1. Depth profiling6.5.2. Imaging and 3-D analysis6.6. Static SIMS (S-SIMS)6.6.1. Review6.6.2. Instrumentation6.6.3. Fundamental studies6.6.4. Analytical methodology6.6.5. Quantitative analysis6.6.6. Imaging6.7. Surface and subsurface analysis with S-SIMS and SIMS7. Sputtered neutrals mass spectrometry (SNMS)8. Stable isotope ratio mass spectrometry (SIRMS)8.1. Reviews8.2. Instrumentation

8.3. Analytical methodology8.4. Sample preparation9. Thermal ionization mass spectrometry (TIMS)9.1. Instrumentation9.2. Calibration9.3. Analytical methodology9.4. Sample preparation10. Other methods10.1. Electrospray mass spectrometry (ESMS)10.2. Gas chromatography-electron ionization mass

spectrometry (GC-EIMS)10.3. New methodologiesAppendix: glossary of terms11.0. References

This Update is part of a series of annual reviews which cover

the various aspects of analytical atomic spectrometry and

follows the same format as last year’s.1 Although an attempt is

made to consider all relevant refereed papers, conference

abstracts, reports, book chapters and patents for inclusion in

this review, the content of the review is highly selective. The

selection of papers is based on criteria applied to focus sharply

on the most significant developments in instrumentation and

methodology or improved understanding of the fundamental

phenomena involved in the MS process. The increasing

importance of speciation and the blurring of boundaries

between atomic and molecular MS require a high degree of

judgement to be made in considering papers for inclusion. The

main ruling criterion for all speciation papers is that the work

should involve or be intended for the study of natural systems.

For example, the study of synthetic metal clusters is generally

not included, whereas the determination of organometallic

compounds in environmental samples is.

Applications of atomic MS are not covered in this Update and

readers are referred to the Updates on Industrial Analysis: Metals,

Chemicals and Advanced Materials,2 Environmental analysis3 and

Clinical and Biological Materials, Food and Beverages.4

Throughout this review, the term molecular ion will be

restricted to denote only the positive or negative radical ion

formed by removal or capture, respectively, of an electron. In

contrast, addition of a proton or cation to a neutral molecule

gives molecular adduct ions. Deprotonated molecules are

considered as fragments.

Although reproducibility or precision is a key figure of merit in

MS, there is no agreed format for quoting it. The reader can

assume that values of precision given in this Update as a

percentage correspond to the RSD unless otherwise specified.

In isotope analyses, however, values of precision are generally

given as the SD of a permil value.DO

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0.1

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It is a widespread phenomenon that analytical techniques ingeneral and MS in particular spawn a large number ofabbreviations and acronyms. A glossary of terms used in thisUpdate appears at the end. Commonly used abbreviations arenot defined in the text: others are defined in the text whenused first and again in the glossary.

1. Accelerator mass spectrometry (AMS)

1.1. Reviews

The overview of AMS by Hellborg et al.5 gave a concise, yetclear, introduction to the design principles of an AMS system,with particular reference to the instrumentation at the Lundfacility. The characteristics of AMS that made it possible tomeasure very low isotope ratios (e.g., 14C:12C down to 10215)were highlighted and emphasis was placed on the importance ofthe ion source. A list of all AMS facilities worldwide waspresented with details of the type of accelerator. Several topicalapplications were discussed, including the use of 14C in dating,environmental and biomedical studies and the measurement of27Al and 59Ni.

Although the use of gas-filled magnets for isobaric separationrequires relatively powerful accelerators, it allows measurementtime to be reduced as a result of the increased beam intensities.The review by Leino6 on gas-filled separators had a relativelybrief section on AMS but included a useful discussion onthe characteristics of the separators and their main designprinciples.

The publication of several application reviews highlighted thewide range of research topics that AMS supports. The review ofMuzikar et al.7 on AMS in geological research can be recom-mended for its overview of AMS techniques and their strengthsand limitations. Production and transport of cosmogenicnuclides (26Al, 10Be, 14C, 36Cl 129I) in the atmosphere, hydro-sphere and lithosphere were discussed and specific applicationsin areas such as geomorphology, tectonics, climatology,hydrology and geochronology were used to demonstrate thewide range of geological studies for which AMS was beingused. In addition to presenting a good overview of the AMStechnique, Lappin and Garner8 highlighted the role of AMS inthe increasing use of human trials for drug development. Therole of AMS in forensic science was described by Tuniz et al.,9

in particular as an ultra-sensitive tool for crime laboratories.

1.2. Instrumentation

Regular upgrades are applied to AMS instruments to improveperformance and increase the range of elements that can bedetermined. As a result, each instrument becomes unique in itsdesign and capability, which are usually described in the formof a ‘‘facility report’’. The Vienna Environmental ResearchAccelerator has been upgraded to a universal facility to allowlong-lived radionuclides from the lightest (10Be) to the heaviest(244Pu) to be measured at natural abundance.10,11 Detaileddescriptions and excellent schematic layouts were given for allcomponents of the system (ion source, low-energy injectionsystem, tandem accelerator, high-energy analyser and heavy-ion beam line). All components were designed to avoid beamlosses, to achieve ultimate sensitivity, and to maximize stability.Additional, moveable slits were installed after the injectionmagnet in order to increase mass resolution to about 900 whilemaintaining high transmission. Good insulation was requiredas the slits were mounted inside the multi-Faraday-cupchamber, which was at 13 kV. A high-resolution electrostaticanalyser (ESA) replaced the Wien filter of the original con-figuration. Charge changing and scattering of ions inside theESA were reduced through maintenance of good vacuum(v1027 mbar). As the detector to be used depended on the ion

to be measured, a switching magnet at the end of the ESA exitbeam line deflected the ion beam to the appropriate detector.Only a TOF detector had sufficient resolution to distinguishbetween neighbouring isotopes of heavy ions. The diamond-like carbon foils used as the start and stop detectors were thethinnest carbon foils available and were mounted on a flatcopper mesh (1.2 mm wire thickness) of 90% transmission.

A well-designed optical system is required in an AMSinstrument to transport the beam from the ion source apertureto the accelerator with a minimum of loss. Older designs of thelow energy system have been seriously limited by the use of asingle-focusing magnet with a bending angle of only 15u. A newinjector installed at the Lund AMS facility included a 90udouble-focusing magnet and a 90u spherical ESA for optimummass and energy separation, respectively.12 The calculationsused for the design of the optics were discussed in detail. Otherimprovements reported for the Lund system included theinstallation of terminal pumping in the accelerator.13 In com-parison with the unmodified instrument, this allowed a highergas density in the stripper and improved vacuum in theaccelerator tubes to be used, and resulted in increased beamtransmission and better measurements. Pressure profiles werecalculated from vacuum conditions obtained for differentstripper gases and gas pressures in the modified system andwere compared with those obtained for the unmodified system.The pressure profiles were to be used to calculate the trans-mission and charge-state distribution for the stripping ofdifferent heavy ions.

Improvements at the University of Arizona facility, where anew 3 MV system was installed in 2000, have centred mainly onsample preparation for radiocarbon dating.14 Details were givenof the new instrument and its advantages for the measurementof 26Al, 10Be and 129I. The processing of samples followedseveral well-defined steps and targets were pressed into aholder designed in-house. A stepped-combustion procedurewas developed for the analysis of bulk sediments with low Cconcentrations.

The report from the Seoul National University facilityhighlighted some of the problems experienced in runningtheir system.15 Sparking in the accelerators was attributed toexcessive moisture present during the monsoon season.Precautions were required to prevent memory effects whichcould occur when both biomedical samples with high 14Cconcentrations and other samples with lower concentrationswere analysed on the system. In order to prevent the detectorfrom becoming flooded with excessive count rates, the strippingefficiency was reduced to a low level by not recirculating thestripper gas (Ar). Use of an elemental analyser to combustsamples was preferred as it gave lower background levels thancombustion using CuO, even though the resulting targets gavea much lower ion beam current.

An AMS facility dedicated to the depth profiling of 3H ingraphite samples from the wall material of fusion experimentswas installed at Rossendorf (Germany) in order to preventcontamination of the existing facility.16 The dedicated facilitywas based on an air-insulated low-energy (100 keV) acceleratorequipped with gas stripper. After gaining experience with thissystem, a second, compact, system was constructed with a SF6-insulated 100 kV tandem accelerator and diamond-like carbonstripper foils. The LOD was 561010 atoms cm23 and the depthrange w50 mm. Although results obtained on the differentAMS systems were consistent, a difference of a few orders ofmagnitude was observed between samples with low concentra-tions analysed by AMS or a conventional procedure involvingcombustion followed by liquid scintillation counting. Thecompact system could also be used for the simultaneous depthprofiling of Be, C, 2H and Li using a Faraday cup at theentrance of the accelerator (SIMS mode).

Methods for on-line AMS analysis continue to receiveattention. A laser-induced sample combustion interface has been

J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 0 2 0 – 1 0 5 7 1 0 2 1

designed for the continuous production and analysis of CO2.17

In this way, a sample such as the effluent stream from achromatographic or electrophoretic separation could beanalysed without the fractionation normally required. Adirect-injection pyrolytic interface, used to produce H2 forthe determination of 3H, was designed for nl min21 flow ratesand operated without desolvation.

Most of the methods developed to overcome the problem ofatomic isobaric interference are variants of the ‘‘range and rateof energy loss’’ methods used to identify heavy ions. Litherlandet al.18 modelled the use of resonant and near-resonant keV ionreactions for isobar separation of 135Ba1 and 135Cs1. Althoughthe method could theoretically be exploited for isobarsuppression, multiple scattering could be a problem andneeded to be taken into account.

A microchannel plate detection system with double delay lineanodes and using custom-designed readout electronics has beendeveloped for AMS.19 The 12 modules, each with a 1062 cmactive area, together covered 80% of the 1.5 m focal plane in thesplit-pole spectrograph. The MeV ion beam passed through athin carbon foil, placed at 40u with respect to the beam, toproduce electrons that were detected by the array. A horizontalresolution of 0.4 mm was obtained for secondary electronsaccelerated to 2.5 keV. To improve this resolution, aHelmholtz-type coil pair was built to confine further thesecondary electrons within a magnetic field. The new detectorsystem was seen to hold the advantages of high-resolution posi-tion measurements, subsequent energy analysis, simultaneousreadings at different positions, tolerance of high-intensity beams,handling a wide mass range and low background when used withother detectors.

1.3. Developments in radiocarbon analysis

Improvements in sample preparation have dominated advancesin AMS radiocarbon analysis. In their overview of radiocarbondating of iron-based artefacts, Cook et al.20 focused mainly onthe simplification of sample preparation and reduction in thesample size requirement. The use of rust as a viable source ofmaterial for dating would allow precious samples to beanalysed with little damage to the artefact but only on thecondition that original carbon remained within the rust matrix.The authors demonstrated that carbon could be extracted fromrust cleanly and gave plausible dates for the artefact, but issuesof contamination and post-deposition carbon exchange stillneeded to be investigated. A summary was presented of all 63previously published results for iron-based materials, togetherwith 29 results previously unpublished. Not all iron-basedmaterials were suitable for radiocarbon analysis, especiallywhen materials were reworked, and Craddock et al.21

questioned in considerable depth the interpretation of AMSdates obtained from C in iron. A notable feature of the paperwas the detailed discussion of the various manufacturingprocesses used to produce iron and steel through history andthe consequences these would have on the accuracy of dating.The authors concluded that any iron samples could have beensubjected to several processes involving heat, reduction,oxidation and carburization with a variety of fuels andfluxes, each containing C in a number of forms. This wouldmake dating unreliable. For most artefacts the technology usedto produce the iron is unknown and there was a high likelihoodthat the final process removed all evidence of earlier processes.The authors were not entirely pessimistic, however, andconsidered radiocarbon dating, when used together withother archaeological information, to be useful for elucidatingthe possible history of the artefact, even if a reliable date wasnot available.

A plasma-chemical procedure for the extraction of 14C fromperishable archaeological artefacts was found to have fourmain advantages over traditional combustion.22 It was

‘‘virtually non-destructive’’, acid pre-treatment to removecarbonate was unnecessary, the background was low and thepresence of oxalates in the sample did not affect extraction.Samples were initially subjected to multiple Ar plasmas toremove loosely adsorbed CO2 and then to multiple O2 plasmasto produce CO2 from the sample. The process selectivelyoxidized organic C in samples (for example, cloth, wood, bone)and was almost non-destructive in that it only removed organicmatter (about 0.1 mg C) and left carbonate, oxalate and othermineral phases unattacked. No visible changes in paint sampleswere discernable. Mass fractionation was low (equivalent to¡32 years) and could be corrected by measuring the d13C in thesame aliquot of CO2 used for the 14C determination. Datesobtained by the procedure agreed well with those obtainedpreviously by combustion.

A major drawback in the direct dating of archaeologicalceramics is that organic carbon in buried artefacts can becontaminated by migrating soil organic matter of a differentage. An important advance in such analyses targeted pureorganic compounds derived from organic residues of commod-ities processed in the vessels during their use.23 Automatedpreparative capillary GC with wide-bore capillary columns wasused to isolate n-hexadecanoic and n-octadecanoic acids fromlipid extracts from archaeological potsherds in high purity(w95%) and an amount (w200 mg) sufficient for AMS analysis.The major advantage of the compound-specific technique wasthat 14C dates could be obtained for the individual compounds,which could be associated directly to the commodities (e.g.,animal fats) processed in the vessels. Analyses of appropriateblanks and controls established that introduction of back-ground 14C contamination was minimal. Initial results yieldedgood correlations between the predicted date of potteryand that measured using n-octadecanoic acid. The use ofn-hexanoic acid could provide anomalously young dates,probably as a result of contamination by soil organic matter.

Kristiansen et al.24 hypothesized that the age of a prehistoricburial mound could be determined by dating components of soilorganic matter from a buried soil surface. The soil organic Cwas probably formed by photosynthesis at, or shortly before,the time of burial. Soil samples were taken carefully usingaugurs, thereby minimizing disturbance to the mounds, and thesoil organic matter extracted using acid–alkali–acid fractiona-tion. The best age estimates were obtained by dating humic acidfractions from the organic-rich layer in the upper cm of buriedsoil. Considerable discrepancies from the reference dates werefound for the age estimates of some samples. This probablyresulted from the migration and recycling of soil organic matterand led to an uncertainty of at least 280 years. The approachcould only be recommended if no more than minimaldisturbance of the earthworks were allowed and no macro-scopic organic remains were available.

The potential problem of contamination in the direct datingof pigments in prehistoric cave paintings was addressed byapplying a sample treatment procedure designed to extract onlythe organic C from the pigment.25 A succession of ‘‘acid–base–acid’’ treatments removed carbonates from the limestone wallsor ground water, humic acids from the degradation of organicmatter and bacteria and other living micro-organisms. Thepurified charcoal or humic acids were heated in a stream of O2

to remove additional organic contaminants prior to analysis.The purification process removed more than 90% of theoriginal mass to produce pellets containing 0.5–1 mg ofC. Dates obtained for paintings in several French cavessuggested that ideas on the evolution of prehistoric art had tobe revised.

The main problem in the radiocarbon dating of bone lies inthe difficulty of extracting material that contains indigenous Cin sufficient amount for analysis. Three methods investigatedfor the pretreatment of old bones were acid treatment toremove carbonate and leave collagen for dating (Longin

1 0 2 2 J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 0 2 0 – 1 0 5 7

method), the Longin method with additional alkali extractionof collagen and reaction of free amino acids with ninhydrinto produce CO2 from carboxyl C atoms (Nelson’s method).26

The modified Longin method was chosen as it gave morereliable results than the Longin method yet was less laboriousthan Nelson’s method. The last method also required animproved vacuum line which was not available. The unrelia-bility of the unmodified Longin method was probably due tothe presence of contamination that was not removed during thepretreatment.

A method for the graphitization of CO2 from carbonaceoussamples was based on the use of septa-sealed vials, togetherwith commercially available disposable materials to eliminatecross contamination, minimize handling and lower costs.27

Samples containing 0.25–10 mg C could be reduced to graphitein about 4 h and about 150 samples could be prepared in aworking day. It was intended that the method would form thebasis for an automated sample preparation system in whichcombustion products from a commercial carbon analyserwould be trapped and reduced to graphite.

1.4. Developments in the measurement of elements other thancarbon

The high sensitivity of AMS has been exploited to measure theextremely low yields of products from super-asymmetricneutron-induced fission of uranium.28 The ability to measure60Fe:Fe ratios down to 1610216 with AMS allowed 60Fe fissionproducts to be detected, even at yields as low as 1610212%.Iron was extracted from high-level radioactive waste concen-trate by a two-step procedure involving complexation on andelution from a column of Tru-spec2 resin followed by cationexchange chromatography. The Fe fraction was finallyextracted into diisopropyl ether and re-extracted into water,which was evaporated to form iron oxide and hydroxide fortarget preparation. The total decontamination factor was w107

and 40% of the total Fe was recovered. Use of a gas-filleddipole magnet with subsequent energy loss measurement gavean overall suppression of the isobaric 60Ni by a factor ofw10.10

Merchel et al.29 developed a procedure for the extraction ofclean nanodiamond samples from primitive meteorites in just aweek. A significant reduction in the processing time of standardprocedures was achieved through the use of microwavedigestion and oxidizing acids to dissolve major minerals. Theuse of AMS to investigate the isotopic composition of traceelements in diamonds was explored by measuring the Pt isotoperatios in the Allende and Murchison diamond samples. Ifsuitable standards were available to correct for fractionationeffects, accuracy on the order of 2% should be achievable. Theupper limit of 161014 atoms g21 placed on the abundance of198Pt-H in nanodiamonds was consistent with what would beexpected from an extrapolation of the apparently massdependent abundance trend of the associated noble gases.

The detection of cosmic c-radioactivity of live nuclides fromsupernovae remnants has had an immense impact on theunderstanding of stellar nucleosynthesis. It is expected thatnuclides with half lives of about 107 years, such as 247Cm,182Hf, 244Pu and 146Sm, should also be present in the interstellarmedium as freshly synthesised matter but they are not detect-able by c-spectrometry because of their low activity. Paulet al.30 have reported an unsuccessful attempt to detect 244Puin a deep-sea sediment sample using AMS. The 239Pu and 240Pupresent in the sample from nuclear test fallout was usedthroughout as an in-situ tracer for the 244Pu determination.Following separation by anion-exchange chromatography, Puwas electrodeposited on a stainless steel disk for a-counting andwas subsequently processed by co-precipitating with Fe forpreparation of AMS targets. Careful calibration of the voltagesof the accelerator terminal and of the Wien filter was necessary

to analyse the different Pu91 isotopes in the beam transportsystem. An overall AMS detection efficiency of 361026 wasestimated from the combined number of 239Pu and 240Pucounts and the cathode content (known from a-countingand chemical recovery). Only one count of 244Pu was detectedover an interval of 3.5 h. This would correspond, withinstatistical error, to the estimated stratospheric fallout contentof the sample. The lack of detectable interstellar 244Pu placedan upper limit on the deposition of interstellar 244Pu of0.2 atoms cm22 year21.

2. Glow discharge mass spectrometry (GDMS)


A new electrostatic quadrupole extraction lens (EQEL) mini-mized the ion beam scattering caused by pressure gradients andspace charge effects in the region between the second vacuumstage (1023 mbar) and the high vacuum (1026 mbar).31

Although the optics were developed for a GD source on aMattauch–Herzog high resolution (HR) mass spectrometer,the design would be also applicable to ICP-MS. The EQELconsisted of a single cylindrical electrode and coaxial inner tubewith a series of slits. Calculations showed that the penetrationof the static field from the outer electrode through the slits gavethe same focusing effect as a rf field in a quadrupole when thepotential difference between the inner and outer electrodes was¢500 V. The EQEL increased the sensitivity in Ar GD analysisof a Ti pin by a factor of 35 in comparison with existing optics.

Oxley et al.32 developed a pin sample holder for a Grimm-typesource in GD-TOFMS. The sample device was mounted on thesource and a raised edge on the cathode surface ensuredreproducible sample positioning. In this way, a quick changefrom pin to flat sample analyses was combined with theadvantages of a Grimm source. Operation in the ms-pulsedmode revealed the occurrence of a diode-type discharge, whichwas in contrast to the usual situation of a discharge confined bythe anode cylinder. Both the cylindrical surface and the flat endof the sample pin were sputter etched. Use of a macor sleeve torestrict the sputtering to the flat surface resulted in unstableoperation. The sputter rate of 1.5 ng per pulse at a currentdensity of 0.07 mA mm22 was 50% higher than that in thenormal Grimm source with 0.15 mA mm22. The intensity ofmatrix ions such as Cu1 was 10% lower than when a commonflat sample source was used. Analysis of NIST SRM 494(copper) gave estimated LOD for Ag, Sb and Sn of 0.7, 1 and0.4 ppm, respectively. The 118Sn:116Sn ratio was measured withan accuracy of 1.9%. Polyatomic interferences were moreabundant than with the Grimm type source but could beminimized by increasing the operating pressure. Preliminaryexperiments on a Ni–Cd coated steel pin showed that the pinsample holder could be used for depth profiling.

A compact GD source, which incorporated a focusing lens,was designed for the analysis of sub-mg quantities of insulatingsamples or atmosphere samples.33 The coaxial-design of thesource featured optical elements behind the discharge surfaceto direct ions efficiently towards the ion sampler. Non-conducting materials such as depleted uranium (DU) dioxidepowder were mixed with indium and rolled between twotitanium plates to form a wire with a diameter of 1 mm. Thewire was mounted into the source by means of a rod connectedto the manifold with Swagelock fitings and Vespel ferrules.Experiments confirmed the predicted tenfold increase intransmission in comparison with the previous source. Isotopesof W were measured by sputtering away only one atom layerusing 2 ms pulses. Use of a capillary (internal diameter of0.125 mm) instead of the rod allowed fluorinated organic gasessuch as freon 12 and perfluorotributylamine in the atmosphereto be sampled.


2.2. Fundamental studies

Potapov et al.34 investigated atomization and ionization inpulsed GD with a thin-walled metal hollow cathode (TWMHC)by collecting dry residues of the introduced analyte solution.Use of high currents (up to 4 A) in the pulsed mode resulted inhigh sputtering and ionization yields due to heating of thecathode to 1200–1400 uC and the low bonding energy betweenthe residue and the cathode surface. A self-consistent descrip-tion of plasma particle distributions and dynamics togetherwith electrical field calculations gave good agreement withexperimental data for the concentrations and spatial andtemporal distributions of plasma species (electrons, ions,sample atoms) and the discharge current. The simulationswere also used for the experimental optimization of thesampling time and transport processes from TWMHC to theTOF analyser. The LOD for Cd and Cu were 20 and 15 ng l21,respectively.

Modeling of the ionization dynamics in atmospheric samplingGDMS identified the key experimental factors for optimizationof the analytical performance.35 The simulations accounted forthe generation and thermalization of secondary electrons, theformation of reagent and analyte ions by electron capture (EC)and ion–molecule reactions, the removal of ions by recombina-tion, diffusion and ion–molecule reactions and the samplingand pumping parameters of the source. Although thermalelectrons and positive reagent ions reached similar numberdensities, the high rate of EC relative to ion–molecule reactionscaused the formation of analyte anions by EC to dominate thegeneration of positive analyte ions by ion–molecule reaction.The high number densities of thermal electrons and reagentions explained the linear response over a wide dynamic range.Simulations predicted an enhancement of the analyte iondensities by increasing the flow rate through the cell ordecreasing the source radius.

The addition of N2 to ms-pulsed Ar GD allowed diagnosticmeasurements of the fundamental excitation and ionizationprocesses to be performed.36 Specifically, Penning excitationinvolving metastable Ar atoms and ground state N2 molecules,as well as charge transfer between Ar ions and the added N2,decreased the ionization of sputtered atoms during the power-on plateau period by a factor of 10 when the discharge gascontained 1% (vol.) N2. Complementary GD-TOFMS andGD-OES experiments on a NIST SRM 1767 (iron disk) with2 ms pulses and a fall time of v45 ns demonstrated the energytransfer from excited argon atoms to nitrogen molecules duringthe voltage-on period. The resulting reduction of the number ofargon metastable states lowered the analyte ionization. In thevoltage-off period, recombination of argon ions restored thenumber of metastable states, which in turn promoted Penningionization and caused Fe1 to be detected several ms after thepulse. The nitrogen pressure could be used to control the after-peak delay and thereby to reduce the interferences byoptimizing the ratio of analyte to discharge ions.

The study of the distinct time regimes for ion formation in ms-pulsed GD revealed the potential of time-gated methods tocombine quantification and speciation of halogenated com-pounds in applications such as GC-GD-TOFMS.37 The GDwas operated at a rate of 100 Hz with a 50% duty cycle. Threeindependent digitizers collected data at w7.5 MHz in separatetime regimes. Atomic ion generation prevailed during the first15 ms after the plasma onset, whereas formation of structuralfragments was at a maximum in the plateau or steady stateregime (about 100 ms into the pulse cycle). Measurement duringthe after-peak period favoured the detection of molecular ionsand protonated molecules, which were formed by inelasticcollisions of neutrals from the analyte with long-lived meta-stables. Structural fragments were formed through electronionization (EI), charge transfer and Penning ionization. Incontrast, Penning ionization was the dominant mechanism for

the formation of analyte ions in the pre-peak regime. As aresult, tuning of discharge parameters in combination withtime-gated data acquisition gave unprecedented control overthe ionization process.

The ion transport in a source by means of diffusion, migrationand convection was studied by Oxley et al.38 by introducing agas sleeve system to force the gas stream over the samplesurface. The in-house designed Grimm GD source wasoperated in the ms-pulsed mode and combined with acommercial TOF analyser. The sample-to-cone distance andsource orifice diameter were 13 and 0.7 mm, respectively, andAr flow rate was up to 500 ml min21. Experiments wereperformed with the NIST SRM 1104 (free cutting brass) and1122 (Be–Cu) and a RM CKD 239 (cast-iron). The gas flowaffected significantly the ratio of argon to analyte ions becausewashout effects in the plasma changed the relative importanceof the ionization processes. Optimizing the pressure yieldedsimilar results. An additional restriction between source andsampler further improved the discrimination between analyteand gas ions. The choice of appropriate delay settings elimi-nated the interferences from C2

1, C2H1 and CN1 on the Mgsignal from cast iron.

2.3. Analytical methodology

The change in signal intensities of discharge gas and analyteions, sputter rate and crater shape due to the addition of 0.5–10% (v/v) hydrogen in Ar dc GD-TOFMS was systematicallyinvestigated for flat discs of RM with copper, copper–zinc andiron matrices.39 It was observed that the pressure had to beraised with increasing hydrogen concentration to maintain thedischarge current (under constant voltage). This confirmed thetheoretically predicted decrease of electron density due torecombination with ArH1 and H3

1 ions. Whereas the Ar21 ionsignals increased by 50% and a factor of 2 upon addition of 0.5and 10% hydrogen, respectively, those of Ar2

1 increased onlyslightly. Analyte ion intensities were enhanced up to four-foldby hydrogen addition and increased with voltage for hydrogenconcentrations w0.5%. On the other hand, the sputter ratedecreased by w50% when 1% hydrogen was added but stayednearly constant at higher concentrations. Chemical reaction ofhydrogen with the surface was believed to decrease the sputterrate. The shape of the crater bottom worsened with increasinghydrogen addition.

The use of collision induced dissociation (CID) of diatomicions was an efficient way to reduce interferences in rf GDMSwith an ion trap modified for the injection of externallygenerated ions through the grounded end cap.40 Neon servedboth as discharge and ion trap bath gas. Slow heatingresonance excitation allowed TaO1 to be dissociated withminimal scattering loss of Au1. Ions of different m/z could beejected or kept in the trap by sequential excitation at selectedvoltages in the fixed frequency mode. In this way, interferencesof, for example, 154GdO1 and 155GdO1 on the 170Yb1 and171Yb1 signals, respectively, could be avoided. Broadbandexcitation eliminated the contributions of 63Cu2

1, 63Cu65Cu1

and 65Cu65Cu1 ions from the signals of 126Te1, 128Te1 and130Te1, respectively. Application of CID experiments toselected Te isotopes did not change the peak intensity ratiosof the other isotopes. It was foreseen that the method could beused to recover atomic ions from polyatomic species.

A systematic investigation of the discharge parametersresulted in optimization of GC-GD-TOFMS with ms-pulsesfor atomic-ion detection with chemical speciation.41 Analyteidentification exploited the GC retention time as well as theatomic ion, structural fragment and molecular weight informa-tion obtained in the pre-peak, plateau and after-peak regimes,respectively. The efficiency of analyte ionization and ionsampling decreased with pressure. Sampling close to thecathode surface maximized atomic ion signals. Increasing the


distance between the sampling orifice and the cathode favoureddetection of structural fragments analogous to those producedby 70 eV EIMS. Strikingly, sampling close to the cathodeincreased the relative importance of molecular ions producedby Penning ionization whereas sampling further from thecathode maximized detection of protonated molecules gener-ated through proton transfer reactions with, for instance,ArH1.

A comparison of atomic ion generation in both double andsingle pulsed GD revealed an analytical improvement throughuse of the additional pulse for the post-ionization of atomssputtered during the first pulse.42 Experiments with NIST SRM1104 and 1122 showed the delay between the pulses to becritical for the signal intensity in GD-TOFMS. The secondpulse was applied at a delay of 10–140 ms and the repeller pulsedelay (ion extraction) was varied between 10–330 ms. Optimizedtiming increased the signals from Ar1 and Cu1 ions in doublepulse GD relative to the single pulse mode by 20% and a factorof 1.5, respectively, at low flow rates. At high flow rates, thesignal improvement was about 20% for both ions. The optimaltime window for ion extraction in the double pulse mode waslarger than in the single pulse mode, facilitating optimal ionextraction. Although the addition of 10% methane to Arreduced the formation of polyatomic ions less than expected,Cu1 ion signals increased by a factor of 5. It was believed thatthe destruction of methane by the first pulse improved theionization of the sputtered neutrals by the second pulse.

2.4. Quantification

A round-robin test evaluated the determination of trace andultratrace elements in the concentration range of 1–20 mg g21

with GD-OES and GDMS using BCR CRM 074 and 075(copper).43 Because all 10 laboratories used HR-GDMSinstruments from the same manufacturer, the study primarilyaddressed the comparability of the procedures. In spite of theuse of pin or disc samples and different relative sensitivityfactors (RSF), quantitative results were found to be relativelyconsistent. However, the application of the same RSF to theraw data further improved the agreement between the resultsfrom different laboratories. Hence, the authors stressed theneed for exchanging calibration data.

Trace levels of Cl compromise the fracture toughness ofZr–2.5Nb alloys in coolant tubes for nuclear reactors. Shekharet al.44 developed a method for the direct determination of Cldown to ppb levels in melt ingots of Zr–2.5Nb using Ar dc GDand a quadrupole analyser. The use of both an electronmultiplier and a Faraday cup for the detection of minor andmajor signals, respectively, resulted in a dynamic range of9 decades. The detectors were cross-calibrated daily bymeasuring the 96Zr1 isotope. The RSF of 35Cl was determinedon a zirconium standard disc with a Cl concentration of 155 ¡5 mg g21 and checked by analyzing samples by both GDMSand pyrohydrolysis-ion chromatography (IC). Surface Cl con-tamination was believed to be responsible for the long stabi-lization time of y50 min. At concentrations v3 mg g21, the35Cl ion was measured because ArH1 interfered with 37Cl.Analytical figures of merit included a LOD of 22 ng g21 andan overall precision of 1% and 10% for concentrations in themg g21 and ng g21 ranges, respectively.

Pulsed-rf GD-TOFMS was used for the determination of Brfrom organic flame retardants in commercial plastics.45 Thegeneration of atomic and structural ions gave both quantifica-tion and speciation analysis. Measurement of the weight lossdue to sputtering revealed a strong correlation between theyield and the sputter rate. The prominence of EI caused the Br1

signal to reach a constant level about 1 ms after the start of the5 ms pulse but it lasted for only 0.2 ms after the power pulse.Low thermal stability and sputter yield prevented directquantification in the bulk plastics, which had to be compacted

in a silver matrix. The reduction of the power loss in the sampleincreased the sputter atomization and ionization yields. Linearcalibration of the signal intensity was achieved for 0–15% Brand the LOD were 0.2 and 0.5% for discharges in Ar andNe, respectively. The precision of the determination was 8%.The method eliminated the laborious dissolution of plasticsnormally required for element analysis.

2.5. Depth profiling

The use of dc and rf GDMS for depth profiling of layers with athickness of y100 nm was investigated by simulations andexperiments.46 Modeling showed that crater shape determinedthe resolution of depth profiles of stacked layers each with athickness of 100 nm. However, the non-Gaussian crater shapesmade the commonly used MRI (mixing, roughness, informa-tion-depth) model inadequate for GD experiments. Hence, thedepth resolution had to be defined as the reciprocal of themaximum slope instead of the distance between 16 and 84% ofthat maximum. Experimental evaluation was performed on10 layers of Cu and Ni40Cr60 (total thickness of 1 mm) onsilicon. Optimized discharge conditions in Ar and use of ananode diameter of 2–4 mm yielded depth resolutions of 25 and100 nm at depths of 0.1 and 1 mm, respectively. The depthresolution was identical for both dc and rf GD. Simulationsof the gas flow in a Grimm-type GD source showed strongcorrelation between the crater shape in a sample of 5 mmaluminum oxide on aluminum and the calculated pressureprofile in the plane 0.1 mm above the sample surface.

Pelaez et al.47 undertook the optimization of analytical depthprofiling using a simple GD source made in-house in combina-tion with TOFMS. The precision and depth resolution wasdetermined for the three operating modes of Ar dc-GD(variation of either current, pressure or voltage while keepingthe other two parameters constant) using Zn-based coatings(thickness of 1 mm) on steel. Variation of current in theconstant pressure–voltage mode resulted in craters with verticalwalls and flat bottoms parallel to the sample surface and gavebest resolution. In contrast to results obtained using a com-mercial rf-GD-OES instrument, the sensitivity of GD-TOFMSwas sufficient for detection of elements such as As and Bi atconcentrations of v0.05%. A subsequent study using steelcoated with galfan (Zn5Al), electroplated ZnNi (Zn12Ni) and agalvanized coating containing lead (Zn0.26Al0.11Pb) focused onquantitative depth profiling.48 Absolute sensitivity factors werederived from the analyte signal intensity as a function of thesputtering rates times the elemental concentration in homo-geneous RM with 0.01–100% concentrations of Al, Fe, Ni, Pband Zn. The constant current–pressure method was found toyield slightly better results than the constant voltage–pressuremethod but the constant voltage–current method was inade-quate. The nominal concentrations and the depth agreed within10–15% with the expected values.

A practical comparison of depth profiling techniques using ion

sputtering showed that Auger electron spectroscopy, X-rayphotoelectron spectroscopy (XPS), GDMS and SIMS all gaveacceptable depth profiles but that sputter times and resolutiondiffered significantly.49 Whereas Auger electron spectroscopyand SIMS were 10 times faster than XPS, they were 1000 timesslower than GDMS. Auger electron spectroscopy and XPSgave the best resolution, with values of 8 and 12 nm, res-pectively, at a depth of 450 nm. The speed of analysis of GDMSwas excellent but quantitative analysis was less impressive.Although SIMS had the best LOD, matrix effects hamperedquantification. No matter how much attention was paid tocalibration of the depth scale, none of the methods couldachieve the accuracy of transmission electron microscopy(TEM) of cross-sections.


3. Inductively coupled plasma mass spectrometry(ICP-MS)

3.1. Fundamental studies and instrumentation

Although not widely used, ICP-TOFMS has attracted suffi-cient interest to warrant a detailed discussion of performance,together with a brief overview of applications.50 The advant-ages of TOFMS detectors over quadrupole mass filters formulti-elemental analysis of fast transient signals were dis-cussed. The applications of ICP-TOFMS to the detection ofsignals from LA, ETV, GC, LC, CE and FI analysis werereviewed.

The poor ionization of the halogens in a conventional Ar-based ICP (Ar-ICP) (e.g., 961024% for F) has led to use of thelow-pressure He-ICP as an ion source in the determination ofBr, Cl, F and I.51 A 5 ml aliquot of an aqueous sample wasplaced on a looped tungsten filament and heated electro-thermally to y2500 uC. The resulting sample vapour wastransported to the ICP with He. The LOD of 0.13, 2.4, 23 and0.05 ng ml21 for Br, Cl, F and I, respectively, were much betterthan those from an Ar-ICP. The absolute system LOD using5 ml of sample were in the sub-pg range and allowed the directdetermination of F at the ng ml21 level in water to be made.

Park et al.52 achieved improved ionization of As, Ge and Se bythe addition of methane (10 ml min21) to the coolant gas. Theanalytical sensitivity of the proposed system was at least two-fold superior to that of a conventional Ar-ICP. The LODobtained for As, Ge and Se were 0.012, 0.014 and 0.064 mg l21,respectively. Recoveries of 103, 99.9 and 96.5% for As, Ge andSe, respectively, were obtained for NIST SRM 2670 (freeze-dried urine).

A conventional ICP consumes about 15 l min21 Ar. A high-efficiency torch (HET) for ICP-MS was developed53 in whichthe plasma could be sustained at an Ar flow rate of 8 l min21.The sensitivity, electron number density, degree of ionizationand ionization temperature of the plasma sustained by theHET were evaluated and compared with those from a con-ventional ICP torch. The HET could be used for ICP-MSwithout significantly degrading the performance of the ICP.Furthermore, the HET reduced the matrix effects whichoccurred in the presence of calcium and sodium species.

The 13C : 12C isotope ratio could be determined in aqueoussolutions of tryptophan, myoglobin and b-cyclodextrin withabout 1% RSD using a prototype ICP-MS instrument with apair of quadrupoles operated in parallel.54 Spectral interferenceon 13C1, thought to be from 12C1H1, occurred with incompletedissociation of myoglobin and/or b-cyclodextrin, but was notapparent with tryptophan. Decreasing the aerosol gas flowrate slightly from that yielding maximum signal eliminatedthe 12CH1 interference. The count rate of the minor isotope(13C1) was artificially enhanced by increasing the voltage of the13C1 detector, and/or by changing the ion beam splitterpotential from a central position. Modifications were made tothe skimmer orifices and ion extraction optics to improvesensitivity.

The performance characteristics of MIP-TOFMS using Ar,Ar–He and He as plasma gases were studied.55 The use of Heplasma gas provided both a very simple background spectrumfree from any significant Ar peaks (but still with NO1 peaks)and good analyte sensitivity. The Ar-MIP, on the other hand,had a background containing many Ar, H2O, N2 and O2

species under a wide range of plasma sampling distances.A simple diagnosis procedure for estimating matrix effects

used both non-analyte and analyte signals.56 Non-analytesignals (e.g., ArO1, N1) were used to monitor changes inplasma conditions. Matrix effects of Al, Ba, Cs, K and Na on19 non-analyte signals and 15 element signals were monitored.Multiple linear regression was used to build the predictionmodels for objective feature selection. The use of non-analyte

elemental and polyatomic signals was evaluated for diagnosingmatrix effects. Models used to diagnose overall matrixeffects were more accurate than those which used the effectof individual analytes. The models presented were able todiagnose successfully matrix effects with at least an 80% successrate.

3.2. Interferences

The formation and elimination of interferences in ICP-MSremains a subject for much discussion. The correct optimiza-tion of experimental conditions and appropriate choice ofskimmer cone materials (aluminium, copper, gold, nickel,silver) was, yet again, re-emphasized in a study of the heat offormation for polyatomic species.57 Segura et al.58 investigatedthe determination of ultratrace levels of Fe in complexmatrices, such as ground water and mineral samples, usingcold plasma conditions (shield torch, 700 W rf power) to reduceAr-based interferences. In order to overcome the increase ofCa-based interferences in a cold plasma, a microcolumn (5.0 cmlength and 3.0 mm id) filled with an Fe-specific chelating agentwas used to separate Ca and Fe. A preconcentration factor forFe of y92 and a LOD of 0.74 mg l21 were achieved using 10 minsampling time and 1 M HNO3 (200 ml) as eluent.

The use of mixed-gas plasmas to reduce the effects of non-spectroscopic interferences was studied using spatial profiling ofion distributions.59 The addition of 5.9% N2 to the outer Ar gasflow greatly reduced the effects of matrix-induced enhance-ment. The formation of analyte oxides decreased by over anorder of magnitude in the mixed-gas plasma, at the expense ofoverall sensitivity. A comparison of the analyte profiles withthose of background ions suggested that EI was the pre-dominant ionization mechanism in Ar plasma whereas, inmixed-gas plasmas, charge-transfer with Ar1 was implied bythe close match between the Ar2

1 profiles and those of theanalytes. The identical radial profiles of background poly-atomic ions also suggested that the Ar-containing ions all hadAr1 as the precursor ion in the plasma. In contrast, thedifferent profiles observed for these ions compared with Ar2

1

in the mixed-gas plasma implied that these ions originated fromthe combination of neutral Ar with an ion. A mixed O2–Arplasma was used by Kralj and Veber60 in an evaluation ofnon-spectroscopic effects caused by moderate concentrationsof organic compounds (methanol, acetonitrile, tris-(hydroxy-methyl)methylamine, glucose and ammonium acetate). Analytesignal changes were ascribed to the effect of C derived fromvolatile organic compounds (VOC). It was concluded that thehigh signal enhancements observed for As, Se and Zn could beattributed to more efficient ionization, caused by the presenceof C1 ions from the modifiers.

The effect of various dynamic reaction cell (DRC) para-meters (collision gas flow rate, reaction gas flow rate andbandpass settings) and the matrix composition on the massdiscrimination of isotopic ratios was studied by Vanhaeckeet al.61 In-cell fractionation resulting from collisional losses,space-charge and kinetic effects differed with various DRCparameters. It was concluded that accurate results could begained by using an external rather than internal correction formass discrimination. The matrix composition also had asignificant influence on the mass discrimination, especiallywhen condensation product ions were used for isotope ratiomeasurement. This effect could be overcome by the use of amatrix-matched isotopic standard for external correction orby isolation of the target element from the major matrixcomponents.

The use of collision cells for the reduction of interferencescontinues to attract attention. The LOD for the radioisotopes135Cs, 137Cs, 210Pb, 226Ra and 90Sr improved by factors of 33,22, 500, 50 and 100, respectively, after optimization of collisioncell parameters.62 It was noted that Cs and Sr measurements


required different collision cell parameters when Ba or Zr,respectively, were present. O’Brien et al.63 used a hexapolecollision cell with a direct injection high efficiency nebulizer(DIHEN) to determine elements such as As, Ca, Cr, Fe and Se.Compared with analysis using a micronebulizer–spray chamberarrangement, improved sensitivities (by a factor of 2–9), LODand precisions were obtained. The use of CID with H2 reducedisobaric interferences, whereas the use of He as a collision gasenhanced analyte ion transmission as a result of collisionalfocusing. The technique was successfully applied to the deter-mination of Ag, Al, Co, Cr, Cu, Fe, Mn, Pb, Sr and Zn (0.49–6.56109 atoms cm22) on silicon wafer surfaces. The methodwas also applied to the determination of Cr in DNA.

The application of ICP-MS with a DRC has been investi-gated by a number of workers for reduction of the Ar2

1 ionpopulation in the determination of Se. Operating parameters,including reaction gas flow and transmission settings, of theDRC using various reaction gases (CH4, H2, N2O and O2) wereoptimized using isotope ratio measurements and the S/B ratioof the analytes.64 The highest reaction efficiency for thedetermination of Se was obtained using CH4 as reactant gas.The use of N2O and O2 also suppressed the Ar2

1 interference,but reaction or scattering losses of Se1 and SeO1 weresignificant under these conditions. The use of H2 led to a highS/B ratio but it was also the least efficient gas for reducing thelevels of Ar2

1 interference. The determination of Se as SeO1

was also investigated using N2O and O2 as reaction gases, butthe slow oxygenation of the potentially interfering Mo1 ionreduced the analytical use of this approach. The best LOD innatural waters (2 ng l21) were obtained by determining 80Se1

using CH4 to suppress the levels of Ar21. The use of a tunable-

bandpass reaction cell for the determination of Se in serum andurine RM was evaluated by Nixon et al.65 Samples were dilutedwith a solution containing 10% ethanol, 1% HNO3, 0.5%Triton X-100 and internal standards (Ga and Y). Reaction cellconditions were optimized for the suppression of Ar2

1 usingCH4 as the reaction gas. It was concluded from the analysis ofNIST SRM (bovine serum and urine) that closer correlationwith reference concentrations was obtainable with DRC-ICP-MS than with conventional ICP-MS.

Further applications of collision cell technology included itsuse to reduce carbon-based polyatomic ion interferences in theETV-ICP-MS determination of Cr, Mg and Si.66 Interferenceson Cr and Mg isotopes were reduced by the use of NH3 as areaction gas. The LOD for 28Si1 and 29Si1 were greatlyimproved over those for conventional ICP-MS; however, NH3

was found to be unsuitable as a reaction gas for the deter-mination of 30Si1, presumably due to the formation of NO1

species. The results of S/B studies indicated that the optimizedexperimental conditions for DRC were similar to those forsolution nebulization and ETV for Cr, but different for Mg andSi. Potential interferences on 28Si1 were found to be reduced bythe use of NH3 reaction gas in a DRC.67 Optimization ofoperating parameters improved the S/N by over an order ofmagnitude for the determination of Si in steel, giving a LODof 2 mg g21. Validation of the method was carried out by thedetermination of Si in NIST SRM 15h (Carbon Steel, 0.1 C)and 368 (Carbon Steel, AISI 1211). Accuracy was better than3% with a precision v5%.

Liu and Jiang68 used DRC-ICP-MS for accurate determina-tion of Cu in coal fly ash samples with high levels of titanium.The polyatomic isobaric interferences caused by the formationof TiO1 at m/z 63 and 65 were avoided by measuring theproduct cluster ion Cu(NH3)2

1. The method was applied to thedetermination of Cu in NIST SRM 1633a and 1633b (coal flyash). The precision between sample replicates was v2.0% andthe results agreed well with the certified values. The LOD forCu was 0.015 ng ml21.

The determination of S and selected trace elements in metallo-thionein (MT)-like proteins using CE-ICP-MS with an

octopole reaction cell was described by Profrock et al.69 Poly-atomic ions that interfered with the determination of 32S wereminimized by the addition of Xe to the collision cell. Themethod was applied to the detection of Cd, Cu, S and Zn incommercially available MT preparations and MT-like proteinsextracted from liver samples of bream (Abramis brama L.). TheLOD were 0.5, 0.3, 3.2, 1.3 and 0.5 mg l21 for Mn, Ni, 32S, 34Sand Zn, respectively. The authors recommended the use ofexternal calibration for the determination of Cd and S incommercially available MT preparations.

3.3. Sample introduction

The continuing development of sample introduction techniqueshas yet again provided a large body of research, somewhat atthe expense of new technology. Again, LA has attractednumerous studies, with the trend continuing towards descrip-tions of applications, rather than novel developments. Thecoupling of separation techniques, such as CE, GC and HPLC,to ICP-MS instruments appears to have almost reached routinestatus, with ICP-MS increasingly being relegated to the role ofan element-specific detector.

3.3.1. Laser ablation. The use of LA for sample introduc-tion remains as popular as ever. Much effort is being put intoelucidating the nature, formation and sizes of particles generatedduring the ablation process. Gunther and his colleagues havebeen prolific in their studies in this field. The use of wavelengths(l) of 266, 213 and 193 nm under identical conditions with asingle solid state laser source (1064 nm Nd:YAG) for harmonicgeneration, together with sum frequency mixing and opticalparametric oscillation, was evaluated.70,71 Experiments werecarried out on the NIST 600 series SRM (silicate glasses).Particle size distributions for all three wavelengths increased inthe order 193 nm v 213 nm v 266 nm and were believed to berelated to the absorption behaviour of the sample at eachwavelength. The change towards larger particle sizes withincreasing wavelength influenced the noise in the transientsignals and their intensity ratios. Ablation at 193 nm produceda smaller number of particles with diameters of w150 nm thanablation at longer wavelengths. Due to the decreased numberof particles with diameters w150 nm, vaporization-inducedelemental fractionation within the ICP was reduced, especiallyfor the more transparent samples. The ICP-MS response forablation at l ~ 213 nm was biased towards that at l ~ 193 nmand l ~ 266 nm for opaque and transparent samples,respectively. The work showed that the choice of wavelengthinfluenced the particle size distribution, which could becontrolled to enhance vaporization, atomization and ionizationeffects within the ICP.

Horn and Gunther72 investigated the influence of ablation cellcarrier gasses Ar, He and Ne on the particle size distributiongenerated and subsequently transported during the LA process.Several glass RM were analysed using l ~ 193 and 266 nm.The particle size distributions were determined using an opticalparticle counter within the diameter range of 0.1 to w2 mm.The results illustrated that the ablation carrier gas stronglyinfluenced the particle size distribution for l ~ 193 nm whilethis effect was far less pronounced for ablation using l ~266 nm. The experiments also showed that ablation at l ~266 nm produced primary particles, which were not principallyinfluenced by the ablation carrier gas environment, whereasablation at l ~ 193 nm produced a larger amount of vapour,which subsequently condensed to form aggregates. Thisexplained the large increase in sensitivity obtainable by ICP-MS when using He carrier gas together with 193 nm ablationsystems, whereas only a minor increase was observed whenusing l ~ 266 nm under similar conditions. The influence ofthe laser energy density on the particle size distribution as wellas on values for transport efficiency was also investigated.


Interaction of the laser with different samples leads todistinct particle size distributions. A particle separation device,developed to minimize particle-size-related incomplete sampleionization in the ICP, gave effective particle separation byusing centrifugal forces in a thin coiled tube.73 The particle cut-off size was varied by changing the number of turns in the coil,the gas flow and the tube diameter. Quantitative analysis withnon-matrix-matched calibration RM, revealed that the particlecut-off sizes had to be chosen according to the ICP conditionsand the instrument used for analysis. Analysis of varioussample materials demonstrated the applicability of the device.The production of ions for silicate matrices in the ICP wassignificantly reduced for particles w0.5 mm in diameter and wasdependent on the element monitored. A washout procedurewas developed to reduce memory effects caused by theseparated particles. This had the additional benefit of allowingtrapped particles to be studied. This work showed the impor-tance of particle-size-dependent ICP-MS signal response andthe potential of the separator for reducing ICP-inducedelemental fractionation.

In a study of the ablation of copper metal, Jackson andGunther74 showed that, whilst significant isotopic fractionationoccurred at the ablation site at low laser fluence, the dominantsource of isotopic fractionation at high laser fluence was thepreferential volatilization of 63Cu during incomplete vaporiza-tion and ionization in the ICP of particles w0.5 mm indiameter. The larger particles (w100 nm) in a laser-producedbrass aerosol were enriched in Cu, whereas the smallestmeasurable particles (v100 nm or in the vapour phase) wereenriched in zinc.75 Solution nebulization analysis on digestedfilter-collected aerosols resulted in a higher Cu : Zn ratio thanthe certified value for the brass sample. A number of conclu-sions were drawn from the study: the ablation process led to nomeasurable Cu enrichment at the ablation crater rim; thedeposition of Zn between the ablation site and the aerosolcollection on filters led to a Cu : Zn ratio up to 37% higher onthe filter in comparison with the certified value; LA aerosolsmeasured by ICP-MS led to significantly lower Cu : Zn ratios incomparison with the certified value. The various sources offractionation were thought to cancel each other out and to leadto an agreement between measured and certified values.

To evaluate a new chemically assisted LA-ICP-MS technique

for elemental ratio analysis of soil samples, Hirata76 determinedPb:U ratios for zircon samples previously analysed by othertechniques. Conventional LA for measurement of Pb : U ratiosshowed considerable elemental fractionation during ablation,mainly due to the large difference between the elementalvolatilities of Pb and U. Introduction of 1,1,1,2-tetrafluoro-ethane into the laser cell caused a reaction of refractory Ucompounds to form volatile UF6 under the high-temperatureconditions at the ablation site and avoided the redeposition ofU around ablation pits. Despite the incomplete reaction,formation of volatile UF compounds improved the transportefficiency of U. Precision of the 206Pb:238U ratio measurementwas 3–5% (2s) for NIST SRM 610 (silicate glass) and NancyRM 91500 (zircon). The U–Pb age data obtained agreed,within analytical uncertainties, with the previously reportedvalues. The observed Pb:U ratio for solid samples wasrelatively insensitive to laser power and ablation time.

The analytical performance of LA-ICP-MS using fs and ns

UV laser pulses was assessed in an attempt to overcomethermally induced chemical fractionation.77 These lasers hadGaussian beam energy profiles. Resulting crater morphologiesand analytical signals showed that the use of a fs laser yieldedablation with minimal thermal effects. Despite the less stableenergy output of the fs laser, the measured Pb, Pb:Th and Pb:Uisotopic ratios were more precise, repeatable, and accuratethan those obtained with a ns laser. Measurements on glass,monazite and zircon RM also showed that fs LA-ICP-MS

calibration was less matrix dependent than when ns lasers wereused and was therefore more versatile.

High-speed photographs and videos of an ICP have beenused to examine the fates of solid particles produced by LA bymeans of the trajectories through the plasma, the lifetimes andemission behaviour of particles.78 Desolvated particles from anebulized Y solution and particles ablated from a Y2O3 pelletwere mixed and introduced simultaneously into an ICP. High-resolution digital photographs and video sequences werecaptured with shutter speeds of approximately 65 ms. Theplasma behavior of particles generated by a frequency-quadrupled Nd:YAG laser (l ~ 266 nm) or an ArF excimerlaser (l ~ 193 nm) was compared, and either Ar or He wasused as transport gas through the ablation cell. No red emissionfrom YO was visible in the ICP when a 20 ml min21 nebulizerwas used to spray a 2000 ppm aqueous Y solution. Thissuggested that the wet droplets from this nebulizer desolvatedalmost completely before entering the ICP. These particlesatomized and ionized like the small dry particulates fromLA. However, many large ablated particulates were observedto pass through the plasma intact, possibly contributing tosignal noise, deposition on the sampler and skimmer cones, andelemental fractionation.

A two-point calibration method for LA-ICP-MS was based onthe mixing of desolvated particles from a Teflon micro-concentric nebulizer (MCN) and particles ablated from alocalized position on a solid sample.79 The comparison ofanalyte signals from the solution with those from the ablatedsolid allowed calibration to be achieved without the use ofmatrix-matched or pelletized solid standards. The loss ofsample weight in the ablation cell was measured on-line with apiezoelectric microbalance for signal normalization. Therelative accuracy of the quantitative analyses of NIST SRM612 (glass) and 1264a (steel) were 3–12%, except for Bi and Pbin steel which were distributed heterogeneously and probablysubject to fractionation. The use of He instead of Ar astransport gas reduced fractionation.

Bizzarro et al.80 determined in-situ Sr isotopic compositions ofapatites and carbonates. The study examined the Sr isotopichomogeneity among coexisting igneous minerals with high Sr(w3000 ppm) and low Rb (vv1 ppm) levels within a singley50 mm thin-section. The precision (y0.005%, 2s) of 87Sr :86Sr values measured by LA-ICP-MS was similar to thatobtained by TIMS analysis but the measurement time of only100 s was much reduced from the w1 h needed for TIMSanalysis. The combined total analyses (n ~ 107) of apatite andcarbonate yielded 87Sr:86Sr ratios ranging from y0.7025 toy0.7031. This variation in Sr isotopic composition was largerthan the estimated external reproducibility (y0.000 05, 2s).The large range of 87Sr:86Sr values suggested to the authors thatapatite and carbonate precipitated predominantly under non-equilibrium conditions.

3.3.2. Thermal and chemical vaporization. A novel electro-static precipitator for the determination of analyte transportefficiencies from a tube furnace ETV-ICP-MS was describedby Ertas and Holcombe.81 The simple experimental set-upcaptured the analyte electrostatically using a negative coronadischarge. The absolute transport efficiencies were determinedfor Be, Bi, Cd, In and Pb to be 26¡3, 35¡1, 39¡8, 35¡1 and50¡11%, respectively. Addition of Pd as carrier enhancedtransport efficiencies by 20–50% for all elements except Bi, forwhich the efficiency decreased by 17%. The addition of NaClproduced transport enhancements of up to 50% for all fiveanalytes. The mean efficiency for collection of the aerosolleaving the ETV was w90% for the five elements studied.Reasonable precision could be obtained with approximately10 ETV firings consuming y20 ng. The transport results werein good agreement with previous studies using similar ETVdesign and analyte masses.


A tungsten filament ETV and ICP-TOFMS system wasdeveloped for the rapid and simultaneous determination of Cr,Cu, Li and Pb in human whole blood and serum samples.82 Thevaporizer, based on a commercial tungsten filament in a 17.3 mlquartz cell with a filament lifetime of w350 firings, wasespecially suited to the evaporation of minute amounts ofsample. The vaporizer was coupled to the plasma torch via an8 cm (3 mm id) glass tube. Sequential and ‘‘modified simplex’’algorithms were used to optimize the voltage/current heatingprograms (drying, pyrolysis, vaporization and filament clean-ing) for sample evaporation as well as the ICP-TOFMSoperational parameters (carrier gas flow rate, modulation pulsewidth) for the simultaneous determination of 17 isotopes. Thecompromise operation parameters of the tungsten filamentETV system differed for aqueous standard solutions, humanwhole blood and serum samples. An Ar-ICP carrier gas flowrate of 1.16 l min21 provided optimum results for the detectionof transient signals. By using 10 ml sample volumes of aqueousmulti-elemental standard solutions, absolute LOD (3s) in therange 0.007–0.1 pg (Ba, Cd, Co, Cr, Cu, Ga, In, Mn, Ni andSr), 0.11–0.19 pg (Ag, As, Bi, Li, Pb and Pd) or 11 pg (Fe) wereachieved. The precision was 2–6% for 10 replicate analyses ofmulti-elemental sample solutions (5 ng ml21 for all elementsexcept Fe, which was at 20 ng ml21). The method accuracy wasverified by the analysis of NIST SRM 909 (Human Serum) andNycomed Pharma 404108 RM (Human Whole Blood) for Cr,Cu, Li and Pb. Other than the addition of a phosphate-basedmatrix modifier, followed by appropriate dilution, no samplepretreatment was necessary. Calibration was achieved byinternal standardization using multi-elemental solutions.Good agreement with the certified values was observed.

Slurry sampling ETV-ICP-MS has been applied to a varietyof materials and analytes. In one study, Cd, Hg, Pb and Tl weredetermined in coal and in coal fly ash.83 An ultrasonic probewas used to homogenize a 25 mg ml21 slurry in the autosamplercup just before introduction of the slurry into the graphite tube.The procedure was optimized for analyte sensitivity, furnacetemperature program, amount of modifier, acid concentration,gas flow rate and particle size. For Hg, the pyrolysis stage wasomitted and a low vaporization temperature (450–1000 uC) wasused. The residual matrix was eliminated in the first step of thefollowing cycle. The modifiers used were: Pd for Cd and Tl; Au,Ir or Pd for Hg; Ir or Pd for Pb. The accuracy of the methodwas assessed by analysing six coal RM and one coal fly ashRM. With the exception of Hg in BCR No. 180, results weretypically within the 95% confidence interval of the certifiedvalues, or close enough (sic) to the recommended values, aslong as the samples were ground to a small enough particle size.The LOD were typically around 0.08, 0.03, 1 and 0.02 mg g21

for Cd, Hg, Pb and Tl, respectively. The precision was v5%. Inanother study, Hg and Tl were determined in environmentalsamples.84 The slurry was homogenized in the autosampler cupwith a constant flow of Ar, just before transferring an aliquotto the graphite furnace. Modifiers and carriers studied includedsolutions of KMnO4, in both untreated and Ru-treated tubes,K and Mn in solution. The best thermal stabilization andsensitivity were obtained with KMnO4 in an untreated tube, inparticular for the determination of Tl. The accuracy of themethod was assessed by the analysis of eight certified RM.External calibration was used with aqueous standards preparedin the same medium as the slurries. The LOD in the sampleswere 0.18 and 0.07 mg g21 for Hg and Tl, respectively. Theprecision (n ~ 3) found for the different samples was 0.8–11and 1–9% for Hg and Tl, respectively. In a third paper thedetermination of As, Pb, Se and Sn was reported in sedimentslurries.85 The ETV tube was treated with Ru as a permanentmodifier, and an optimized mass of 1 mg of NaCl was addedto the slurry (1 mg ml21) as a physical carrier. The optimumpyrolysis and vaporization temperatures were 800 uC and2300 uC, respectively. The effect of different acid concentrations

in the slurry on the analyte signal intensities was evaluated. Theconcentrations measured for six sediment RM were within the95% confidence level of the certified values. The LOD were:0.17, 0.3, 0.05 and 0.28 mg g21 for As, Pb, Se and Sn,respectively. The precision (n ~ 5) for the different sedimentsamples was 1–8, 29, 6–12 and 3–8 for As, Pb, Se and Sn,respectively.

The influence of instrument operating conditions and slurrypreparation was investigated for ultrasonic slurry samplingETV-DRC-ICP-MS for the determination of Cr and V in soilsamples.86 Ascorbic acid was used as the modifier to enhancethe ion signals. The intensities of ions isobaric with Cr and Vwere significantly reduced by using NH3 as the reaction cell gas.Results obtained for Cr and V in NIST SRM 2709 (SanJoaquin soil) and 2711 (Montana soil) using the method ofstandard additions agreed with the certified values. The pre-cision between sample replicates was better than 3% and themethod LOD, estimated from standard addition curves, wasapproximately 0.16 and 0.24 mg g21 for Cr and V, respectively.

Elemental signatures of rare earth elements (REE) in fishotoliths were determined by Arslan and Paulson87 using a solidphase extraction protocol in combination with ETV as asample introduction procedure for ICP-MS. Various para-meters, such as carrier gas flow rate, atomization temperatureand chemical modification, were optimized. Severe memoryproblems were experienced at atomization temperatures ofv2800 uC due to incomplete vaporization. An increase in thePd chemical modifier concentration up to 0.5 mg in the 70 mlinjection volume led to a four-fold enhancement of the inte-grated signals. This phenomenon was attributed to the carriereffect of Pd rather than stabilization, since no significant losseswere observed for drying at around 700 uC, even in the absenceof Pd. Preconcentration was performed on-line at pH 5 byusing a mini-column of Toyopearl AF-Chelate 650M chelatingresin, which also eliminated the calcium matrix of otolithsolutions. After preconcentration of 6.4 ml of solution, theconcentrate was collected in 0.5% (v/v) HNO3 (0.65 ml) inautosampler cups, and then analyzed by ETV-ICP-MS. Themethod was validated with the analysis of a fish otolithRM. Results obtained from otoliths of fish captured in thesame habitat indicated that otolith REE concentrations weremore dependent on environmental conditions of the habitatthan on species differences.

Anderson and Pergantis88 developed a novel sequentialpneumatic nebulization (PN) hydride generation (HG) ICP-MS method for the determination of As and Se species. Themethod offered the advantage of sample introduction via eitherPN or HG by simply rotating a 4-way switching valve while thesystem was in operation. In PN mode, the liquid sample wasaspirated into the ICP, allowing the total amount of eachelement to be determined. In HG mode, only the As and Sespecies that formed volatile hydrides were measured. Con-veniently, this differentiated between the four most toxicarsenic species (arsenate, arsenite, monomethylarsonic acidand dimethylarsinic acid) on the one hand, which formedvolatile hydrides, and the virtually non-toxic forms (arsenobe-taine, arsenosugars, etc.), which did not, on the other. Thisallowed the amounts of toxic and non-toxic As species presentin a sample to be estimated. For As, the technique gave LOD of36 and 1 ng l21 in PN and HG modes, respectively. For Se, theLOD were 1.50 and 220 ng l21 in PN and HG modes,respectively. The technique gave good long- and short-termstabilities of v6% RSD for both elements.

3.3.3. Nebulization. A DIHEN was used by Baca et al.89 tocouple a thin-layer electrochemical flow cell with an ICP-MSinstrument to perform on-line anodic stripping voltammetry(ASV). The ASV-DIHEN-ICP-MS technique was capable ofanalysing selected heavy metals present at ultratrace levels(sub-pg g21 levels) and with LOD lower than that available


from conventional ICP-MS. In addition to good analyticalperformance, the technique offered other attractive features,such as the ability to eliminate matrix effects and the possibilityof probing electrode reactions involving trace amounts of metalspecies. For conducting ASV on-line with ICP-MS, theDIHEN offered advantages over the MCN, such as reductionof both memory effects and potential artifacts from the erosionof the Hg film into the flowing solution stream. Compared witha direct injection nebulizer (DIN), the DIHEN was easier tooperate and its simpler design and the lack of back pressure inthe DIHEN capillary made it more compatible with couplingto the thin-layer electrochemical cell.

The most critical operating parameters for the determinationof Hg present in organic solvents using a DIHEN with FIsample introduction were the nebulizer gas flow rate andamount of O2 added to the plasma.90 A nebulizer gas flow rateof 0.3 l min21 together with O2 (50 ml min21) added to theplasma auxiliary gas flow gave stable conditions and highanalyte sensitivity. The recovery of Hg from HgCl2, CH3HgCl,(CH3)2Hg and HgO in hexane was 99¡4, 101¡4, 95¡4and 104¡7%, respectively. The LOD for Hg in hexane was85 pg ml21 based on 3s on the signal for a 201Hg-spiked blank.A gradual deposition occurred at the nebulizer tip and partlyblocked the gas annulus and, under the optimized conditionsused in this work, the nebulizer could be used for only y50hexane samples before cleaning was necessary. The rate ofdeposition seemed to be related to the amount of carbonintroduced through the nebulizer.

A DIHEN-ICP-MS system was used by Kahen et al.91 toanalyse petroleum samples dissolved in volatile organic solvents.To minimize solvent loading, the solution uptake rate wasreduced to 10 ml min21, far less than the 85 ml min21 commonlyused for aqueous sample introduction to the DIHEN, and O2

was added to the nebulizer gas flow and outer flow of theICP. Factorial design was applied to investigate the effect ofnebulizer tip position within the torch and of the nebulizer andintermediate gas flow rates on the precision and the net signalintensities in multi-elemental analysis. Cluster analysis andPCA were performed to distinguish between the differentisotopes, oxide species and doubly charged ions. Acceptablerecoveries (100 ¡ 10%) and precision (v3%) were obtained fortrace elements in a certified gas oil sample and a custom-madeRM using FI analysis. Because of high blank levels, LOD were1–3 orders of magnitude poorer for organic sample introduc-tion than those obtained for aqueous solutions.

Jensen et al.92 investigated the suitability of a DIN and aMCN for the analysis of drug substances containing Br, Cl and I.Using the DIN at flow rates of 25 and 50 ml min21, the influenceof 0–50% methanol and 0–25% acetonitrile on the sensitivitywas studied. The relative sensitivity of Br and Cl decreased withincreasing amounts of organic solvent, whereas that for Ireached a maximum at low concentrations of organic solventand then declined with increasing amounts of organic solvent.Unlike the response of the DIN, that of the MCN depended onthe analyte structure because many Br-, Cl- and I-containingcompounds were partially lost in the desolvating unit. Forthose compounds that were not lost, the sensitivity wasindependent of methanol concentration in the solvent.

When self-aspirating nebulizers are used for coupling CE toICP-MS, it is important to offset the suction effect to avoiddegradation of the separation. Sample uptake rates for threeMCN of the same model, in combination with a cyclonic spraychamber, were characterized and compared for utilization inCE-ICP-MS interfaces.93 The specific model studied had anominal uptake rate of 100 ml min21 at 1 l min21 Ar gas flowrate. Sample uptake rates at various nebulizer gas flows weremeasured by aspirating water from a weighed container andcalculating the uptake rate in ml min21. The nebulizers studiedprovided good day-to-day reproducibility, but a comparisonof the three nebulizers reflected a significant difference in

performance. The characteristic of decreasing uptake rates withincreasing nebulizer gas flow could be exploited for sampleintroduction for CE-ICP-MS. Uptake rates showed strongdependence on Ar gas flow rates and the dimensions of thesample uptake tubing.

3.3.4. Flow injection. A novel low-volume electrolysis cellwas developed specifically for use in a system with electro-chemical HG (EcHG) sample introduction for the rapid andsimultaneous determination of the hydride forming elementsAs, Bi, Ge, Hg, Sb and Se.94 The EcHG conditions (electrolyteconcentrations and flow rates, electrolysis voltage and current),and ICP-TOFMS operational parameters (carrier gas flow rate,modulation pulse width) were optimized for the simultaneousdetermination of 12 selected isotopes. The absolute LOD (3s)using a 200 ml sample loop were in the range 10–160 pg for As,Bi, Ge, Hg, Sb and 1.1 ng for Se. The precision was 4–8% forseven replicate injections of 20–100 ng ml21 multi-elementalsample solutions. The analysis of NIST SRM 1643d (TraceElements in Water) showed good agreement with the certifiedvalues for As and Sb. However, the value obtained for Sewas dramatically different from the certified value. This wasbelieved to be due to the presence of hydride-inactive Se speciesin the sample. Recoveries of better than 93% for Ge and Hgand 83.9% for Se were achieved on a spiked SRM sample.The method was successfully applied to the simultaneousmulti-elemental determination of hydride forming elements inspring water samples originating from two different regions inHungary.

A FI system combining electrolytic dissolution and program-mable isotope dilution (ID) ICP-MS was proposed by Packeret al.95 for the determination of Pb in high-purity copper. Bestelectrolytic conditions were achieved using 1.5 M HNO3 aselectrolytic solution and applying 2.5 A (dc) current via a goldcathode for 4 s. The 206Pb:208Pb ratio was measured before andafter on-line addition of the 208Pb-enriched solution. Precisionsfor isotope ratio measurements and the determination ofconcentration in samples containing 7–70 mg g21 Pb were0.5% (n ~ 3) and v4% (n ~ 9), respectively. No significantdifference at the 95% confidence level was observed when theresults were compared with those obtained by conventionaldissolution.

Band broadening due to laminar flow and spray chamber deadvolume is a potential problem in FI-ICP-MS. Sonke et al.96

studied these two dispersion effects with a sheath flow CE-ICP-MS interface. A numerical model was used to simulateadvection diffusion processes in the CE-capillary and disper-sion in the spray chamber. Experimental results of FI with thisCE-ICP-MS interface agreed well with numerical modelingresults. Dispersion due to laminar flow depended strongly oncapillary diameter and analyte diffusion coefficient, and to alesser extent on laminar velocity and capillary length. Thistypically amounted to a peak width increase of one order ofmagnitude. Use of three different spray chambers of 5, 20 and150 ml dead volume demonstrated an increase in bandbroadening and peak tailing with increasing dead volume.The use of standard Scott-type spray chambers (w90 mlvolume) increased peak widths by 5–10 s regardless of theinjection time. A low-dead-volume spray chamber was recom-mended for experiments for which resolution was critical. Theauthors felt that this modeling approach could be extended tothe coupling of other FI techniques, such as micro-LC andnano-LC with ICP-MS.

3.4. Separation and speciation

The coupling of chromatography to ICP-MS remains aspopular as ever, with the emphasis still strongly on the separa-tion science. The review of Montes-Bayon et al.97 focused onthe basic principles of LC-ICP-MS, its historical development


and the many ways in which this technique could be applied.Different LC separations were discussed as well as the factorsthat had to be considered for their coupling with ICP-MS.Recent applications to the speciation of environmental, bio-logical and clinical samples were also presented.

The coupling of CE with ICP-MS allows high separationefficiency to be combined with good sensitivity, low sampleconsumption and good LOD. The interface between CE andthe nebulizing system of the ICP-MS plays a key role in theircombination. The most important factors in interface design,considered by Ye et al.98 in a review with 35 references,included electrical connections, nebulizer designs and flowcompensation in the nebulizer system. Methods for achieving astable electrical current within the capillary included theemployment of concentric sheath flow, stainless steel compo-nents, platinum wire and silver paint. The structures andperformance of various nebulizer designs (concentric, micro-concentric, high efficiency, ultrasonic and direct injection) werereviewed and compared. Four compensation systems suggestedfor elimination of nebulizer suction were negative pressure atthe inlet of the capillary, a sol–gel frit at the outlet of thecapillary, the pumping of an appropriate make-up flow tothe interface and the make-up flow rate. An overview of theevolution of CE-MS focused on its application to speciationanalysis.99 Different interfaces of CE to ICP-MS, as well asvarious nebulizer designs, were described and their charac-teristics presented. The efficiency of CE coupled to MS inspeciation analysis was illustrated by a number of relevantapplications to the study of inorganic, organometallic(Hg speciation) and bio-inorganic (As and Se speciation)compounds.

A simple, rapid and accurate multicapillary GC-ICP-TOFMS method was developed for speciation analysis ofmethylmercury (MeHg1) and inorganic mercury (Hg2

1).100

The potential of ICP-TOFMS for transient multi-isotopedetection of very short signals with full width at half maximum(FWHM) of 0.4 s was evaluated. Purge-and-trap (P&T) andsplit injection systems were compared for species separationand profile of transient signals. The use of P&T injection, afterin situ derivatization of the ionic mercury species with sodiumtetraethylborate, gave a baseline chromatographic separationof MeHg1 and Hg2

1 within 35 s. To correct for matrix-inducedion signal variation and instrumental drift, propylmercury(PrHg1) was used as internal standard. The LOD were 16 and257 fg g21 for MeHg1 (as Hg) and Hg2

1, respectively, and theanalytical precision for 10 successive injections of a standardmixture containing 10 pg MeHg1 (as Hg) and Hg2

1 were 1.2and 4.1% for MeHg1 and Hg2

1, respectively. The method wasvalidated by analysis of NRCC RM DORM-2 (dogfish muscle)and BCR CRM 464 (freeze-dried tuna fish).

A sheathless interface based on a total consumption micro-nebulizer operating at flow rates of 0.5–7.5 ml min21 wasdeveloped as an interface between capillary HPLC and ICP-MS.101 Mobile phases containing up to 100% organic solventcould be nebulized and transported into the plasma efficientlywithout the need for cooling of the spray chamber or additionof oxygen. The coupled system was applied to selenopeptidemapping in a protein fraction isolated from a selenized yeastextract. The LOD of 150 and 200 fg for 80Se and 82Se, res-pectively, were a factor of 100–150 better than those reportedpreviously for HPLC-ICP-MS. The minimal peak broadening(y5 s at FWHM) allowed baseline resolution of more than30 selenopeptides to be made. This is not feasible usingconventional HPLC-ICP-MS coupling.

Yang et al.102 described a method for the accurate andprecise determination of dibutyltin (DBT) and tributyltin(TBT) by species-specific ID-GC-ICP-MS. The analytes wereextracted with acetic acid using open microwave digestion,derivatized with sodium tetraethylborate and extracted intoiso-octane. Mass bias correction was based on the expected

ratio of 120Sn:117Sn to that of the mean 120Sn:117Sn ratiocalculated from the inorganic Sn peaks detected in all chro-matograms. The precision of calculated 120Sn:117Sn ratios forboth DBT and TBT in RM improved by a factor of two when asector field instrument was used instead of a quadrupoleinstrument. The method was validated using NRCC RMPACS-2 (marine sediment). The three-fold enhancement in S/Bobtained with the sector field instrument led to LOD for DBTand TBT of 0.3 and 0.4 ng g21, respectively. An on-line mode,based on continuous nebulization of a Sb solution, wassuccessfully applied for mass bias corrections in the rapid andprecise determination of TBT in sediments and biologicaltissues.103 A simultaneous microwave extraction and spikingprocedure was rapid (2 min) and gave a precison of 0.7–13.7%.

Encinar et al.104 reviewed the innovative approach of usingisotopically-labeled butyltin species for sea-water and sedimentspeciation analysis. The capability of the ‘‘multi-isotope-labeled multispecies’’ spikes to control every step in speciation,even the initial solid–liquid extraction of the species from asolid sample, was discussed. This strategy allowed detectionand correction for species degradation during the extractionprocess to be achieved. This method was applied to the studyand validation of the solid–liquid extraction of butyltin speciesfrom a sediment RM using mechanical shaking and microwave,ultrasonic and pressurized liquid extraction.

Application of a large volume sample stacking technique

improved, by a factor of 9–13, the LOD obtained with twodifferent CE-ICP-MS interfaces (a Babington and a highefficiency nebulizer).105 The proposed methodology wassuccessfully applied to metal speciation studies of MT isoformspresent in eel liver from both control and Cd-induced groups.Not only was there an increase in Cd isoform signals followingexposure to Cd, but differences in the profiles and signalintensities of the Cu and Zn isoforms were also observed.

Smith et al.106 described the use of HPLC-ICP-MS for thequantitative determination of ZD0473, a platinum anticancer

drug, and its related biologically active ‘‘aqua’’ compounds inbiofluid samples. Comparison of the method performance withthat of a conventional HPLC-triple quadrupole MS (HPLC-MS/MS) system showed that both methods were capable ofproviding accurate and precise results but that the HPLC-ICP-MS method had advantages of extended linear range andsuperior sensitivity. The LOQ for ZD0473 of 0.1 ng ml21 wasconsiderably better than that (5 ng ml21) obtained using theHPLC-MS/MS method. Use of a single combined HPLC-ICP-MS/MS system made it possible to determine the relative MS/MS response of the ‘‘aqua’’ compounds for the first time.

An ICP-MS instrument equipped with a DRC was used foron-line detection of V in serum after size exclusion chromato-

graphy.107 Use of a buffer at physiological salinity (0.15 MNaCl) was necessary to ensure the stability of the V compoundsduring chromatography but resulted in ClO interferences on Vin the ICP-MS analysis. This was overcome through use of theDRC, which resulted in unhindered detection of V withoutadapting the conditions of the chromatographic separation.The merits of CH4, CO, NH3 and O2 reaction gases werecompared, as well as those for the combination of Ar (collisiongas) and H2 (reaction gas). In each instance, the reaction cellparameters were optimized in order to obtain the lowest LODfor 51V (as 51V1 or as 51V16O1 with O2 as the reaction gas) inthe Cl-rich solution. The best LOD for vanadate of 4 pg V in100 ml serum injected was obtained with NH3 reaction gas. Therepeatability was 7%.

The use of HPLC-ICP devices for the determination of non-metals is less common than with metals. Peters and Jones108

reviewed the applications of HPLC-ICP techniques for the

determination of compounds containing C, P, S and the halogens.The instrumentation used for each application was described,and the performance of the systems summarized.



The high resolution and sensitivity of HR-ICP-MS has beenexploited by Chipley et al.109 to determine very low concen-trations of elements in soils by a new, on-line, continuous-leachtechnique, producing real-time data. Solvents (deionized H2O,1% HNO3, 10% HNO3 and 30% HNO3), were sequentiallypumped through a column loaded with 20–50 mg of thesample. The mass spectrometer was scanned continuously atlow, medium or high resolution, depending on the element andthe interferences to be resolved. The authors found that releasepatterns of the trace elements could be matched to the majorelement composition of specific minerals.

The provision of timely information on the distribution andlevel of contamination following a nuclear incident has beeninvestigated using HR-ICP-MS combined with ultrasonicnebulization and on-line ion chromatography (IC).110 Amethod for the determination of Am, Np and Pu in foodwithin 3 h, without compromising accuracy or LOD, waspresented. Samples were prepared by HNO3 closed-vesselmicrowave digestion, evaporated to dryness and diluted into amobile phase of 1.5 M HNO3 and 0.1 mM 2,6-pyridinedi-carboxylic acid. Separation of 239Pu from 238U was achievedusing a commercially available polystyrene–divinylbenzeneIC column. Instrumental sensitivity for 238U was found to bew2 6 107 cps ppb21, giving a LOD for Pu of 0.02 pg g21.

The analysis of both aqueous and organic phases followingsolvent extraction is often a time-consuming process. Ekberget al.111 presented a technique whereby Zr in both aqueousand organic phases, contained in an ethanol carrier phase,could be measured using the same calibration curve. Themethod was successful as long as the organic and aqueouscontent in the carrier phase was v10% and was suitable forenvironmental studies involving the analysis of organicfractions, such as metal fingerprinting of petroleum fluids.The ability to use readily available and traceable aqueousstandards led to significantly improved confidence in theanalytical results.

A study of single particle analysis of colloids in water usingICP-MS showed that the transient signal induced by the flashof ions due to the ionization of a colloidal particle in the plasmatorch could be detected and measured.112 The intensity of thesignal was recorded and the peaks analysed as a function ofboth the particle size and the fraction of the total elementconcentration in the colloid phase. The frequency of the flasheswas found to be directly proportional to the concentration ofparticles in the colloidal suspension. After developing thetheory on detection, composition and intensity of ion flashesgenerated by colloid ionization in the plasma torch, tests wereperformed on model colloids (rutile, alumina, goethite) and ona natural clay (montmorillonite). The experimental conditionsand isotopes used to detect natural colloids in a single particleanalysis mode were described.

3.6. Isotope ratio measurement

A new approach to the estimation and correction of mass biaswas based on modelling the instrument response function.113

Isotope ratio measurements of Cd and Sn were used to studythe variation of mass bias with time, absolute mass and massdifference between isotopes in multiple collector (MC) ICP-MS. No statistically significant variations were seen over thefirst 20 min of analysis, after which the data deviated signifi-cantly from the original measurement. The pattern of thepoints about the mean value suggested spectral interferenceand/or inaccurate values for the true isotope ratios as the causeof the deviation. However, the variation of bias with massdifference between isotopes did show a linear relationship.When a true instrument-response function was investigatedusing a multi-element solution, a second order polynomial

provided the best fit to the data. The mass bias correctionderived from such a model was used to calculate corrected Cdisotope ratios that were closer to the natural values than thoseobtained from other, commonly used, correction expressions.Increasing the concentration of a matrix element (bismuth orcalcium) significantly affected the value of Cd and Mg isotoperatios. The direction and magnitude of the effect wasdependent on the position in the MC array with the heavierisotopes suffering higher levels of suppression. This effect wasinstrument dependent and was not observed for an instrumentwith a different MC array. A method of semi-quantitativeanalysis was developed that used the bias of 16 isotope ratiosacross the m/z range to define the parameters in a quadraticinstrument-response function. The function was then appliedto calculate the concentration of 24 analyte elements based onknowledge of ionization efficiencies and the concentration of asingle internal standard. This approach gave errors in thecalculated concentrations that were comparable with theresults obtained by using 6 internal standards, and did notrequire separate measurement of a standard solution to pre-determine the instrument response.

The use of transient and steady state signals for thedetermination of isotope ratios was discussed by Carrionet al.114 The transient signals were generated using a FI systemwith four different loop volumes coupled to an ICP-TOFMSvia a Meinhard or an ultrasonic nebulizer. No matrix effect onthe measured isotopic ratio and its precision was observed inthe analysis of solutions (0.5–100 ng ml21 in 4% HNO3) ofisotopic Pb and Rb, with or without added matrix (110 mg l21

Na). Under steady state conditions (30 s integration, n w5), thebest possible precision (RSD) for isotopic ratios was 0.08, 0.49,0.04 and 0.07% for 85Rb:87Rb, 204Pb:206Pb, 207Pb:206Pb and208Pb:206Pb, respectively, using the analogue detection mode.In transient mode, the corresponding precisions were 0.2, 0.1,0.3 and 0.3% RSD, respectively. Best accuracy and precisionwere achieved with calculations using the peak height at aminimum of five data points. The quality of the measurementsimproved for longer integration times, higher concentrationsand with transient peak profiles that reached a steady statelevel. The data obtained for the two smallest loops were ofpoorer quality since the analogue signal intensities wererather low. The ion counting data obtained for concentrations¢10 ng ml21 and the two larger loops had to be dead-timecorrected. Use of ultrasonic nebulization gave better precisionthan the Meinhard nebulizer due to a 10-fold increase in signalintensity.

Capillary GC coupled to either MC-ICP-MS or ICP-TOFMS, with simultaneous aspiration of aqueous samplescreating wet plasma conditions, was investigated for the isotoperatio determination of the transient signal from trimethylsti-bine.115 In addition, the influence of different mathematicalmodels on the precision of the isotope ratio determination oftransient signals was investigated. The trapezium integration offull or half peak gave precisions of 0.08 and 0.02%, respectively,using MC-ICP-MS, whereas ICP-TOFMS gave values ofaround 0.2%. Use of MC-ICP-MS revealed a chromatographicisotope fractionation of trimethylstibine with (CH3)3

123Sbeluting earlier than (CH3)3

121Sb. As a consequence, the mea-sured 123Sb:121Sb ratio shifted by about 2%. Two differentapproaches were attempted to correct instrumental mass biaseffects in order to be able to measure natural isotope frac-tionation. First, Cd and Sb in aqueous solution were simul-taneously introduced with the GC carrier and ICP Ar gas flow,and secondly, a standard–sample bracketing approach with agas standard introduced via the GC was used. The co-elution ofchloroform had no significant influence on the isotope ratio ofSb in trimethylstibine determined by GC-ICP-MS. Samplesfrom a laboratory sewage sludge fermentor were analysed andisotopic fractionation of the biologically produced trimethyl-stibine was found.


A new technique for the precise and accurate determination ofCd stable isotope compositions using MC-ICP-MS was appliedto geological materials and meteorites.116 Mass bias correctionwas made using external normalization to Ag. The accuracyof the new procedure was ascertained by comparing resultsfor meteorites with published TIMS data or by externalnormalization using a different MC-ICP-MS instrument.Separate dissolutions and multiple analyses of several rockand meteorite samples gave a long-term reproducibility of¡1.1 eCd u21 (2s) where eCd u21 represents the deviation of aCd isotope ratio in a sample from that in the JMC Cd standard,in parts per 104, and normalized to a mass difference of 1 u. Aslittle as 5–20 ng of Cd was sufficient for the acquisition ofprecise and accurate data.

An ‘off line’ method of data integration overcame problemsexperienced with the determination of Cr isotope ratios usingsingle collector ICP-MS and led to a nearly ten-fold improve-ment in the repeatability of measurement of natural 52Cr:50Crratios.117 The reproducibility of the ratios was improved bymore than a factor of 4 and the variability in average ratio from12.5 to 0.75%. Under these conditions, direct ID could beapplied as a primary method of measurement for the certifica-tion of the Cr content in a 51Cr-enriched material. The isotoperatios were measured with a repeatability of 0.1–0.7%,depending on the value of the ratio, and were corrected formass discrimination effects using the certified isotopic RMIRMM-625.

A time-resolved-analysis data acquisition technique was usedto monitor the time profiles of 65Cu:63Cu and 56Fe:54Fe isotoperatios in the LA-ICP-MS isotopic analysis of two NIST metalRM.118 An ArF excimer laser (l ~ 193 nm) produced pit sizesof 16–63 mm in the ablation of Cu and Fe from the metalsamples. A systematic increase (1–2%) in the measured 65Cu :63Cu ratio under prolonged ablation could be reduced to v1%by applying a new correction technique for the slow response ofa Faraday preamplifier. Compared with that obtained byconventional measurements, the precision was improved two-fold to 0.7%. This correction technique also improved themeasurement of 56Fe:54Fe isotopic ratios in iron and reducedthe systematic increase in the measured ratio to v1%.

The first reported cold plasma ICP-MS Fe isotope studyrelied on reduced temperature operation of the ICP source toeliminate Ar polyatomic interferences.119 Instrumental massbias corrections were performed by sample–standard bracket-ing and using Cu as an external mass bias drift monitor.Repeated measurements of a terrestrial basalt RM gave anexternal reproducibility (1s) of ¡0.06 and ¡0.25% for d56Feand d58Fe, respectively. The measured Fe isotopic composi-tions of various bulk meteorites were identical, within error, tothe composition of the terrestrial basalt RM, suggesting that Femass fractionation during planet formation and differentiationwas too small to be measurable.

An alternative approach was the use of HR-MC-ICP-MS forthe measurement of Fe isotopic composition.120 Spectral inter-ferences (ClOH1 or ArN1) resulting from the use of HCl orHNO3, respectively, were completely resolved and did notaffect the accuracy of the determined Fe isotope ratios.External reproducibilities of 56Fe:54Fe and 57Fe:54Fe isotoperatios were better than 50 ppm (1s) at a concentration of5 mg l21. The instrumental mass discrimination observed forraw ratios drifted by as much as 0.09% per u over an analyticalrun, but could be corrected on-line by simultaneous monitoringof the 62Ni:60Ni isotope ratio. Variations in the Fe concentra-tions or the acid strength of the solutions affected the apparentmass discrimination. A relative decrease in the raw 56Fe:54Feand 57Fe:54Fe isotope ratios with increasing Fe concentrationruled out space charge effects as an explanation for thisphenomenon. The authors suggested that the larger dry aerosolparticles formed at higher Fe concentrations were not com-pletely vaporized until later in the plasma, thus reducing the

relative rate of diffusional losses of lighter 54Fe from the centralchannel. Virtually identical work on high precision Fe isotoperatios was undertaken by Weyer and Schwieters.121 Variousinterface set-ups were investigated, using both wet and dryplasma techniques, and a wide range of concentrations (50 ppbto 10 ppm) measured to demonstrate the flexibility, robustnessand sensitivity of the technique. It was concluded that precisionand accuracy were independent of both the sample introduc-tion system and the sample concentration.

The determination of 129I in environmental samples atultratrace levels is very difficult by ICP-MS because of 129Xeimpurities in the Ar plasma gas, 127I1H2

1 interferences and thelow abundance sensitivity of ICP-MS for 129I:127I isotope ratiomeasurement. These problems were overcome through use of ahexapole collision cell with a mixture of He and O2 reactiongases to reduce the background intensity at m/z 129.122 Afteroptimization of measurement procedures the LOD for 129I1 inaqueous solution was 0.8 pg ml21. The LOD for direct 129I1

determination in contaminated environmental (soil) samplesvia gas-phase desorption without any additional sample pre-paration was 30 pg g21.

Crowe et al.123 presented a method, using quadrupole ICP-MS with a Nd : YAG (l ~ 266 nm) LA sample introductionsystem, for the precise (0.2% relative standard error on 206Pb :207Pb) determination of common Pb isotope ratios at low(y2 ppm) Pb concentrations. Focusing the laser above thesample surface significantly improved the ratio measurementprecision as a result of increased count rates and improved thehomogeneity of ablated particulate matter. The use of a mixedAr–N plasma significantly increased sensitivity and reducedmass bias.

Measurement of Li isotopic ratios by MC-ICP-MS using‘‘cold’’ plasma conditions was characterized by fewer baselineinterferences and improved reproducibility when comparedwith conventional hot plasma techniques.124 The 2s precisionsfor 1200, 800 and 680 W ICP forward powers were con-servatively estimated as 1.1, 0.7 and 0.5%, respectively. Matrixeffects were significant at the coldest plasma temperatures, withdeviations of w0.5% in d7Li being observed at 680 W forsolutions containing 100 and ¢500 ng g21 Li and Na,respectively. However, no matrix effects were detected at 800 Wfor solutions containing up to 5 mg g21 Na. Measurementsunder all conditions were strongly sensitive to variations in acidconcentration and required precise matching of sample andstandard matrices. Self-induced Li matrix effects were notevident for Li concentrations below 120 ng g21. Blankcorrections were, however, necessary.

Variations in Hg isotope ratios in cinnabar ores obtainedfrom different countries were determined by MC-ICP-MS.125

Values of d198Hg (relative difference of 198Hg:202Hg from theratio in NIST SRM 1641d (Hg solution)) varied from 0.0 to1.3%. The typical external uncertainty of the d values was 0.06–0.26%. The Hg was introduced into the plasma as elemental Hgafter reduction by sodium borohydride. The Hg isotope ratioscould not be normalized using the measured 208Pb:206Pb ratiobecause of a significant fractionation of Pb isotopes in the leadhydride generated with the mercury hydride. Instead, the Hgratios were corrected using the 205Tl:203Tl ratio measuredsimultaneously. The internal precision of the isotope ratiomeasurement was as good as 14 ppm for 60 ng Hg generatedfrom a 10 ng ml21 Hg solution over 10 min. A deviation of#50.2% per u from IUPAC values was explained by theinadequacy of the exponential law to correct for mass bias inMC-ICP-MS measurements. The absence of a precisely charac-terized Hg isotope RM prevented unambiguous determinationof the absolute Hg ratios of the ore samples.

Wombacher and Rehkamper126 investigated the applicationof the generalized power law (GPL) for mass bias correction asthe large instrumental mass discrimination of plasma sourcemass spectrometers could not be accurately corrected with the


common mass fractionation laws. The Nd isotope ratio dataacquired with two MC-ICP-MS instruments from the samemanufacturer were compared with published Nd referencevalues determined by TIMS. The GPL applied a variable, n,that could be used to describe different mass-dependencies. Inparticular, the GPL is equivalent to the power law for n ~ 1and approaches the exponential law as n approaches 0.Whereas the exponential and power law corrected datadisplayed significant (w100 ppm) deviations from the referencevalues for some Nd isotope ratios, the instrumental massdiscrimination was accurately described by the GPL. For threedata sets acquired under optimal operating conditions this wasachieved with n # 20.23, but during routine operation of theinstrument the n-values were observed to vary between 20.2and 20.4. It was concluded that the GPL was a valuable toolfor data evaluation in isotope ratio mass spectrometry and forthe characterization of instrumental mass-dependencies. Theobserved mass-dependencies were in accord with isotopefractionation by kinetic transport processes.

A MC-ICP-MS instrument fitted with multiple ion countingdetectors in addition to the array of Faraday detectors was usedto measure Pu isotopes.127 Use of the multiple ion countingdetectors avoided the need to cycle a number of small ionbeams through a single collector. This increased efficiency ofion beam usage, combined with the elimination of ion beaminstability, made multi-channel analysis ideal for low-level Puisotope analysis. Mass and inter-detector bias could becorrected for by using an external U- or Pu-based standardsolution. Using minimized analytical times and desolvatingnebulization, 240Pu:239Pu ratios could be reproduced to betterthan 3% (2s) with less than 10 fg Pu.

Precise Si isotopic fractionation in silica (quartz), opal(diatomite, sponges) and standards has been determined byemploying MC-ICP-MS in dry plasma mode, using a desolvat-ing nebulization system.128 Instrumental mass bias wascorrected by measurement of Mg isotopes in dynamic modeand application of the fractionation exponential law andexternal normalization. The repeatability and the internalprecision on the d29Si measurements of a 1 ppm Si solution indiluted HF/HCl were better than ¡0.1% (2s). The accuracywas assessed by comparison with data obtained by analysis ofan in-house standard using laser fluorination SIRMS. The dryplasma methodology improved the sensitivity more than tenfold compared with wet plasma MC-ICP-MS.

Deschamps et al.129 examined the optimum conditions forrapid, precise and accurate determination of 234U:238U ratios ingeological materials by MC-ICP-MS. Isotopic measurementswere performed using Faraday collectors in static mode only, inorder to eliminate the difficulty of proper calibration of theDaly/Faraday gain ratio. However, since the instrument usedhad a poor abundance sensitivity, the major issue to beaddressed was the tail correction, for which a modified versionof that described by Thirlwall130 was used. This was based oncorrection of the actual tail contribution under each peak,as assessed by the tail shape measurements on mono-isotopicion beams, instead of the usual half-mass zeros baselineestimation. A 236U:233U double spike was employed to correctfor mass discrimination bias. A desolvating-nebulizer sample-introduction system, consuming a minimum of 200 ng U, gavea 234U signal of y4–5 mV for 50 measurement cycles of 5 seach. Replicate measurements of an RM using this time-consuming, 10-min procedure, yielded a mean d234U value of236.42¡0.80% (2s, n ~ 19), which was highly consistent withvalues reported by other laboratories. The total reproduci-bility, including both chemical separation and spectrometricmeasurement, was assessed to be y1.3% (2s) for geologicalsamples.

Hellstrom131 used the parallel ion-counting capability ofan appropriately configured MC-ICP-MS to measure simul-taneously 230Th:234U and 234U:238U ratios in a single solution.

The method fully integrated the measurement of elementalfractionation, instrument mass bias and ion counter gain, andallowed U/Th ages to be calculated on-line on a cycle-by-cyclebasis. Permil-level precision could be achieved for both230Th:234U and 234U:238U ratios on as little as a few tens ofng U. As it was not necessary to collect separate U and Thfractions, a simple Tru-spec2 ion exchange resin procedurecould be used to give a high sample throughput. Repeatanalyses of RM gave excellent agreement with results fromother laboratories.

A series of experiments was undertaken by Segal et al.132 todetermine the isotopic composition of Yb as accurately aspossible. The 171Yb:172Yb ratio, measured in an erbium–hafnium–ytterbium mixture and normalized using the knownvalues for both 167Er:166Er and 179Hf:177Hf ratios, was used tocorrect all Yb ratios. The mean Yb ratios measured in thisstudy were in fair agreement with data reported over the last25 years and were considered by the authors to be closest to thetrue values. The precision (2s) was at least 3–8 times bettercompared with that obtained by TIMS.

A new technique for the precise and accurate determinationof Zr isotopic composition in geological samples depended oncomplete separation of Zr from Cr, Fe and Ti using a two-stageanion-exchange procedure to avoid the presence of argides onthe Zr isotopes.133 Analyses of synthetic standard solutionsshowed that isobaric interferences of Mo and Ru could beadequately corrected. Replicate dissolutions of the Allendecarbonaceous chondrite with v100 ng Zr yielded a long-termreproducibility of ¡39, 25 and 82 ppm for 91Zr:90Zr, 92Zr:90Zrand 96Zr:90Zr, respectively. The method was successfullyapplied to terrestrial igneous rocks, meteorites and mineralseparates including samples with high Ti contents.

4. Laser ionization mass spectrometry (LIMS)

This section covers the one-step laser irradiation–ionization ofsolids as well as the two-step method involving laser post-ionization of laser-generated neutrals. The section on RIMSwill be confined to publications dealing with the ionization ofatomic beams or gas phase neutrals under resonant conditions.Laser microprobe mass spectrometry (LMMS) refers to the useof a focused laser with a spot diameter on the sample of lessthan 5 mm. The application of LMMS to suspended particles incombination with TOF analysis is called aerosol-TOF LMMS(aTOF LMMS). Laser ionization using irradiation spot sizeson the sample with a diameter above 5 mm is denoted as laserplasma ionization (LPI). To summarize the characteristicsof the laser, the wavelength and nominal pulse duration aredenoted by the symbols l and t, respectively.


Uchimura et al.134 developed a rugged TOFMS with supersonicjet (SSJ) or effusive molecular beam (EMB) sample introduc-tion for the speciation and determination of Cl in organiccomponents of flue gases from waste incinerators. The SSJsystem injected the analytes through a pulsed nozzle with Arcarrier gas while the EMB interface used a capillary withoutcooling. An excimer-pumped tunable dye laser or a frequency-doubled narrowband optical parametric oscillator pumped bya frequency-tripled Nd:YAG was used for resonance-enhancedmultiphoton ionization. The selective ionization of differentchlorophenol isomers illustrated the high degree of speciationthat could be achieved. The molecular cooling in the SSJ inter-face improved the selectivity and sensitivity in comparisonwith the EMB and resulted in a LOD of 56 ng l21 for 2,4-dichlorophenol. The sensitivity decreased with increasingnumber of Cl atoms in the analyte because of the shorter lifetimeof the excited-state and the need for three-photon-ionization.



The TOF LMMS study of the ion emission from neodymium andpraseodymium carbides at fluences between 0.5 and 16 J cm22

with a frequency-doubled Nd:YAG (l ~ 512 nm, t ~ 6 ns,repetition rate ~ 10 Hz) used an intensified charge-coupleddevice camera to detect the light of the plume with a spatial andtemporal resolution of 37 mm and 5 ns, respectively.135 Thesignal intensity ratio of MCn

1:M1 (M ~ Nd or Pr) ions was ashigh as 0.5, regardless of whether carbides or metal oxide–graphite mixtures were irradiated. Measurement of the velocitydistributions for the emitted ions and neutral species at adistance of 0.5 mm above the sample showed mean velocities of3 6 105 cm s21 for neutrals and 86105–8.76105 cm s21 forC1, Pr1 and Pr21 ions. The data suggested the occurrence ofhigh temperature vaporization followed by gas phase reactions.However, the onset of LA involved complex processesincluding in situ reaction of oxides with graphite to formcarbides and the selvedge formation of MCn

1.The relative importance of the neutral and ionized Si atoms

desorbed by laser irradiation was used to investigate theredistribution of photon energy in the solid.136 In principle, thisredistribution can lead to melting of the material in thesubsurface over a depth corresponding to the diffusion length,or to the direct ejection of particles through distortion of theelectronic structure of the sample components. The desorptionwas achieved with a frequency tripled Nd : YAG laser beam(l ~ 355 nm, t ~ 5 ns, repetition rate 10 Hz) in the fluencerange between 85 and 165 mJ cm22. A tunable dye or opticalparametric oscillator laser was used for the post-ionization ofSi under resonant conditions (l ~ 243.877 nm), allowing theemission of only 100 sputtered neutrals (v0.1% of a mono-layer) to be detected. Energy filtering before the quadrupolediscriminated against directly ejected ions in the SNMS mode.The number of ions and neutrals emitted as a function of thedeposited laser energy (mJ per pulse) followed functions of e11

and e6.84, respectively. The kinetic energy (KE) distributions ofthe neutrals, measured by post-ionization at different distancesto the target, were bimodal and the derived temperaturesreflected the occurrence of both thermal and electronic emis-sion mechanisms. Additionally, the threshold laser energycorresponded to that of molten rather than of crystallinesilicon.

The effect of the laser power density on the distributionbetween ions and neutrals generated from a-iron disilicide wasstudied with KrF excimer LA with and without post-ionizationby an ArF excimer at a distance of 65 mm from the sample.137

An adjustable delay between the two lasers allowed KEdistributions of the neutrals to be measured. The data reflectedthe existence of distinct regimes at low, moderate and highfluence. Specifically, a fluence of v0.7 J cm22 produced onlyFe, Si and Si2 neutrals. The Maxwell–Boltzmann equationdescribed the velocity distribution of Si atoms. The mean KE of0.35 eV and the derived target surface temperature of 4000 Kwere consistent with thermal emission. Singly and doublycharged Fe and Si ions appeared only at higher fluence andtheir mean KE ranged from several eV to 100 eV at a moderatefluence up to 2.5 J cm22. Under these conditions, surfaceheating no longer explained the results and effective accelera-tion of the ions in the dense plasma was believed to occur.

A study of the ion composition generated by pulsed LPI ofSe targets provided evidence for cluster growth in the plasma.138

Irradiation was performed by excimer XeCl laser (l ~ 308 nm,t ~ 20 ns) at a fluence of 5 J cm22. The gas phase componentswere sampled at distances of between 1.5 and 2.5 cm from thetarget. It was found that the plume closer to the samplecontained Se21, Se1, SeO1 and Se2

1 as the major componentswith minor contributions from Sen

1 cluster ions (n ~ 3–5). Atgreater distances from the target, however, Se1 ions wereaccompanied by a series of Sen

1 cluster ions with n as high as

12. The Se121 ion was the most abundant. Interestingly, all

the ions with low m/z values except H1 disappeared almostcompletely.


A significant breakthrough for speciation in frozen hydratedcells could result from the novel use of infrared (IR) matrix-assisted laser-desorption ionization (MALDI).139 In contrast toconventional UV-MALDI, in which the added matrix absorbsUV radiation, irradiation of samples at a l in the mid-IRmolecular fingerprint region allows IR chromophores intrinsicto the sample such as H2O and NaNO3 to be excited and resultsin the expulsion of ions. The approach was demonstrated forfrozen sodium chloride solutions as a model for frozen cells.The sample was irradiated with 100 ns pulses of a free-electronlaser, tunable between 2 and 10 mm. The diameter of the laserspot (100–250 mm) on the sample allowed a power densityof 106 W cm22 to be achieved. A commonly used TOFMSinstrument with an ion reflector together with a laser tuned to al ~ 5.9 mm, was used to detect the Na1 cationized H2O andNaCl (mixed) adducts up to m/z 600. The relative contributionof the NaCln(H2O)mNa1, (H2O)nNa1 and (NaCl)nNa1 ionsin single shot spectra reflected the local concentration andformation of NaCl-enriched domains due to phase separationduring freezing. This study offered the prospect of quantitativespeciation in sample volumes about that of a single biologicalcell.

A mobile aTOF LMMS instrument called laser massanalyser for particles in the airborne state (LAMPAS) wasdeveloped for the chemical analysis of surface components inindividual soot particles.140 Particle introduction was detectedthrough light scattering, which triggered a pulsed ionizationlaser (l ~ 337 nm, N2 laser). The positive and negative ionsproduced from the same particle were recorded simultaneouslyusing two separate TOF tubes with a mass resolution of 400.The feasibility of speciation analysis by the analysis of sootparticles (diameter of 0.2–0.5 mm) generated either from agraphite spark discharge in Ar or by a diesel engine and dilutedin a chamber with 60 ppb nitrogen oxides was investigated.Quantitative analysis of diesel soot in the presence ofammonium sulfate particles revealed difficulties in distinguish-ing between neat diesel, neat ammonium sulfate and mixedparticles.

The use of laser ionization TOFMS for depth profiling hasbeen evaluated in a study on the diffusion of phosphor andtitanium dioxide coatings in solar cells.141 The absence ofcharge build-up on the sample was a particularly attractivefeature of laser ionization in contrast to the use of SIMS. A Nd :YAG laser beam (l ~ 1064 nm, t ~ 6.5 ns, repetition rate 2 Hz)impinged upon the sample at an incidence angle of 60u.Application of 300 shots at fluence between 1.5 and 7.1 J cm22

yielded elliptical craters with a diameter of 1.5–2 mm. Theaverage erosion rate was 0.3 nm per shot for titanium dioxidecoatings. The top-hat intensity profile of the laser beameliminated the typical crater effects of Gaussian beams. Com-parison of LIMS with SIMS for the depth profiling of siliconwafers doped with different levels of P showed good agreementfor the shape and slope of the distribution. The LIMS analysiswas, however, 64 times faster than SIMS analysis. The resolu-tion of a few nm at the interface of a 1.2 mm titanium oxidecoating on silicon demonstrated the feasibility of fast depthprofiling of thick non-conductive layers.

5. Resonance ionization mass spectrometry (RIMS)

5.1. Reviews

A thorough treatise on RIMS instrumentation and applica-tions to rare isotope research concluded that the method is now


mature.142 Despite the steady development and refinement ofhigh-power pulsed and continuous-wave (CW) lasers, copper-vapour pumped dye systems retained their work-horse statusbecause of their high power and repetition rate. Although therepetition rate of only 1 kHz constrained the use of excimerlaser pumping, the ns pulse length perfectly suited combinationwith TOFMS and allowed the LOD of 106 atoms to beachieved at a mass resolution up to 1500. The introduction oftunable, solid-state lasers offered compact, inexpensive andmaintenance free alternative systems for the copper-vapourpumped dye lasers for ultra-trace analysis of Pu. The use ofHR CW lasers for the determination of trace isotopes withrelative abundances of 10210 allowed RIMS to be used forapplications normally undertaken by AMS, such as environ-mental protection, biomedical tracer studies, cosmochemistryand radiodating.

The state-of-the-art and prospects of RIMS for isotopic ratiodeterminations in presolar grains were compared with those ofmagnetic sector SIMS with MC detection.143 The 40-referencereview on the study of presolar grains critically assessed the useof resonant post-ionization of neutrals sputtered by primaryion beam bombardment for isotope analysis of elements suchas Ba, Mo and Zr in single silicon carbide grains. Presolarmaterial typically had 95Mo:96Mo and 97Mo:96Mo ratios thatwere about a factor of 2 higher than those in solar systems,while the excess of 138Ba and 96Zr could exceed solar levels by afactor of 3–5. A HR SIMS instrument, which allowed a Cs1

gun to be focused on a spot with diameter as small as 30 nm,was considered as a valuable complement to RIMS for theisotope imaging of interplanetary dust samples with a sub-mmresolution.


A new ETV source with 6 filaments was used for the tracedetermination of Vitamin B12 in fruit juice using a 1-colour2-photon RIMS scheme at a l of 298.71 nm to detect Co.144

The new vaporizer allowed sequential analysis of ml aliquots tobe performed without breaking the vacuum. Only filtrationwas required in the preparation of the juice for analysis. Themethod of standard addition was used for quantification in therange 1–1000 ng g21, for which precision was about 10–12%.Although the LOD for Co was v5 pg, typical juices contained10–30 pg ml21.

Upgrading a TOF LMMS instrument with independent lasersfor desorption and ionization allowed145 traces of Pd and Rh inminerals to be determined with a LOD of 15 ng g21. Pulsed ionextraction reduced the noise from directly emitted ions. Thesolid was ablated with a frequency-quadrupled Nd : YAG (l ~266 nm) laser and the neutral species in the gas phase werepost-ionized using a Nd:YAG pumped tunable dye laser. Thisinstrumental combination was suitable for analysis of allelements except He and Ne. The ion signal measured as afunction of the ion fluence showed that saturation of theionization step required application at two different l. Calibra-tion was performed with a series of NIST SRM, containing1.3–119 mg g21 Pd and 7.2–49.5 mg g21 Rh in a gold or silvermatrix. The precision of the quantitative analysis was v15%and measured values agreed within 10% with those obtainedusing SIMS.

The use of high-repetition rate (10 Hz) tunable Ti:sapphirelasers with short pulses (t ~ 100 ps) improved the overallefficiency of trace element analysis by a factor of 100 incomparison with the use of conventional dye lasers producing10 ns pulses at 1 Hz.146 Instead of the traditional multi-laserconfiguration, a single laser was used in the split mode for bothirradiation with a spot of 50 mm and post-ionization of the gasphase neutrals. In this way, high sensitivity and isotopicselectivity could be achieved with a simple set-up, allowing aTOF analyser with mass resolution of only 200 to be used. The

LOD of Al was 3 ng g21 per laser shot. Compared withconventional systems, the combination of higher repetitionrate and more efficient post-ionization lowered the concentra-tion that could be measured per second.


The Chicago–Argonne resonant laser ionization mass analyser(CHARISMA) upgraded with frequency-tripled diode-pumped Nd:YAG (l ~ 355 nm, t ~ 8 ns, repetition rate1 kHz) and Ti:sapphire laser systems for desorption and post-ionzation, respectively, was evaluated for the isotopic analysis

of heavy elements in individual presolar grains as small as a fewmm.147 A lateral resolution of 1 mm was achieved through theuse of a Schwarzschild cassegrain microscope. The applicationof suitable RIMS schemes circumvented the numerous isobaricinterferences that would normally require HR, such as thecontributions from Al2, AlSi, CaO, Cr, FeH, Ni, Si2, SiC2, TiC,and hydrocarbon ions on the signals from Fe, Mo and Zr. Theanalytical performance was checked by the isotopic analysis ofBa in a terrestrial sample of barium titanate and in a singlesilicon carbide grain with a diameter of 2 mm, isolated from theIndarch meteorite. Using 36105 laser shots, the d130Ba andd138Ba values (both normalized on d136Ba) in the meteoritegrain were 2888¡161% and 2266¡50% (2s), respectively.These results confirmed the extra-terrestrial origin of the grain.The method was applied to the analysis of 19 single presolarsilicon carbide grains (diameter 2.3–5.3 mm) recovered from theMurchison meteorite and pressed in a thin sheet of gold.148 The2s uncertainties of v15% (relative) for d135Ba, d137Ba and d138

Ba (normalized on 136Ba) compared well with results obtainedpreviously using SIMS or TIMS.

6. Secondary ion mass spectrometry (SIMS)

The bombardment of a sample with primary ions in the1–25 keV range causes the development of a collision cascadeover a distance of several tens of nm below the surface. Alongits trajectory the projectile imparts sufficient energy to thelattice to break all intermolecular forces and to move individualatoms from their initial positions in the lattice. The generationof the analytically used secondary ions is believed to occurthrough the recoil of momentum towards the surface. While theextensive atom relocation destroys the molecular structures inthe subsurface, a substantial fraction of the analytes in theupper (few) monolayer(s) remain intact because of the ultra-fast penetration. In static SIMS (S-SIMS) experiments,sputtering is limited to only a fraction of the upper monolayerto maximize the contribution of ions that are representative ofthe molecular form of the element. Speciation and character-ization of organic surface analytes forms the major applicationof this technique. Depth profiling experiments are excluded bythe principle of S-SIMS. This review restricts the use of theacronym SIMS to the dynamic mode that uses continuoussputtering for the erosion of subsequent layers. As a result, theelemental composition in the subsurface can be monitored as afunction of depth. To clearly distinguish between the essentiallydifferent analytical features of the two methods, SIMS istreated in sections 6.2–6.5 and S-SIMS in a separate section 6.6.Both types of instrumentation are covered in section 6.1. AsTOF mass analysers provide advantages for dynamic as well asstatic experiments, explicit distinction will be made betweenTOF SIMS and TOF S-SIMS. The incidence and emissionangles, mentioned throughout this section, are expressed as theangle between the beam and the normal to the sample surface.Unless otherwise specified, the surface roughness is referred toby the root mean square (rms) parameter.



Takanashi et al.149 combined focused ion beam milling andsecondary ion analysis for 3-D analysis. A high intensity,focused Ga1 ion beam created a flat surface by the so-called‘‘shave off’’ technique while another pulsed Ga1 primary ionbeam generated secondary ions. The set-up was evaluated bythe 3-D analysis over a depth of 8 mm of insulating polyesterfibers with diameters up to a few tens of mm. The surfaceroughness, large depth range and sample charging duringanalysis made this a demanding test case. The imaging of eightplanes 0.9 mm apart took about 30 hours, which included thelong time required for ‘‘shaving off’’. The distribution ofpolymerization agents such as calcium stearate was imaged bymeans of Ca1 and CaO1 secondary ions while Cn

2 ionsallowed an organic UV stabilizer to be located.

The bombardment of solids with a gas-cluster ion beam(GCIB) showed significant potential for ultra-shallow depthprofiling.150 The GCIB was generated by EI of the neutralclusters in a supersonic jet of argon, oxygen or nitrogen. Spacecharge effects limited the diameter of the primary ion beam onthe sample to 1 mm. Generation of 20 keV primary ion clustersfrom argon at a current density of 300 nA cm22 allowedmetals and semiconductor targets to be etched at a rate of0.5–1 nm min21. The KE distributions of secondary Aln

1 ions(n ~ 1–6) generated from aluminium foil showed a maximumfor ions with energy of only 1.5 eV while the FWHM was about2.5 eV. Apart from atomic ions, argon GCIB typically gener-ated cluster MAr(n)

1 ions with n ~ 1,2, depending on the metalunder study. The use of oxygen or nitrogen gas in the GCIBproduced strong signals of SinOm

1 or SinNm1 (n ~ 1–5, m ¢ n)

secondary ions, respectively, from silicon. The yield of high m/zions significantly exceeded that obtained when using poly-atomic projectiles such as SF5

1. Analysis of native oxide filmson metals required a minimal acceleration of the Ar cluster ionsup to about 7 keV, equivalent to y3 eV per incident Ar atom.Depth profiling of a Ni–Fe permalloy film clearly distinguishedbetween the native oxide and the alloy while the sputter-induced topography was minimal.

Nittler and Alexander151 developed an analytical system forthe fully automated determination of isotopic ratios in mm-sizedparticles on the basis of ion imaging and new algorithmsto localize the particles. Scanning of a 75675 or 1506150 mmarea with a Cs1 or O2 primary ion beam, focused to a spotwith diameter of 1 or 0.5 mm, respectively, localized theparticles of interest, which were then individually analysed witha focused ion beam scanned over an area of 363 mm. Use ofoptical encoders circumvented the poor reproducibility of thecommercial sample stage and positioned the beam within 1 mmof a predefined point. The statistical error on d28Si and d29Siusing the peak-jumping mode was 2.6 and 2.7%, respectively,while the intrinsic uncertainty remained within 3%. Theaccuracy of Al isotope determinations was within the countingerror of 10–15%. The automated analysis typically processed300 particles per day, depending on the number of grains perarea and their size.


Ultra-shallow depth profiling requires insight into ionizationunder low energy primary ion bombardment in the transientregime.152 Specifically, the effect of variations in the workfunction and valence bands on the emission of 28Si2 ions wasstudied for silicon, with and without native oxide layers,sputtered by 1 keV Cs1 primary ions at an incidence angleof 60u. Measurement of the potential difference between thesample and a Kelvin probe head at zero current showed asignificant decrease of the work function for oxide-free siliconwafers and XPS data revealed no appreciable change inthe valence band close to the Fermi edge. Furthermore, the

exponential relationship between the work function and theintensity of Si2 ions produced from oxygen-free silicon wasconsistent with the resonance charge transfer assumed in theelectron-tunneling model. In contrast, the exponential depend-ence of the Si2 ion emission on the work function for Si2 ionemission was no longer seen for wafers with native oxide layersand XPS data showed growth of sub-band features under Cs1

ion bombardment. The results indicated the creation of newresonance charge transfer channels which caused the observedenhancement of the Si2 ion yield from oxide-covered silicon.

The formation mechanisms of MCs1 and MCs21 ions (M ~

Cu or Ni) were studied by measurement of the KE distributionsof the secondary ions generated from pure metal foils underCs1 primary ion bombardment.153 While the Cu2 secondaryion intensities reflected the expected matrix effect, oxygenflooding did not influence the yield of CuCs1 and CuCs2

1

secondary ions. Furthermore, the signal intensity of CuCs21

was higher than that of CuCs1, regardless of the energy of theCs1 primary ions (1–5 keV). In contrast, the yield of Ni2 ionsgenerated from nickel targets decreased with increasing energyand surface concentration of the Cs1 primary ions. The energyof NiCs1 ions at the maximum of the KE distributiondepended on the energy of the Cs1 primary ions and wasalways higher than for that for NiCs2

1, reflecting the lowersurface binding energy of NiCs2

1 than of NiCs1. The datapointed to the formation of MCs2

1 ions through recombina-tion of sputtered M2 and two Cs1 ions rather than throughCs1 attachment to sputtered MCs neutrals.

Molecular dynamic based simulations of sputtering of a singlesilver crystal by 15 keV C60 projectiles used sophisticatedinteraction potential functions to describe the behavior of165 000 atoms in a time domain of 4–10 ps after impact.154 Theenergy deposition in the immediate near-surface region wasfound to be responsible for the ejection of secondary particles,while the generated pressure wave in the subsurface did notcause damage or atom displacement. The diameter of thesputtered crater was about 10 nm. The yield of Ag and Ag3

particles using C60 projectiles exceeded that using Ga1 primaryions by a factor of 10 and 25, respectively.

The feasibility of speciation analysis during depth profilingdepends on whether the detected ions are formed throughdirect emission or selvedge recombination of sputtered speciesin the dense gas phase above the sample. Sarkar et al.155

addressed this fundamental question by depth profiling theinterfaces of Au–Cu multilayers on silicon or glassy substrates.As the interface was approached from the Au layer, theAu2Cu1 and Au1 ion signals reached a maximum closer to theinterface than AuCu1 and AuCu2

1 ion signals. The reverse wasobserved when the interface was approached from the Cu layer.The dependence of Au2Cu1 or AuCu2

1 ion formation on theposition relative to the interface was seen as evidence againstion-beam-induced damage and selvedge recombination.Furthermore, the integrated intensities of the different clustersagreed very well with those for Cu3Au alloy, whose localpresence in the multilayer was confirmed with X-ray diffractionand HR TEM data.

A new statistical 3-D damage accumulation model for ionimplant simulation featured automated reduction of the statisti-cal noise by dividing the dose into sections as well as a newscheme for parallel calculations to speed up the simulations.156

Calculations of the particle trajectories accounted for thepartial amorphization of a local region caused by a priorimpact. The model needed only one fitting parameter forelectronic stopping and used 3-D electron density distributionsfor different targets, including semiconductors. The modelcould also simulate the implantation of molecular andcluster projectiles. Simulated and experimental depth profilesagreed well for silicon implanted with As, B and BF2 projectilesat energies of 15–100 keV, angles of 0–22u and doses of 1013–1015 atoms cm22.



A systematic study of atomic ion yields for Ca, Fe, La, Lu, Mg,Sc, Si and Y from synthetic silicate glasses showed theimportance of energy filtering for the reduction of matrix

effects.157 Pyroxene samples were bombarded with O2 primaryions (0.5–3 nA) while energy filtering with a bandwidth of 20 eVwas applied between 0 and 130 eV. The absolute sensitivity ofthe atomic ions from the minor elements depended non-linearlyon the Fe content, and matrix effects decreased with increasingsecondary ion energy. The dependence of the yield of the majorcomponents on the Fe concentrations and selected energyrange was different for each element. The detection of ions withenergy between 60 and 100 eV provided the best compromisefor minimizing matrix effects.

The yield of Sin1 (n ~ 1–11) and Sin

1XlYk1 ions (X, Y ~

Au, B, C, N) secondary ions generated from single siliconcrystals by polyatomic Aum

2 (m ~ 1–5) projectiles (4–18 keV)increased158 non-linearly with m. In contrast, the yield ofhetero-atomic clusters such as Si3B1, Si3C1 and Si2N1,increased with the projectile energy for a given m. Thestrong emission of Si3B1 ions under Au5

2 bombardment waspartly due to the non-additive sputtering and partly to thelowering of the formation energy by the saturated valencebonds. Polyatomic projectiles improved the LOD for B, C andN impurities by a factor of 100–1000 in comparison withatomic-primary-ion bombardment. Important applicationswere foreseen in the field of shallow depth profiling becauseprimary ions with only a few hundreds of eV per impactingatom could be used without significant effect on the sensitivity.

The combination of shallow bevelling and use of an isotope

tracer (18O) improved the study of self-diffusion in yttria-stabilized zirconia.159 The 18O diffusion was measured as afunction of temperature (650–1200 K) and yttria content(8–24 mol%). Because of the deep penetration of oxygen,samples were polished to form a new surface at an angle of 1–3uto the original surface. The shallow bevel allowed the depthrange to be ‘‘magnified’’ by a factor of 10 in comparison withthe perpendicular cross section while beam-induced artifactswere avoided. The agreement between the measured depthdistributions and predictions from simulations showed thevirtues of converting a depth profile experiment into surfaceanalysis.

A new procedure to deduce bulk and boundary diffusivities of

oxygen in polycrystalline oxides from shallow depth profilingexperiments was developed by Fielitz et al.160 Traditionally, the18O:16O peak intensity ratio is plotted as a function of depth(z1.2) and the boundary diffusivity is derived from the slope ofthe linear tail. The volume diffusivity must be deduced from thedata obtained close to the surface. Both the initial surfaceroughness and that introduced by coating the sample with aconductive layer often hampered the procedure. The improved,alternative procedure used scanning electron microscopy(SEM) and appropriate imaging software to determine thelength of the grain boundaries per cm2 which was needed tocalculate the volume diffusivity from the depth profile data atgreat depth. The approach was successfully applied to oxygengrain-boundary-diffusion experiments on high-purity densepolycrystalline 3/2-mullite material.

The use of matrix ion signal intensities for the correction of

topography effects exploited the similar m/z shifts for matrixand analyte ions generated from rough surfaces.161 Topogra-phy effects caused variations in the flight time of given ions ofup to 50 ns. Application of PCA to the images of the matrixions showed that all the principal components were associatedwith different aspects of the local topography. Mapping of thetopography-related m/z shifts could be used to deconvolute theanalyte ion signals and thereby to increase the mass resolutionof the reconstructed spectra. It was considered that this

approach would also improve the analytical precision andsensitivity.

6.4. Quantitative analysis

Careful optimization of the primary-ion-beam density and useof analytical expressions for recorded signal intensities as afunction of bulk concentration, memory effect, and adsorptionfrom the residual vacuum allowed Pivarov et al.162 to achieve aLOD of 661014 atoms cm23 for N in silicon carbide. Epitaxiallayers with 1018 and v1015 N atoms cm23 were analysed byscanning the 10 keV Cs1 primary ion beam over a 45 6 45 mmarea and extracting the secondary ions from the central field of30 6 30 mm. Cluster ions (12C14N113C14N)2, 12C11B2 and12C13C2 were used for detection because their narrow KEdistributions made energy filtering independent of small KEshifts during analysis. Variation of the raster size at constantcurrent allowed the relative signal contributions from thebackground, surface and bulk to be optimized.

The determination of Al, B, Be, H, N and O in silicon carbidewas systematically studied using 6H–SiC standards implantedwith 1014–1015 ions cm22 at 30–100 keV energy.163 Sampleswere bombarded with Cs1 or O2

1 primary ions, depending onthe element under study. Energy filtering was applied to thesecondary ions with a band pass of 130 or 30–50 eV in the lowand high mass resolution mode, respectively. Serious matrixeffects for all cluster ions except MCs1 and for HR measure-ments with the narrow band pass energy filter were revealed bycalculating RSF under different analytical conditions. Theintensity of SiO2 ions was inadequate for assessing the localoxygen distribution in silicon carbides but O2 or CsSiO1 ionscould be used as an alternative. The discrepancy between thedepth profiles obtained using O2 and SiO2 ion beams except atthe beginning of the experiment, was explained by the breakingof Si–C and formation of Si–O bonds when the number ofbeam-induced vacancies exceeded 3.3 6 1021 cm23.

Optimized determinations of the isotopic composition of B, Cand O with a lateral resolution of 30–50 mm in coral skeletonsallowed the pH variations during growth to be assessed with atime resolution as good as 12 hours.164 The isotopes of C and Owere measured with a mass resolution of 2000, MC detectionand Cs1 primary ions. A mass resolution of 5000, electronmultiplier detection and O2 projectiles were used for B iso-topes. The internal 1s error on 12C2 and 16O2 was ¡0.1–0.2%and about 3 times higher for 11B2. The average externalreproducibility estimated from replicate measurements oncarbonate standards over 4 years was ¡0.4, 0.65 and 0.9%for B, C and O, respectively. The external reproducibility for Oduring one day was ¡0.17%. The instrumental mass fractiona-tion (IMF), determined by analysis of various carbonatestandards, showed a systematic difference between calcite andaragonite of 4.5 and 2.7% for C and O, respectively. Duplicated18O measurements of the same spot in different sessionsagreed within 0.5%. The isotopic compositions of B and O atthe mm scale could be substantially different from thosemeasured by conventional techniques.

The d18O in stalactites is used to delineate the temperature ofprecipitation and climate change. Kolodny et al.165 demon-strated the benefits of SIMS with high lateral resolution forisotopic analysis of O in stalactites in comparison with con-ventional procedures of micro-drilling and SIRMS analysis.The resolution provided by the SIMS procedure (25 mmdiameter) was a significant improvement over that of micro-drilling (0.5 mm diameter). The 16O2 and 18O2 secondary ionswere generated by Cs1 primary ion bombardment and simul-taneously collected on Faraday cups. The IMF was determinedfrom repeated analysis of a standard (optical calcite) with achemical composition similar to that of the stalactite. Theoverall precision of about 1.0% for 45 measurements wasprimarily limited by slow drift effects. The analysis of spots


only 100 mm apart yielded a time resolution of between 4 and100 years, depending on the stalactite growth rate.

A comprehensive study of the analytical errors in zirconU–Pb geochronological measurements with the sensitive high-resolution ion microprobe (SHRIMP) showed the advantage ofusing the 206Pb1:238UO2

1 signal intensity ratio instead of the206Pb1:238U1 ratio because of superior counting statistics andunreliable correction of the inherent mass discrimination bymeans of 238UO1:238U1 ratios.166 The NIST SRM 610 (glass)and a natural zircon RM (Z6266) were used to delineate therelative importance of the uncertainties on the 206Pb1:238U1

isotope ratio measurements with SHRIMP. The within-spotanalytical errors of 0.58% (relative) for the 208Pb1:206Pb1 and232ThO1:238UO1 ratios accounted for the standard deviationof the whole data set. An unaccounted uncertainty (1s) ofabout 1% occurred in the measurement of 206Pb1:238 UOx

1

(x ~ 0,1,2) ratios in both materials. By correcting the inherentdiscrimination using the 206Pb1:238U1 ratio, the unexplainedvariation in the commonly used 238UO1:238U1 ratios wasavoided.

The generation of secondary ions with lower energy than inmost ion microprobes facilitated the measurement of oxygenisotope ratios in meteoritic phosphate minerals.167 The 16O2 and18O2 secondary ions generated with 20 keV Cs1 primary ionsfrom 25 mm diameter spots were detected in a MC system.Count rates of 106–109 cps reduced the analysis time to 5 min,including the pre-sputtering step. The IMF determined forwhitlockite and apatite agreed within 1.19% despite the largedifferences in the composition of the minerals. The in situmeasured d18O of between 2.8 and 6.4% was similar to valuesmeasured for Martian meteorite whole-rock and terrestrialigneous phosphate.

Mojzsis et al.168 found deviations from the mass dependentfractionation for d33S in Archean sedimentary sulfides. Themineral-specific IMF was derived from analysis of standardgrains whose matrix composition was quantified by wavelengthdispersive spectroscopy electron probe microanalysis. Spots(25 mm diameter) were analysed with 20 keV Cs1 primary ions.Ion beam pre-cleaning of the samples eliminated preferentialsputtering effects in the transient regime. Setting the massresolution to 4000 reduced the contribution of 32SH2 ions tothe 33S signal to v10 ppm. Variations in the baseline current inthe amplifiers of the MC detection system restricted the preci-sion of the deviations from the mass-dependent fractionationline (D33S) to 0.1%.

Another study of the S isotope composition on the mm scalefocused on the textural variations in sphalerites.169 Polisheddisc sections were bombarded with Cs1 primary ions over anarea of 15 6 30 mm. The 34S : 32S intensity ratio was measuredby magnetic field switching and extreme energy filtering(200 eV). Standards of similar matrix were used to determinethe IMF. The precision of 0.8% for d34S measurements allowedsphalerites and associated pyrites to be categorized into pre-sphalerite, diagenetic pyrite and syn-sphalerite pyrite, inter-grown with sphalerite. Furthermore, two textural varieties ofsphalerite mineralization could be characterized.

Imaging was used for the determination of N and its isotopiccomposition in interplanetary dust particles containing 2H-richmacromolecular organic components.170 The Cs1 primary ionbeam was focused to a 1.5 mm diameter spot. Images of C2, O2

and Si2 secondary ions identified the grains of interest forsubsequent analysis of the N concentration and isotope com-position in the spot mode. Detection of N as 12C14N2 and12C15N2 ions overcame the problem of low N2 yield.Overcoming interferences from 10B16O2, 11B16O2, 12C2H2,13C12CH2 and 13C14N2 required a mass resolution of 7000.Count rates were 1.2 6 104–6.6 6 103 and 25–55 cps for 12C14Nand 12C15N, respectively. Analysis of kerogen standards and amixture of olivine with kerogen showed that errors due tomatrix effects were v5%. An accuracy within 5% was

achieved for 30 mm kerogen grains with known 15N : 14Nratio. The counting statistics made the precision of mm-scalemeasurements a function of the size and N content of the grainstudied.

The O isotopic composition in presolar spinel grains as smallas 150 nm from the Murray and Murchison carbonaceouschondrites was determined with an instrument which featured a16 keV Cs1 primary ion beam with a spot size of 50–100 nm.171

Miniaturized electron multipliers, of which four could bemoved into the focal plane of the instrument, allowed MCdetection of O isotopes to be used. The grains were depositedon gold foils along with standards of aluminium oxide particlesand a Murchison matrix sample. Separation of 16OH2 and17O2 ions required a mass resolution of 5000, whereas 3000 wassufficient for the detection of 16O2, 18O2 and MgO2. Images of16O2 ions and secondary electrons in areas of 20620 mmlocated the grains. Subsequently, the beam was focused to0.8 mm for isotope analysis. The SD on 17O:16O and 18O:16Oisotope ratios were 26–34 and 32–49%, respectively. A scatterplot of the d17O:16O as a function of the d18O:17O locatedmeteoritic and presolar grains both inside and outside a centralelliptical area.

6.5. Single and multi-dimensional analysis

6.5.1. Depth profiling. The sputter rate variation in ultra-shallow depth profiling was studied by analysis of 5–10 shortperiod d-layers of boron nitride between silicon layers (1–5 nmthickness).172 Matrix effects were avoided through use of a 0.37partial monolayer of boron nitride with a nominal thickness of0.05 nm. Oxidation of the first d-layer and the high sputteringrate of 1 keV O2

1 primary ions prevented detection of the firsttwo d-layers in samples with silicon spacers up to 3 nm thick.The differences between the d-layer positions measured with250 and 1000 eV primary ions at incidence angles of 0 and 9u,respectively, decreased with depth and disappeared at the 10thlayer in the sample with 3 nm thick spacers. Compared withanalysis in the steady state regime, the sputtering rate increasedby a factor of 1.5 and 3.5 for bombardment with primary ionsof 250 and 1 keV, respectively, over a depth of 2–3 nm. Thisdepth corresponded to the atomic mixing range.

Ohr et al.173 demonstrated the advantages of TOF SIMS fordepth profiling 1–5 nm thick carbon layers at the top of severalfunctional layers on glass-based hard discs. Application of thedual-beam method in TOF SIMS with 1 keV Cs1 and 15 keVGa1 ions for sputtering and analysis, respectively, allowed 100data points to be collected for a 1–5 nm layer with simultaneousdetection of CrCs2

1, CoCs21 and CCs2

1 secondary ions. Theauthors stressed the importance of using X-ray reflectivity datato determine accurately the thickness of carbon films as thin as1 nm.

A comparison of SIMS and low energy ion scattering (LEIS)for the analysis of 5 nm silicon nitride films demonstrated thedifferent degree of ion-beam-induced broadening for the Ndepth profile.174 In comparison with SIMS, the LEIS methodoffered superior sensitivity for low-mass elements such as Nand O. On the other hand, information on the depth distribu-tion of N measured with LEIS was confined to about 13monolayers (5 nm), whereas SIMS analysis yielded depthprofiles to a depth of 7–12 nm. The Cs1-beam-induced atommixing over a distance of 10 nm was believed to explain theincreased width of the depth profiles in SIMS.

The panoramic detection of TOF-SIMS together with thelimited erosion typical of S-SIMS instruments allowed Jakschiket al.175 to determine simultaneously the depth distribution ofmany elements in thin layers. Atomic layer deposition was usedto grow 5 nm thick ammonium oxide films on either silicondioxide or silicon nitride layers (1 nm thickness) on the top of asilicon wafer. Application of 1 keV Cs1 primary ions at anincidence angle of 45u in TOF SIMS allowed depth profiles for


Al2, C2, H2 and Si2 to be recorded with 10 data points pernm. Unlike data from sequentially scanning SIMS instruments,the sensitivity and resolution of the TOF SIMS depth profiledata were unaffected by the number of elements measured. Inaddition, there was no depth offset for the different ions.

Depth profiling of alternating aluminium oxide and alumi-nium films with an individual layer thickness of 20 or 40 nmdeposited on a (100) silicon substrate showed unexpectedlyincreasing variations in secondary ion signal with depth.176 Byusing signals from Al4

1 secondary ions generated under Cs1

primary ion bombardment it was possible to distinguishbetween Al in a metallic or Al in a ceramic oxide environment.The increasing amplitude of the periodic intensity variationwith depth excluded ion-beam-induced mixing effects. Detailedinvestigation of the surface at different stages of the layerdeposition showed that the surface roughness rms parameterincreased with the number of layers on the substrate andcorrelated well with the intensity variations produced bysuccessive layers in the SIMS depth profile.

The generally observed effect of ion-beam-induced damagein the subsurface did not exclude the feasibility of quantitativespeciation by measurement of selected cluster ions.177 Specifi-cally, the intensity ratio of AlOSi1 and Al2O1 secondary ionsdetected during depth profiling with 5 keV Ar1 primary ionsallowed the binding between organosilanes and the aluminiumsurface to be selectively addressed. Measurements with XPSconfirmed the validity of the approach. Strong adsorption ofthe silanes to the metal surface without effective binding madethis example an excellent test case for speciation in the sub-surface. The 5% precision of the ratio measurement allowed theeffect of different preparation procedures and the initial surfacetopography of the treated metal on the silane binding to bestudied.

Comparison of SIMS and Rutherford backscattering spectro-metry (RBS) for investigating the diffusion of Ag from Mg–Agcathodes of organic light-emitting diodes based on tris(8-hydroxyquinoline) aluminium (Alq3) revealed the importanceof ion-beam-induced artefacts.178 The samples consisted ofsuccessive 50 nm thick layers of Mg:Al, Alq3 and an organichole transport polymer on indium tin oxide. Experimental andsimulated RBS data pointed to the existence of an abruptAg–Alq3 interface with v0.5 at.% Ag in the first 20% of theorganic layer. In contrast, SIMS depth profiles showed asignificant tail of Ag extending deep into the organic layer.Furthermore, in spite of careful calibration of the sputter ratewith profilometry, quite different depth profiles were obtainedusing 1 keV Ar1, 12.5 keV O2

1 or O2 primary ions. In additionto ion beam mixing and matrix effects, field-induced ionmigration had to be considered to explain the discrepancybetween SIMS and RBS data.

The depth profiling of Se in organic semiconducting films ofpolyaniline emeraldine base required179 separation of 80Se2

ions from isobaric interferences with m/z difference of only0.060. The use of a TOF instrument provided sufficient massresolution and the high transmission gave fast depth profilingof Se at total concentrations in the low mg g21 range.Quantitative analysis using the RSF of Se in silicon showedsurface concentrations of around 1011 atoms cm22, far belowthe LOD of many surface analysis methods. Imaging revealedthe formation of sub-mm Se-clusters, which could be of interestfor quantum dot studies.

Nanoscale assessment of wear and corrosion of inconel tubesrequired depth profiling of 59Co in nickel films on galliumarsenide substrates over a distance of w400 nm.180 A massresolution of 300 was sufficient because the contribution of58NiH1 to the signal of 59Co1 was v0.3%. Application ofatomic force microscopy showed a roughness of 7 nm for nickelfilms implanted with 260 keV Co. The roughness of the craterbottom sputtered with 1.5 keV O2

1 primary ions at anincidence angle of 45u was 14 and 58 nm at a depth of 90 and

620 nm, respectively, when no oxygen flooding was applied.Use of oxygen flooding virtually retained the initial surfaceroughness up to a depth of 620 nm and allowed a linearrelationship to be obtained between crater depths and sputtertime.

The comparison of SIMS and Auger electron spectroscopywith ion beam sputtering for the depth profiling of multilayerson LiNbO3 focused on speciation and quantification.181

Specifically, a 150 nm thick Cu layer, sandwiched eitherbetween a 20 nm tantalum and a 30 nm titanium layer orbetween two 50 nm layers of Ta30Si18N52, was sputtered with12.5 keV O2 primary ions in SIMS analysis or 1.5 keV Ar1

projectiles for Auger electron measurements. Significant matrixeffects and variations in the sputter rate occurred in spite of thein-situ oxidation by primary ions in SIMS. In contrast, peakshape analysis in Auger electron spectroscopy allowed varia-tions in the binding energy, shape of the valence band andrelocation effects as well as intrinsic or extrinsic losses to beaccounted for. Although SIMS was, in general, inferior toAuger electron spectroscopy for speciation and quantification,its better LOD was necessary to probe elements with con-centrations of v0.1%. Data on the position of the Ti/LiNbO3

interface obtained by Auger electron spectroscopy could beused to calibrate the SIMS depth profile.

Determination of the depth distribution of H across thesilicon dioxide–silicon interface at a depth of 600 nm, aftertreatment with low temperature hydrogen annealing, requiredcareful optimization to avoid generation from the rim of thecrater of H1 ions by electron bombardment.182 The H levels atthe interface approached the trap density of 1010–1012 atomscm22. Alignment of the 2 keV electron and 5 keV Cs1 primaryion beams was critical as misalignment even over a distance assmall as only a few 100 mm caused the signal intensity to varyby a factor of 2. A top-loaded sample holder was preferred asit gave a precision of 5.4% for the measurement of Si2 signalsin comparison with 10% obtained with a spring-back-loadedsample holder. Another key experimental parameter was thecontribution of background hydrogen from surface contami-nants and residual gas.

The properties of novel bounded and hot pressed magnetsdepended on the hydrogen binding and electronic surface effectsin hydrogenated Nd2(Fe/Co)14B alloys.183 Partial filling of thesample chamber with 2H2 allowed the material to be hydro-genated in-situ. The reduction was measured by the Nd1 :NdO1 ratio which was particularly high when samples ofNd2(FexCo12x)14B with x ¢ 0.5 were deuterated. The fastdecay of the Nd2H2

2:Fe22 ratio for x w 0. 5 pointed to

the conversion of Nd2Fe14B2 into Fe, Fe2B and NdH2. TheFe2H2:Fe2H2

2 and Co2H2:Co2H22 ratios produced from

Nd2Fe14B and Nd2Co14B, respectively, showed the preferentialemission of Fe2H2 and Co2H2

2 rather than Fe2H22 and

Co2H2, respectively. This indicated that the capability of Feto dissociate H2 was superior to that of Co, a trend that wasconfirmed by XPS.

The benefits of backside depth profiling for the analysis ofultrashallow layers were demonstrated on silicon samplescontaining four B d-layers (5 nm separation).184 The aim wasto mimic the depth profiling of samples implanted with 100–200 eV B at a dose of 105 atoms cm22. To avoid the hightransient sputtering rate and surface roughness problemsresulting from the use of low energy primary ions, backsidedepth profiling with 0.2–1 keV O2

1 primary ions at anincidence angle of 45u in conjunction with oxygen flooding wasinvestigated. A layer of amorphous silicon was deposited on thetop of the sample to avoid sputter rate variations at theinterface with the adhesive in the backside mode. A 40 nm thinsection was polished to a surface roughness of v0.1 nm in thearea to be analysed (3 mm diameter). The 10B contamination ofthe surface allowed the original surface to be localized in thebackside experiments. A depth resolution of 0.7 nm was


achieved for both sides of the distribution when 200 eV O21

primary ions were used.A new sample preparation method for the backside depth

profiling of wafers with silicon-on-insulator (SOI) structure wasbased on the use of the dimpling technique in combination withpolishing and etching.185 The sample (6 6 6 mm) was coatedwith 50 nm of palladium to improve conductivity and mountedwith its front side on a glass plate. Subsequent polishingreduced the thickness from 720 to about 100 mm. Furtherthinning made use of the dimpling method used in TEM.Examination of the dimpled bottom provided accurate controlof the thickness of the silicon layer because red light becamevisible through the layer at a thickness of 20 mm. SubsequentKOH etching removed residual Si without attacking theoxide layer. The roughness of the buried oxide was as goodas 0.4 nm. The exposed area with a diameter of 800 mm wassufficient for SIMS analysis. Four samples could be prepared ina day. Results showed sharp interfaces in stack samples thatwere representative for current complementary metal-oxide-semiconductor (CMOS) technology.

The improvement of depth resolution by backside depthprofiling was demonstrated by a study of a B-implanted poly-crystalline silicon layer with a thickness of 160 nm anddeposited on a 1.5 nm thick nitrated silicon dioxide layer at thesurface of a SOI wafer.186 The substrate was etched withtetramethylammonium hydroxide and subsequently treatedwith dilute HF. Backside depth profiling of B with 0.5 keV O2

1

primary ions at an incidence angle of 56u together with oxygenflooding gave a decay length of 1 nm, whereas front sideexperiments gave a decay length of 2.7 nm. This was partlyexplained by the surface roughness of 4.5 nm for the poly-crystalline layer whereas that of the etched backside surfacewas as low as 0.3 nm. Use of 2 keV Cs1 primary ions at anincidence angle of 44u and MCs1 (M ~ B, N or O) detectiongave a decay length of B profiles of 8 nm in front sideexperiments and 1.3 nm in backside profiling. Front side andbackside experiments yielded decay lengths for N of 7 and2.6 nm, respectively. The additional improvement for N incomparison with that for B came from the ion-induced diffu-sion of nitrogen in oxynitrides.

6.5.2. Imaging and 3-D analysis. Use of benchtop SIMSprovided a lateral resolution of 10–100 mm in the imaging ofelement distributions in filamentary trails of corrosion pro-ducts on steel.187 The coating formed on steel by treatment withan organic corrosion inhibitor was delaminated by exposure toCsCl solutions. After peeling off the organic layer, images fromsecondary electrons and Cl2 and Cs1 ions could be acquiredfrom areas in the range 1.561.5 to 4.564.5 mm. Whereas Cs1

was homogeneously distributed over the surface, Cl2 imagesshowed minimal levels at the delamination front and enrich-ment in the head of the filiform corrosion tails. The low cost,easy operation and high throughput of the bench-top instru-ment were considered to compensate for the relatively poorlateral resolution.

The superior sensitivity of TOF SIMS in comparison withSEM was exploited for the imaging of metals at the soil–rootinterface of trees from the vicinity of a copper smelter.188 Thelateral distribution of Al, Ca, Fe, K, Mg, Mn, Na and Tisecondary ions was recorded over a 2506250 mm area.Normalization of the atomic ion intensities to the totalnumber of secondary ion counts in each pixel corrected forthe matrix effect. Topography effects in embedded samples dueto height differences of up to 1–2 mm were responsible for thelimited image resolution of 1 mm.

Combination of imaging and depth profiling allowedGammer et al.189 to perform a 3-D analysis of the non-metallicinclusions in a low-carbon chromium-steel (0.5% C, 13% Cr).Bombardment with Cs1 (for Al, Ca, Mg) or O2

1 (for O and S)primary ions was used to produce secondary ion images from a

central area of 1506150 mm in the sputtered region of350 6 350 mm and to a depth of 15 mm. The lateral resolutionwas about 2 mm in the stigmatic mode and 0.2–0.5 mm in ionmicroprobe experiments on a restricted raster of 64 6 64 mm.Depending on the steel manufacturing process, spherical oxideor ring shaped particles with a size of 3–5 mm were formed withhigh levels of Al, Ca, Mg, O and S.

6.6. Static SIMS (S-SIMS)

6.6.1. Review. Belu et al.190 comprehensively discussed thepresent state-of-the-art and potential of TOF S-SIMS for thecharacterization of biomaterials. In addition to consideration ofthe basic principles, operational features and complications ofquantitative analysis, attention was paid to the imagingcapabilities, which were considered to represent the primeasset of the method for biomaterial research. Numerousexamples were cited to assess critically the trade-off betweenion yield and lateral resolution. Although the wealth ofinformation obtainable through the use of atomic ions,structural fragments and molecular (adduct) ions was recog-nized, interpretation of the resulting complex spectral patternswas claimed to require multivariate statistical methods, whichwould additionally improve the LOD and precision of quan-titative analysis.

6.6.2. Instrumentation. Use of fine rhenium powder (635mesh), mixed with barium sulfate in small tubes of rhenium orplatinum, as the emitter in a polyatomic primary ion sourceincreased the current of perrhenate projectiles by a factor of1000 compared with the previously used mixture of bariumperrhenate and europium oxide in a ceramic matrix.191 Heatingthe emitter to 1100–1300 uC yielded primary ion currents ashigh as 200 nA mm22 for w100 h. This performance wasmaintained for barium sulfate concentrations in the range of 22and 70% by volume. Interestingly, a freshly prepared emitterproduced perrhenate ions from the central mixture but ageingcaused the primary ions to come from the edge of the metaltube. A magnetic field separated the 10 000-fold excess ofelectrons from the perrhenate beam. The high output currentwill need careful design of the optics to avoid space-chargeeffects.

6.6.3. Fundamental studies. Li and Hirokawa192 resurrectedthe concept of fragment valence to describe the systematictrends in the composition of secondary ions produced frominorganic samples bombarded with Ga1 primary ions. Specifi-cally, binary salts or oxides denoted as MpAn, were consideredto generate secondary ions of the type MxAy with nx ¢ py 1 1for positive ions and nx ¡ py 1 1 for negative ions. This simpleapproach was unsuccessful even for oxysalts, which requiredmore complicated formulae to describe the detected ions. Evenin this relatively simple case, the initial aim of predicting themajor ions in a spectrum of a given analyte could not beachieved.

In contrast to the previous study, Van Ham et al.193 used acomprehensive database for w100 compounds to demonstratethat ion formation could be explained by considering initialdesorption of the analyte molecules in the form of neutrals by,for instance, ultra-fast thermal processes followed by ioniza-tion in the selvedge through electron interaction and adductformation. All assignments were supported by m/z determina-tions typically accurate to within 50 ppm. The precision andaccuracy of isotope ratio determinations were within 2%.Hence, even small deviations from expected isotope ratioscould be used to detect interfering ions. The developed con-cepts allowed modification in surface composition to bedeductively studied.

The formation of secondary cluster ions from metals by Ga1

primary ions was related to the formation of eutectic phases


with elements such as Al, Si and Zn.194 Specifically, the atomicand cluster ion signal intensities for Al, Co, Cr, Fe, Mn, Mo,Ni, Sb and Si were dependent on whether Ar1 or Ga1 primaryions were used and varied in a characteristic manner. Phasediagrams of Ga and the element under study revealed thatformation of eutectic phases, intermetallic compounds andsolid solutions could explain the detection of Ga-containingclusters for elements that form alloys with Ga, whereas metalswith low solubility of Ga emitted primarily metal clusters. Theauthors believed that some kind of surface alloying wouldoccur and govern the subsequent ion ejection.

6.6.4. Analytical methodology. The empirical calibration ofthe relative intensities of secondary ions from the substrate andthe surface coating allowed the thickness of ultra-thin carbonfilms on magnetic disks to be determined.195 The approach waselaborated for a multilayer system on glass, of which the twoupper coatings consisted of a carbon layer with a thickness of1–12 nm and a Co–Pt magnetic alloy. The ultra-smooth surfacecontributed to the successful calibration of the Co1:C3H7

1

ratio as a function of the independently determined thickness ofthe carbon film. Optimum emission of Co1 ions was obtainedwith a primary ion beam incidence angle of between 41 and 75u.Because of the generally accepted notion that the informationdepth of S-SIMS does not exceed one monolayer, the authorspreferred to assume the presence of nanoscale holes in thecarbon coatings even though TEM measurements did notreveal such defects.

Two consecutive studies focused on the use of S-SIMS toobtain direct information on the chemical species in the uppersurface layer of involatile getter (NEG) materials for MSpumps.196,197 An instrument with in situ heating capabilitieswas exploited to study the reaction kinetics of the activation ofZr–V coatings on steel. Use of 2 keV Ar1 primary ionbombardment caused oxidized NEG surfaces to emit M1 andMO1 (M ~ V, Zr) ions. Minor signals from MC1, MH1 andMOH1 ions indicated the binding of hydrogen, hydroxyland carbon to the metal. The activation involved reductionof the oxides by hydrogen diffusion from the bulk and/orhydrogen from the ambient atmosphere by decomposition ofwater and hydrocarbons at high temperature. The M1:MO1

and MH1:M1 intensity ratios measured as a function of thetemperature were used to follow directly the activation process.The precision was sufficient to determine the ZrH1 contribu-tion by subtraction of the different isotope signals. The secondstudy focused on the NEG activation in Ti, V, Zr and Ti–V–Zrsystems. The MX1 : M1 (X ~ C, CO, CO2, H, O, OH)intensity ratio could be used to measure coverage of the surfacewith specific oxygenated species.

The differentiation of sp2 and sp3 C in activated carboncatalysts represents a speciation task far beyond the capabilitiesof most microprobes.198 A study of ASTM (American Societyfor Testing and Materials) grades of carbon showed that therelative intensities of the CH2, C2

2, C2H2 and C2H22 ions

generated under bombardment with 5 keV Ar1 ions allowedthe surface ratio of C : H to be linked to the crystallinity and thecontent of sp2 and sp3 C. Quantitative analysis requiredindependently characterized standards. Knowledge of theelectron density and reactive sites was essential to optimizecatalyst performance.

A study of surface reactions on magnetic recording disksdemonstrated the use of speciation capabilities for a completedescription of a material with inorganic and organic compo-nents.199 Ceramic sliders and the magnetic disk were mountedinside the vacuum chamber to allow the wear tracks and sliderheads to be studied without further exposure to ambient con-tamination. The slider contained an aluminium oxide, titaniumnitride, diamond-like-carbon or cubic boron nitride coatingwith a thickness of 0.5 mm and the disks were composed of amultilayer of diamond-like carbon, Co-containing magnetic

alloy and glass. A perfluorinated alkyl polyether film with athickness of 10 nm served as lubricant. The detection of AlF4

2

ions from the wear track bombarded with Ga1 primary ionswas direct evidence for the local presence of AlF3, known tocatalyze lubricant decomposition. Furthermore, organic frag-ments showed that lubricant degradation started with the endgroups. In contrast to aluminum oxide and titanium nitridecoatings, layers of diamond-like carbon and cubic boronnitride on the slider suppressed the decomposition of thelubricant.

An example of refined speciation analysis was the detectionof covalent bonds between (100)GaAs and an ultra-thin (5 nm)S-containing polymer layer deposited by rf plasma to passivatethe surface of GaAs.200 The simultaneous presence of signalsfrom ions such as AsS2

1 or GaS1 and RS1 (R ~ organicchain) clearly pointed to the covalent binding of the sulfidefunctionality in the coating and the GaAs. The identificationwas confirmed with XPS data. Attempts to quantify the TOFS-SIMS signals were hindered by the typical complexity of themass spectra from plasma-functionalized materials.

An ultimate test for a speciation method is the detection ofinterphase interaction products. Specifically, an organic layer ofheptahelicene on Ni(111) was studied because of its potential asa high-efficiency emitter of circularly polarized light in opticaldata storage and liquid crystal displays.201 Unambiguousidentification of interphase interactions is always a challenge toMS, for which the analytes have to be released from the solid.Therefore, the intermolecular bonds within each phase mustbe broken but those between analytes in different phasesshould remain intact. The detection of heptahelicene–Ni1 andheptahelicene–Ni2

1 showed that it is feasible to detect inter-phase interactions using S-SIMS. This application will becomeincreasingly important with the growing use of chemicalsurface engineering and nanotechnology.

One of the first S-SIMS studies in the field of biomedicaltissue analysis dealt with the location of various elements andsubstances in epiretinal proliferative tissues excised duringsurgery.202 To preserve the distribution of the componentswithin the tissue, samples were fast frozen in liquid nitrogenwithout any fixation and freeze-dried on silicon. The simulta-neous detection of atomic ions such as Al, Ca, Cu, Fe, K, Mg,Mn, Na and Zn and organic substances such as vitamins,palmitic, oleic and stearic acid allowed correlations of sub-stantial interest for medical diagnosis to be made. Despite thesuperior lateral resolution of electron probe X-ray micro-analysis in comparison with that of TOF S-SIMS, the lattermethod was clearly preferred because of its lower LOD andability to provide information on organic structures.

The speciation of an interface layer buried deep under a porousoxide layer is a demanding analytical task. Development ofcathode ray tubes using thermionic oxides required the chemi-cal composition of a nickel–aluminum or nickel–magnesiumalloy under a 50–100 mm porous layer of barium and strontiumoxide to be investigated. Instead of dealing with depth profilingartifacts, Jenkins et al.203 focused on the development of asample preparation method suitable for exposing the interface.The difference in mechanical properties between the porousoverlayer and nickel prevented cross sectioning. Either use ofadhesive tape to remove the bulk oxide in combination withphysical scratching or delamination of the oxide layer undermoisture attack proved to be adequate. The concentrations ofNi as well as of the activators Al and Mg were quantified bymeans of the atomic ion signal intensities. The interferencefrom hydrocarbon ions on the signal of Al1 was corrected onthe basis of the hydrocarbon fragments at m/z 29.

The in situ study of the reaction between thin layers of sulfuricacid monohydrate with gaseous ammonia allowed the kinetics ofthis environmentally important process to be determined.204

Self-assembled monolayers (SAM) of sulfuric acid on gold weremounted on a cold stage at 200 K and exposed to 1029 mbar of


ammonia in the TOF S-SIMS sample chamber. Bombardmentwith 7.5 keV Ar1 primary ions generated molecule-specificadducts of sulfuric acid, water and ammonia up to m/z 250. TheLOD were significantly better than those obtained usingattenuated total-internal-reflection IR spectrometry. The massspectra contained no evidence for ion-beam-induced damage ofthe surface layer.

The separation of isobaric contributions from atomic ions andorganic fragments was the key issue for the successful assess-ment of the geological and mineralogical origin of pyrites incoal seams in China.205 Whereas previous studies using lowmass resolution instruments had failed, state-of the-art TOFSIMS provided sufficient mass resolution up to m/z 100 toseparate the atomic ion signals from the organic interferencesand thereby allow adequate quantification to be attempted.Once separated from the atomic ions, the organic fragmentscould be additionally used as fingerprints for cells and bacteria.As a result, distinction between bacteriogenic, framboidal,massive, cell-filling, fracture-filling, and nodular pyrites becamefeasible. For instance, bacteriogenic pyrites yielded high signalsfor Al1, Ca1, Cu1, Fe1, Ni1, Si1, Zn1 and C3H7

1, whereasframboidal pyrites primarily generated Al1, AlO1 and Si1 ionstogether with organic contributions from C3H3

1, C3H51 and

C4H71.

6.6.5. Quantitative analysis. Takeda and Fukuwa206 deter-mined the surface concentration of hydroxyl groups in thin filmsof silicon oxide doped with Al, Ti or Zr and deposited on glassby rf magnetron sputtering. Although Fourier transform IRspectrometry was the most common method for characteriza-tion of silanol groups, the poor range over which informationcould be taken prevented selective measurements of the thinfilm surface. The chemical shift in XPS could not distinguishbetween oxide and hydroxide functions. Application of a15 keV Ga1 primary ion beam to an area of only 80680 mmallowed the absolute concentration of the hydroxyl groups tobe determined from the 16O2:OH2 intensity ratio andcalibration with RM. The v1% precision of the method ledto an excellent correlation between contact angle measure-ments and the surface F-concentration after reaction with aperfluoroalkyl isocyanate silane.

6.6.6. Imaging. The use of noise removal and multivariateimage analysis allowed the distinction between surface featuresof similar composition to be significantly improved.207 Threeestablished noise removal algorithms (down-binning, boxcarand wavelet filtering) were applied to the images of 20–100 mmthick patterned layers containing circular or square regions ofphotoresist or protein SAM and separated by 10–60 mm widesubstrate channels. Increasing the pixel size by down-binningappeared to the most appropriate method for combining withPCA. The loadings of PCA models for the filtered imagesfacilitated tracking back ions that caused the contrast in thetotal ion current image. Strikingly, high m/z organic ions weremore useful than the atomic ions for explaining the contrastdifferences, even though the atomic ions had significantlyhigher intensities.

Development of multi-gas sensors required both the removalof organosilanes from and the deposition of tin and iron oxides onmicro-hot-plate sensing arrays to be characterized. The simul-taneous recording of images and mass spectra in TOF S-SIMSwas a significant advantage for probing the surface concentra-tions in layers as thin as a few nm.208 As the mass resolution inthe imaging mode was inherently restricted to the separation ofnominal masses, the presence of isobars was checked by takingspectra with primary ion pulses of 1 ns. The high yield ofatomic ions facilitated imaging with high contrast and lateralresolution.

Imaging and speciation of metal oxides by means of bothatomic and cluster ions was exploited in a study on the

microbially and electrochemically induced pitting of stainlesssteel ennobled by the microbial deposition of manganeseoxides.209 Imaging revealed local spots where the Cr1

secondary ion intensity was anomalously low without acorresponding increase of the Mn1 ion intensity. Exposureof the steel to NaCl revealed the start of pitting at these sites.Analysis of Fe, Fe2O3, Fe3O4 and FeOOH reference samplesshowed that measurement of the FeH1:Fe1 ratio allowed theoxidation state of the iron in the pits to be determined. WhereasFe2O3 was prevalent in ennobled steel samples, an oxidationstate between Fe2O3 and Fe3O4 was found in untreated metal.

The simultaneous imaging of atomic ions and organicfragments was exploited to study the grade and stage of prostatecancer in biopsies with TOF S-SIMS.210 Prostate cancer cellcultures were fast frozen between steel plates and subsequentremoval on one plate allowed cells to be cleaved in differentplanes, exposing the inside for analysis. After freeze drying, ionimages over areas of 1006100 mm were taken with a Ga1

primary ion beam focused on to a 1 mm diameter spot. Use ofthe Na1:K1 ratio, as well as the intensity of the Ca1 ions,pinpointed the fractured cells. Further confirmation could beobtained from the detection of phosphatidylethanolamine andcholine fragments from the cell membrane components infractured and unfractured cells, respectively. The specificitywith which fractured cells could be located contributed signifi-cantly to correlating histology and the presence of elementssuch as Cu and Mg at specific sites in the cells.

6.7. Surface and subsurface analysis with S-SIMS and SIMS

The capability of TOF SIMS to generate in the same experi-ment both molecular information on the surface and depthprofiles of elements in the subsurface allowed Socha andVayrynen211 to study the process in which silicon carbide istreated with HF and oxidized with H2O2. The co-deposition ofnickel and sub-mm silicon carbide particles depended onoxidation conditions adequate for making the surface of thecarbide particle hydrophilic. Application of Ga1 primary ionbombardment under static conditions generated intense signalsfrom CxFy fragments, typical for Teflon, but none from SiFx

species. Subsurface analysis at above the static ion dose con-firmed the complete removal of SiFx and the formation of aSiOxCy layer under the Teflon.

The combination of imaging under static conditions with depthprofiling was exploited for the characterization of sphalerite(ZnS) and pyrite (FeS2) flotation samples.212 The 25 keV Ga1

primary ion beam was focused to a 200 nm diameter spot. Thestructural relevance of adduct ions generated in the static modeallowed interpretation of the image information to be refined.The agreement between Fe and FeOH images suggestedadsorption or precipitation of ferric hydroxide on the surface.Interestingly, the change of the Cu1:Zn1 ratio from 3.6 to 1 inthe subsurface reflected the substitution of Zn atoms by Cuatoms when Cu migrated into the sphalerite mineral.

The feasibiliy of surface and subsurface speciation of calciumphosphates was explored in a study of plasma-sprayed hydro-xyapatite coatings on titanium implants.213 Fast plasma-annealing and subsequent cooling in layers of differentthickness caused the initially generated metastable calciumphosphates to convert into different molecular forms. Applica-tion of 8 keV Cs1 primary ion bombardment on a 2006200 mmsample area showed that the ratios of low m/z cluster ions suchas PO2

2:PO32 and Ca1:CaOH1 could be used to distinguish

adequately between hydroxyapatite, a- and b-tricalciumphosphate, calcium phosphate and orthocalcium phosphate.Careful calibration of the distinguishing ratios as a function ofthe ion dose allowed speciation to be extended to the transientregime and depth profiling mode. A study on uranium oxideparticles confirmed the feasibility of speciation in the sub-surface.214 Surface analysis in the static mode showed that


normalization of the characteristic signals from uranium oxideions to U1 or O2 in the positive or negative ion mode,respectively, could be used to distinguish between UO2, U3O7,UO3 and U3O. Applications of dual-beam TOF SIMS depth-profiling over a distance of 2 nm on the oxidized samplesshowed that the normalized UO3 signal intensities agreed withthe average UO3 concentration determined by XPS. Therefore,it was believed that, in spite of the ion beam damage, adequatespeciation could be achieved in the subsurface.

Moller et al.215 achieved the analytically demanding distinc-tion between reacted, adsorbed and dissolved oxygen in polymers.Low-density polyethylene was exposed to an atmosphereenriched with 18O2 for up to 288 h at a temperature of 90 uCto speed up ageing. The surface and cross sections wereanalyzed with TOF S-SIMS using a Ga1 primary ion beam.Oxidation of the polymer structure was followed by measuringthe CH3

18O1 and C2H318O1 ions. Interestingly, the signal

intensities of 18O2 and 18OH2 detected as a function of theoxidation and ageing conditions correlated well with those ofthe organic ions, indicating the absence of adsorbed oxygen.Furthermore, analysis of cross sections confirmed that thereaction occurred homogeneously in depth terms, excluding thepossible role of dissolved oxygen. Relative quantification wasfacilitated by using the corresponding 16O ions to normalize thepeak intensities.

7. Sputtered neutrals mass spectrometry (SNMS)

An EI source was designed specifically to allow a quadrupoleSIMS to be used in an alternative SNMS mode.216 Use of alarge ionization volume in combination with a relatively lowelectron current minimized space charge effects. The highacceptance angle of the source, mounted at a distance of 20 mmfrom the sample, ensured that about 0.9% of the emittedneutrals was collected. About 1 in 107 of the total number ofneutrals sputtered was detected as a post-ionized species. Aretarding lens with a potential slightly above that of theionization chamber discriminated against the residual gas ions.The energy-independent transmission of secondary ions in theEI source meant that it could be left in place during SIMSexperiments.

Electroceramics, such as perovskite, offer a wide potentialfor novel nanoscale devices such as multi-GBit memories,transistors and nano-actuators. Breuer et al.217 exploited thecombination of SNMS and SIMS for the depth profiling ofBaxSr12xTiO3 dielectric films on multistack layers of Pt/TiO2/SiO2/Si or Pt/ZrO2/SiO2/Si, where TiO2 or ZrO2 served as anadhesion layer. Use of Cs1 sputtering and EI gave depthprofiling measurements of interfacial reactions in the virtualabsence of matrix effects. Hence, SNMS was the method ofchoice for the quantitative analysis in the interface region.However, the SNMS LOD was poorer than that of SIMSanalysis, which therefore had to be used for determination ofthe depth distribution of trace elements.

8. Stable isotope ratio mass spectrometry (SIRMS)

8.1. Reviews

The excellent review of Ghosh and Brand218 on the use ofSIRMS in global climate change research presented a detailedyet clear description of the principles of the MS methods (withplentiful schematic diagrams), sample preparation and severaltypical applications. There were considered to be three mainobjectives of studying isotopes in relation to past climate—reconstruction of past natural climatic variability, testing ofclimate models using past environment conditions andimprovement of model calibrations for predicting futureclimate.

The review of Asche et al.219 on the sourcing of organic

compounds based on natural isotopic variations discussedthoroughly the principles of this widely used approach toestablish the origin of materials. A brief introduction toinstrumentation associated with the high precision MSdetermination of stable isotope ratios was followed by anin-depth discussion of applications to establishing the authenti-city, adulteration or source of foods, petroleum and endogen-ous steroids. Petroleum samples from different geographicallocations could be distinguished on the basis of isotopiccomposition even though they were otherwise identical.Climatic conditions induced isotopic fractionation whichcould be characteristic for a particular region or even, forexample, a specific vintage of wine. Intramolecular or chiralisotope analysis could be used to detect the origin of flavourcompounds. Use of illicit, endogenous drugs could be detectedby SIRMS analysis.

Pouteau et al.220 discussed methods for the measurement ofcholesterol absorption in humans and charted the switch fromthe use of radioisotopes to the application of stable isotopemethods. The most promising procedure was considered to bea dual-stable-isotope dual-tracer approach based on the simul-taneous administration of oral (enriched 2H) and intravenous(enriched 13C) differentially labelled tracers followed byplasma sampling for 3–4 days. The use of on-line GC-combustion-SIRMS and GC-pyrolysis-SIRMS allowed muchsmaller amounts of tracers to be detected than would have beenpossible using GC-EIMS.


Normal background compensation or source cleaning wereinadequate for accurate correction of short- and long-termmemory effects which caused discrepancies of 0.1–0.2% per10% difference in d45(CO2) values between sample andreference gases.221 These effects were attributed to ionimplantation and sputtering of the focusing lenses, in particularthe ion source slit. Replacement with tantalum componentsminimized sputtering phenomena. In addition, the acceleratingvoltage was reduced to reduce implantation depth and changeswere made to source conductance, inlet pressure, electronemission and inlet changeover equilibration time. The decreasein acceleration potential did not affect sensitivity provided thatthe instrument was calibrated and focused correctly. Significantimprovements in measurement accuracy were realized and,after modification, changes in inlet equilibration time (w15 s)no longer influenced the measurement. Different settings of ionsource conductance resulted in a five-fold improvement in theshift in d13C to about 0.04% per 10% measurement differencebetween sample and reference.

A system for the automated measurement of 13C:12C ratios inCO2 and dissolved inorganic carbon (DIC) in ecological andenvironmental studies consisted of an automated headspacesampler linked to a commercially available CO2 preconcentra-tion system and continuous flow (CF)-SIRMS instrument.222

The objectives of the study were to test the performance of thesystem and to develop sampling procedures that minimizeddisturbance and off-line preparation while giving accurateanalysis and high sample throughput. In the field, gas sampleswere injected directly into N2-filled autosampler vials, therebyallowing small samples to be analysed directly without the needfor additional preparation. The system demonstrated goodaccuracy, precision (236.6¡0.06% for d13C in a standardsample) and linearity with standard errors of v0.1% forreplicate gas standards over a five-fold concentration range. Upto 40 samples with concentrations ranging from the ppb to the% level could be analysed in a 10 h run.

A system for the unattended collection of 100-ml air samplesin remote areas was designed primarily for collecting samplesfor research into CO2 fluxes between an ecosystem and theatmosphere.223 Although it was concluded that isotopic


exchange between sampler components and air samples, heldfor up to 5 days at between 10 and 40 uC, was largely within theoverall precision limits of the CF-SIRMS system used,unacceptable errors were found for samples held for longeror at higher temperature. The error observed for d18O wassubstantially higher than that observed for d13C. The primesource of error was stainless steel tubing. At this stage ofdevelopment, the sampler described is clearly limited in itsapplication, in particular for the measurement of d18O inatmospheric CO2.

The performance of an elemental analyser (EA)-SIRMSsystem for the measurement of both d13C and d15N in a singlesample was improved by intoducing a six-port valve after theGC column and inserting a trap of CO2-absorbing materialinto a loop.224 This arrangement allowed the N2 peak to beswitched through the trap and residual traces of CO2 from theprevious sample to be removed. The CO2 peak, on the otherhand, was switched directly to the analyser, thereby by-passingthe trap. Serious errors were observed in the measurement ofd15N if the traces of CO2 were not removed. The lifetime of thecombustion reactors was enhanced greatly, especially for theanalysis of samples with high salt content, by partiallyreplacing some of the Cr2O3 in the oxidation tube with CeO2

separated from the remaining Cr2O3 by a plug of quartz wool.Sacks et al.225 reported an experimental and theoretical

evaluation of quantization error in CF-SIRMS systems.Samples of CO2 (0.1–30 nmol) were injected into a GC-combustion-SIRMS system and the analogue signal digitized(24-bit at 10 Hz) and post-processed to simulate 12, 14 and16-bit data sets. The value of d13C was calculated for all datasets using either the conventional ‘‘summation’’ method or bycurve-fitting the chromatographic peaks to the exponentiallymodified Gaussian function. Data reduction using curve-fittingwas more robust than the conventional method when evenmodest levels of quantization error were present. Curve-fittingachieved a precision (1s) of v0.3% for injections of 0.76 nmol,some 20-fold smaller than the 15 nmol required for the‘‘summation’’ method. The poor performance of the summa-tion algorithm was particularly noticeable at 12-bit resolution,at which an SD of v1% could not be reached even at themaximum injection size possible.


An investigation into the amount-dependent isotopic fractiona-tion in compound specific isotope analysis (CSIA) using GC-combustion-SIRMS paid particular attention to wherefractionation occurred within the system.226 Injection experi-ments with varying amounts of gases (CO2, n-hexane andtoluene) revealed that the main source of fractionation was thesplit/splitless injector and that neither the combustion reactornor the detector contributed. Although optimization of injec-tion parameters reduced fractionation from 3 to 1%, completeelimination could not be achieved. It was suggested thatcorrection for the remaining and unknown amount-dependentfractionation could be made by analysing, with each batch ofsamples, standards of varying analyte concentrations andknown d13C values.

The calculation of d13C from raw CO2 isotope data involvescorrection of the ratio of the ion beams measured at m/z 45 and44 for the contribution arising from 17O-containing molecules.The correction is based on the 18O abundance (measured at m/z46:44) and an assumed relationship between the 17O and 18Oabundances. Assonov and Brenninkmeijer227 demonstratedthat the three correction algorithms generally in use producedbiased d13C values and that the biases were larger than theprecision of modern SIRMS instruments. The three algorithmswere based on different determinations of the 17O absoluteabundance ratio and different values of ‘‘l’’, the main charac-teristic of the oxygen isotopes relationship. The authors

proposed that the slope of the fractionation line for waters(‘‘l’’ ~ 0.528) be used as a default value for the d17Ocorrection. In addition, there was considered to be a pressingneed for redetermination of 17RVPDB-CO2

(17O:16O calculatedon the VPDB-CO2 international scale) as the considerablediscrepancy (80%) between values currently used introducedbias into published data. An approach for this redeterminationwas discussed but results were not presented.

The analytical frontier for SIRMS now lies in the develop-ment of automated position-specific isotope analysis (PSIA)systems for intramolecular studies. Sacks and Brenna228 used aGC-pyrolysis-GC preparation system coupled to a combustionSIRMS system for the high-precision PSIA of C in amino acidanalogues. The C isotope ratios were measured in the gas-phasepyrolysis fragments from multiple sources of 3-methyl-thiopropylamine (3MTP) and isoamylamine (IAA), thedecarboxylated analogues of methionine and leucine. Dec-arboxylation had the advantage over alternative derivatizationmethods of adding no exogenous C, which would interfere withthe fidelity of the pyrolysis fragments. The characteristicpyrolysis products over a temperature range of 620–900 uCwere CH4, C2H6, HCN and CH3CN for 3MTP and propylene,isobutylene, HCN and CH3CN for IAA. Optimum pyrolysistemperatures were established for each fragment so that thesignal was at a maximum and the fragments produced retainedtheir structural integrity. The PSIA results presented high-lighted the intramolecular variability of isotope ratios and theinformation lost in CSIA. The pyrolysis efficiency was,however, only about 5% and so PSIA required several ordersof magnitude more sample than CSIA.

The two N atoms in the linear N2O molecule are notequivalent and the 15N abundance may vary independently atthe two positions. A method for the automated PSIA ofatmospheric N2O was based on the fragmentation of the N2O1

ion in the ion source to produce a significant number of NO1

fragment ions.229 A prerequisite for the method was that thecollector configuration should allow both the NO1 ions at m/z30 and 31 to be monitored simultaneously and the N2O1 ionsat m/z 44, 45 and 46 to be measured conventionally. It was notnecessary to measure both sets of ions at the same time but thelatest generation of instruments do, in fact, allow collectors tobe configured to monitor all masses of interest simultaneously.Such a set-up would make it possible to measure both 15N and18O together. The analysis was complicated by the non-linearityof the fragment ion measurement and by isotopic scrambling inthe ion source. Ideally the NO1 ion should contain only the Natom originally bound to the O atom but 8.5% of the NO1

fragments were found to contain the other N atom. Thereproducibility for a 125 ml sample at ambient N2O con-centrations was ¡0.1 and ¡0.2% for d15N and d18O,respectively.

The determination of the isotopic composition of themicrobial biomass C in soil is an increasingly important toolin the study of soil microbial ecology and soil C turnover. Atpresent there is no standard procedure for the extraction of soilsamples and preparation for isotope analysis. Potthoff et al.230

discussed the advantages and disadvantages of differentmethods for the determination of 13C:12C in soil microbialbiomass and proposed a new method based on UV-catalysedpersulfate (Na2S2O8) oxidation of fumigated and non-fumigated soil extracts, cryogenic trapping of the releasedCO2 and GC-SIRMS. Comparison of the new procedure withan off-line dual-inlet SIRMS procedure and an on-line EA-SIRMS procedure showed that there were no significantdifferences in the d13C values measured by all three procedures.

Stable isotope approaches can help to improve our under-standing of trophic interactions in complex systems such assoil. The most widely used approach for determining d13C andd15N in soil invertebrates, bulk analysis by SIRMS, has themajor shortcoming of requiring tens to hundreds of organisms


for a single determination. The application of on-line, ther-mally assisted hydrolysis and methylation reactions (usingtetramethylammonium hydroxide) allowed the compound-specific determination of d13C in fatty acids in soil mesofaunato be applied to a single organism.231 The derivatization tookplace in the pyrolysis-GC inlet of a pyrolysis-GC-combustion-SIRMS system. The fatty acids were converted rapidly to theirmethyl esters in a single step reaction, thereby avoiding thecomplicated protocols associated with wet chemical derivatiza-tion methods. Fatty acid profiles and corrected d13C valuesshowed excellent agreement with those obtained by conven-tional methods. Although a kinetic isotope effect preventeddirect calculation of the fatty acid d13C values by simple massbalance equations, the effect was reproducible so thatcorrection factors could be applied and the associated error(¡0.5%) fully quantified. Results were presented for the firsttime for the d13C values and fatty acid composition ofindividual soil mesofauna.

The novel linking of fluorescence-activated cell sorting withSIRMS using in-line pyrolytic methylation greatly improvedthe resolution in discriminating and tracing the differential Cpathways at the base of the pelagic food web.232 Naturalphototrophic populations could be probed individually fortheir in-situ d13C signatures. Concentrated cell suspensions(4 ml) were deposited on to the tips of ferromagnetic wires(Curie-point temperature, 480 uC), together with a methanolicsolution of trimethylphenylammonium hydroxide (0.25 M,1 ml). A release of the cellular fatty acid fraction from thesample by in-situ methylation was achieved with a 3 s pyrolysistime. The monitoring of phytoplankton for their in-situ d13Csignatures showed that algal and cyanobacterial clustersdiffered significantly in isotopic composition.

Although CSIA has become a powerful tool for inferring theVOC in various systems, a major drawback is the relatively poorsensitivity. Enrichment prior to analysis is necessary as 0.8 nmolC in each individual compound is required. The use of on-column or splt/splitless injection combined with solid-phasemicroextraction (SPME) or P&T preconcentration has beeninvestigated in the development of a CF-SIRMS procedure forthe determination of d13C values of commonly reportedgroundwater contaminants at mg l21 concentrations.233 Thebest LOD (0.25–5.0 mg l2l) were obtained with P&T, which alsohad a higher reproducibility and smaller isotopic fractionationthan SPME. As P&T and SPME are solventless methods, theiruse resulted in better chromatographic separation and LODthan those obtained with the liquid injection techniques.Although small but highly reproducible isotopic fractionationwas observed for all the techniques, the magnitude was muchsmaller than that of natural fractionation resulting frommicrobial or chemical transformation reactions. The methodcould also be used for the determination of 2H:1H ratios,although the LOD were 10–20 times poorer than those for13C:12C ratios.

The first successful use of a total organic carbon (TOC)analyser in a CF-SIRMS system has been reported for the 13Canalysis of DIC and dissolved organic carbon.234 For the TOCanalyser to be coupled to the SIRMS instrument in CF mode,He replaced N2 as carrier gas as it could be used at a flow ratetolerated by the mass spectrometer. The SIRMS software wasused to control the sample trigger. Sampling was performedusing either a loop (low concentration, automatic mode) or asyringe (high concentration, single injection mode). Tests werecarried out on various compounds ranging from those oxidizedeasily (sugars) to those oxidized with difficulty (humic acids).Typical analytical precision (2s) was better than 0.20% andsample reproducibility 0.10–0.35% depending on the reactivityof the sample.

Methods for the determination of isoflavones in foodmatrices are generally hampered by the lack of suitable analy-tical standards. The first demonstration of quantitative analysis

by GC-combustion-SIRMS, using intrinsically labelled isofla-vone standards, indicated that the method is as sensitive aspublished GC-EIMS methods for the determination ofdaidzein.235 Standards containing phytoestrogens with 13Cabundances of up to 20% were produced by intrinsicallylabelling soya plants grown in an atmosphere enriched in13CO2. Although incorporation of 13C into the plant moleculescould not be controlled, combustion to CO2 removed thecomplexity of the isotopomeric distribution within the mole-cules. Isotope ratio measurements were made for the sample,the standard and an accurately prepared mixture of the two. Amajor advantage of the new technique was that the analyticalstandard matched the sample closely and could be expected togive realistic estimates of losses from all stages of the analyticalprocedure. The method was not, however, universally applic-able. The instability of the silyl ether of genistein prevented itsdetermination.

Little has been reported previously on the use of CSIA forcompounds highly enriched in 13C. Mottram and Evershed236

investigated the potential of GC-combustion-SIRMS as a verysensitive detector for enriched compounds, in particular byexploiting the amplifier gain applied to the m/z 45 cup inSIRMS instruments, which is higher than that applied to theother cups. Analysis of two 13C-enriched fatty acid methylesters with d13C values of y500% demonstrated that responsesin the m/z 45:44 intensity ratio could be detected even thoughthe compound was at such a low concentration that no peakwas observed in the m/z 44 ion chromatogram. The LODwere 30 and 20 pg injected for 13C-hexadecanoate and13C-octadecanoate, respectively, using the m/z 45:44 chroma-togram, whereas the equivalent LOD using the m/z 44 ionchromatogram alone were 180 and 200 pg, respectively. Resultswere, however, both inaccurate and imprecise at amountsbelow 5 ng injected component. It was also found, perhapsunsurprisingly, that a significant within-run carryover effectexisted due to tailing of chromatographic peaks. The isotopevalues recorded for compounds eluted immediately after anenriched component were changed significantly. Although theprocedure could be used for qualitative analysis, considerablelimitations remain for the quantitative analysis of real samples.

An extensive new set of high precision measurements of d13Cand d2H of stratospheric and upper tropospheric CH4 was madepossible by the recent development of an on-line CH4 pre-concentration system coupled to a CF-SIRMS instrument.237

The procedure required about three orders of magnitude lessvolume of whole air at ambient CH4 concentration andsignificantly less preparation time than analysis by conven-tional dual-inlet SIRMS with off-line preparation. Ambient air(60 ml) was introduced into a system consisting of three traps topreconcentrate the CH4 and to remove interfering gases. TheCH4 was further purified on a capillary separation column andthe eluate was either quantitatively oxidized to CO2 and H2Ofor carbon analysis or quantitatively pyrolysed to H2 and C forhydrogen analysis. The average precisions (1s) were ¡0.04 and¡1.25% for d13C and d15N, respectively.

The increasing burden of atmospheric CH4 has a significantimpact on both global warming and atmospheric chemistry, yetrelatively little is known about the provenance of atmosphericCH4. Methodology for the measurement of d2H in atmosphericCH4 is less well developed than that for measurement of d13Cbecause of the large sample size requirement (100 l air) andtime-consuming sample preparation. An on-line extractionsystem coupled with GC-pyrolysis-SIRMS was faster (1 h persample) and less laborious than previous methods, requiredonly 120–360 ml of air containing 10–30 nmol CH4, and gave aprecision of ¡3.1%.238 The same system could be used for thedetermination of d13C when the pyrolysis furnace was replacedwith a combustion reactor. By using a combination of d13C andd2H values, it was possible to show that 70% of CH4 in anurban atmosphere came from a landfill site.


Determination of d2H in keratin provides a novel means oftracking geographical movements of birds and other species. Akey requirement of the analytical procedure is that it shouldprovide a high sample throughput at low cost and at the sametime take into account the fast hydrogen exchange betweenlaboratory air moisture and exchangeable hydrogen inkeratins. The method of Wassenaar and Hobson239 met theserequirements by using an equilibration procedure in whichkeratin standards and samples of bird feathers were deliber-ately exposed in the laboratory for w96 h to equilibrate withambient air moisture prior to analysis by EA-SIRMS with aflash pyrolysis furnace. The sample throughput of 120–160samples per day was limited by the need to clean the pyrolysiscolumn after about 180 samples due to accumulation of Agfrom the sample capsules. The method was based on the not yetverified assumption that the powdered keratin standard and thesolid feather would exchange hydrogen to the same degree.

Headspace samples are usually transferred directly toseptum-capped vials using a gas-tight syringe and storedprior to analysis. In a test of the reliability of screw-cap glasscontainers (12 ml) fitted with bromobutyl rubber septa forstorage of samples containing 15N-labelled N2 and N2O, theconcentration and isotopic composition were determined byautomated CF-SIRMS at monthly intervals for 1 year.240 Aftera year the N2O concentration had decreased by 35 and 34% fortest and standard mixtures, respectively, but there was nodetectable change in 15N enrichment. The loss process thereforehad no effect on enrichment. Losses of N2 were less than thosefor N2O because of the smaller pressure gradient between theinside and outside of the vials. Accurate determination ofconcentration was possible if a calibration gas was stored andanalysed together with the sample.

An elemental sulfur RM (IAEA-S-4) has been shown to beisotopically homogeneous in amounts as small as 41 mg andtherefore suitable to be used as an isotopic sulfur RM.241 Thed34S vlaue was measured as 116.90¡0.12% (n ~ 66) on theVCDT (Vienna Canon Diablo troilite) scale for both the coarse(w0.25 mm) and fine (v0.25 mm) fractions using 56 ¡ 3 mgsample.

8.4. Sample preparation

Although the effects of derivatization of amino acids for GC-combustion-SIRMS analysis have been studied extensively, fewif any studies have investigated the possibility of isotopicfractionation during the hydrolytic cleavage of amino acids fromtheir parent proteins or peptides. Jim et al.242 demonstrated thatheating (100 uC, 24 h) in 6 M HCl to cleave amino acids frompeptide standards did not result in significant isotopic fractiona-tion. In addition, d13C values calculated for bulk collagensamples from the values measured for the constituent aminoacids were consistent with measured values for bulk collagen.

The use of trimethylsilylation to derivatize sugars, alcoholsand acids for the measurement of 13C has the fundamentaladvantage of not involving carbon bonds and therefore doesnot cause isotopic fractionation. The main disadvantage,however, is that three C atoms are added per derivatizedhydroxyl group and must be accounted for in the measured 13C: 12C ratios. Evaluation of the method for sugar analysisrevealed that the number of added C atoms was actually lessthan expected from stoichiometry.243 This was attributed toincomplete derivatization or incomplete oxidation of methyl-silyl C in the analysis. The isotope excess of enriched samplescould be determined with a relative error of 5% using acalibration of the number of analysed added C atoms. Naturalabundance d13C in sugars could be measured with an accuracyof ¡1.5%. The sensitivity of the method was limited by thedilution of the sample signal by the added carbon.

For many ecological applications of stable isotope tech-niques it is necessary to separate lipid and lipid-free fractions

yet there has been no reported testing of the effect of lipidextraction on measured d13C values. A comparison of a Soxhletextraction method using petroleum spirit with two liquid–liquid extraction methods (methanol with chloroform andisopropanol with cyclohexane) for the extraction of lipids fromfish feeds concluded that all three methods were suitable andgave values within 1% of each other.244 However, the amountof lipids extracted varied depending on the method used anddata could only be compared if the same method were used forall samples. The extraction of lipids from diatoms requiredfurther investigation as marked differences in measured d13Cvalues were found dependent on the extraction method used.

An improved procedure for determining 13C and 2H ratios inapple juices and related products involved chemical trans-formation of fructose into hexamethylenetetramine, whichretained position-specific 13C and 2H isotopic information fromthe C and non-exchangeable H atoms in the sample.245 The 13C12C and 2H:1H ratios were measured by GC-combustion-SIRMS and GC-pyrolysis-SIRMS, respectively. In comparisonwith the use of nitro-sugar derivatives, the new method reducedanalysis time and improved sensitivity. The addition of low costcommercial sugar syrups to apple juices could be detected.

Chloroacetic acids are a major class of environmentalpollutants widely distributed throughout the world and areof concern due to their phytotoxicity. In a new approach ofusing stable isotopic signatures to determine the sources of andprocesses affecting chloroacetic acids, the acids were reactedwith acidic methanol to form the corresponding methylchloroacetates for GC-combustion-SIRMS analysis.246 Thederivatization reaction and other steps in the analyticalprocedure did not result in isotopic fractionation so the d13Cvalues in the chloroacetic acids were easily calculated from themeasured values in the methyl esters. A major limitation of thecurrent procedure was that chloroacetic acids are often presentin the environment at concentrations (mg l2l to ng l21) wellbelow those detectable so samples needed to be concentratedprior to analysis. It was demonstrated that the isotopicsignatures of chloroacetic acids differed depending on theirorigin, so the method could be used to trace chloroacetic acidsin the environment.

Spotl and Vennemann247 improved the preparation ofcarbonate minerals to provide a high sample throughput withhigh precision even for small sample sizes. Provided that thereaction time between acid and carbonate and the amount ofacid were kept constant, temperatures (e.g., 72 uC) well aboveambient could be used for the reaction. This, together with useof a heated GC column, resulted in fast chromatographicseparation and high sample throughput (72 samples in 13 h).The entire calibration was based directly on standards run withthe samples rather than solving fractionation equations for theacid–base reaction. Precisions of 0.12 and 0.16% were obtainedfor d13C and d15N, respectively, in 50 mg samples.

Urea tracer experiments can help to assess metabolic res-ponses to physiological stress, nutritional status, diet andmedication. Whereas the determination of 13C-urea enrichmentby GC-combustion-SIRMS requires a minimum of C atoms tobe added during derivatization, analysis by GC-EIMS requiresa stable molecule giving little fragmentation. Both theseconditions were met by converting urea into 2-methoxypyri-midine.248 In a combined 13C- and 15N2-urea experiment, lowresolution GC-EIMS did not discriminate between 13C-ureaand 15N1-urea, whereas SIRMS analysis excluded detectionof the 15N analogue. Combination of results from the twomethods gave the abundances of 13C- and 15N1-urea. Overall,13C-urea enrichments based on single point measurements byGC-combustion-SIRMS could be obtained as values ¡0.02%atom percentage excess (range 0–1%) and 15N2-urea enrich-ments by GC-EIMS were obtained as values ¡0.15 molepercentage excess (range 0–7%).

For the first time, H2/H2O equilibration using a platinum


catalyst has been reported for the determination of hydrogenisotope ratios in small volumes of water by CF-SIRMS.249 Anequilibration of a few seconds could be achieved by vaporizingthe water completely and by increasing the exchange tempera-ture to 100 uC. Measurement precision and accuracy of v1%were achieved for 5 ml samples with an analysis time (includingequilibration and processing) of 5 min. Ice samples could beanalysed continuously by coupling a melting device to theinstrument.

Two different approaches have been proposed for thenitrogen isotope analysis of ammonium in KCl soil extracts forwhich current methods require relatively large sample size(100 ml), removal of the matrix and preconcentration. Norlinet al.250 demonstrated the potential of combining headspaceGC-combustion-SIRMS with SPME extraction using fibrescoated with Nafion1 (a perfluorosulfonated ionomer). Theammonium was converted to ammonia, absorbed from theheadspace and injected into the GC. Isotopic fractionation of1.0117 ¡ 0.0009 occurred between the fibre and the gas. Anammonium concentration of 10 mM was needed to obtainsufficient absorbed ammonia for analysis. Preliminary resultsindicated that d15N analysis was possible for only 0.4–0.5 mmolammonium with an SD of 0.8% (n ~ 5). Johnston et al.251

derivatized ammonium with o-phthaldialdehyde and sodiumsulfite at pH 11.2 to produce 1-sulfonato-iso-indole, which wasreadily concentrated from dilute solutions by reversed-phaseSPME. This derivatization procedure, complete within 2 h,avoided the use of N-containing reagents and had severaladvantages (simpler chemistry, more rapid, insensitive tosalinity) over the alternative (synthesis of indophenol).Although partial derivatization of potentially interferingamino acids occurred, these derivatives were not extracted bySPME. The d15N values of ammonium in KCl soil extractscould be measured within 48 h by automated CF-SIRMS with aprecision of 0.23% (1 SD). Parallel measurements of ammo-nium standards with known d15N were made to correct for low-level contamination of the reagents by the absorption ofambient ammonium.

The addition of water to fruit juices is better detected by useof 18O in both water and organic compounds than in wateralone. A procedure for the measurement of 18O:16O ratios in thesugars of orange juice involved fermentation and distillation toproduce alcohol, which was placed in silver containers for EA-SIRMS analysis.252 Isotopic fractionation occurred during thefermentation and distillation but was reproducible if analyticalconditions were well controlled. In addition, particular carewas needed to ensure that the ethanol samples did not becomecontaminated with water or organic matter. Due to the robust-ness of the method and the fact that most 18O-enrichment in thesugars remained in the end-product, ethanol was considered tobe a convenient internal reference to circumvent the spatial andtemporal variability observed for 18O:16O in water. Adultera-tion of juices with water was identified by a deviation from thestrong correlation between the d18O values of the juice waterand ethanol.

The conventional procedure for the determination of d37Clinvolves the separation of CH3Cl from CH3I by GC. Wu andSatake253 based their separation procedure on the difference invapour pressures at the melting point of 2,2,4-trimethylpentane(2107 uC), which was contained within a sealed cold trap toprevent contact with the atmosphere. The recovery rate forCH3Cl was 100% when the operation was carried out twice.The isotopic fractionation in d37Cl was v0.04%.

9. Thermal ionization mass spectrometry (TIMS)


The noise level of Faraday detectors has become a limitingfactor in high precision analysis of small samples. Multicollector

instruments are generally fitted with a single ion countingchannel so peak jumping procedures are required for very smallsamples. Wieser et al.254 have developed a new multi ion-counting system consisting of up to eight specially miniaturizedsecondary electron multipliers identical in size with andinterchangeable with Faraday cups. The detector system hasbeen used for the measurement of Nd and U isotopes butinformation on the performance characteristics is awaited. Inparticular, the linearity and stability of the detectors need to beexcellent and attention to proper cross calibration is important.

9.2. Calibration

Two different zircon samples have been proposed as RM forU–Pb geochronology suitable for use not only for TIMSbut also LA-ICP-MS and SIMS analyses. The TEMORA 1zircon from eastern Australia was homogeneous from sub-mmto intergranular scales and had an ID-TIMS age of(416.75¡0.24)6106 years.255 A total of 21 zircon fractionswere analysed over a period of two years. The analysis of alarge set of single zircons (n ~ 27) from the Duluth Complex(North America) gave an age of (1099.1¡0.2)6106 yearswhich was indistinguishable from previously publishedresults.256 Both materials were considered to be suitable foruse as benchmarks, in particular for establishing systematicerrors such as instrumental fractionation, laboratory blanks,sample–tracer equilibration and the calibration of ID tracersolutions.

The 143Nd:144Nd and 87Sr:86Sr ratios in the NIST replace-ment geological RM BCR-2, BHVO-2 and AGVO-2 werefound to be indistinguishable from the corresponding ratios inthe original RM (BCR-1, BHVO-1 and AGVO-1).257 In addi-tion, the same ratios were measured for eight Max PlanckInstitut–Dingwell glass RM. There was excellent isotopichomogeneity for all the samples analysed, so that they could beused as isotope RM. In the case of the NIST RM GSP-2, theratios were not the same as those measured in GSP-1.


No detailed investigation of the physicochemical conditions ofsilica gel has been made even though particle size, size dis-tribution and specific surface area can have large effects onionization efficiency, ion beam stability and mass fractionation.A new silica-gel activator, prepared by the hydrolysis oftetraethoxysilane, gave a stable 208Pb1 beam current of4610211 A for 200 ng Pb.258 The ion beam current decayedless than 40% over a period of 30 min. The physicochemicalconditions of the synthesized silica particles could be controlledthrough selection of hydrolysis conditions (temperature, pH,ratio of reagents), making the material ideal for a systematicstudy of ionization.

In an iterative exact-matching technique for ID analyses, thesame amount of isotopically labelled spike solution was addedto both the sample and a calibration standard and, in addition,the amount of determinand in the calibration standard wasmatched to that in the sample.259 Each successive iteration ofthe procedure involved adjustment of the amount of calibra-tion standard taken, to match the newly calculated deter-minand concentration, and adjustment of the amount of spikesolution added to both sample and calibration blends. Typi-cally, the determinand concentrations converged on the finalvalue within three iterations. This procedure was applicable toID analyses of both organic and inorganic samples.

Isotope analyses of B can be performed using either positiveor negative ionization TIMS procedures. Whereas the formermethod gives good precision (¡0.01%) yet requires largesample size (several mg), the latter is much more sensitive yetsuffers from poorer reproducibility because of problems incontrolling fractionation. Furthermore, the negative ionization


method gives a consistent but unexplained offset of 10% inthe 11B:10B ratio measured in the NIST SRM951 (boric acid).Shen and You260 made precise measurements using negativeionization by determining the BO2

2 ion abundance at m/z 42,43 and 45 simultaneously and correcting for instrumental massfractionation on the basis of empirically derived 18O:16O ratios.The mean value for the 11B:10B ratio in NIST SRM951 (boricacid) was measured as 4.0430¡0.0015 (2s ~ 0.36%, n ~ 11),which matched the certified value of 4.043¡0.003. This level ofprecision was of the same order as that obtainable using thepositive ionization procedure measuring Cs2BO2

1 but thesample requirement was three orders of magnitude smaller.Mass fractionation was dependent on the chemical form of Banalysed so it was essential to ensure that all samples andstandards had the same chemical form. In addition, theactivator also affected fractionation. Best precision wasobtained with Ba(OH)2 or B-free sea-water as activator.

The use of MC ion detection was essential to obtain thedegree of precision required to measure the naturally-occurringmass-dependent isotope fractionation of Mo in molybdenites atthe 0.1% per u level.261 The fractionated molybdenites wereisotopically lighter than the reference samples. A double94Mo–100Mo spike was used for the analysis. Solutions of Mowere mixed with an ascorbic acid solution (4 ml), loaded onto single rhenium filaments and covered with a silica gelsuspension (3 ml). A strict heating and analytical procedure wasfollowed to ensure identical conditions for all samples. Furtherstudy was required to eliminate the additional and variableisotopic fractionation introduced by isolating Mo from moly-bdenite solution using ion exchange. The authors identified theneed for a Mo isotopic RM that could serve as the standard forall Mo isotope abundance measurements.

A nine-collector dynamic procedure was an improvementover single collector or static MC measurement of Ce isotopiccomposition and provided the precision required for the La–Cedating of geological materials.262 Oxygen gas was leaked intothe ion source chamber to maintain a stable beam (1610211 A)of 142CeO1 ions. Data were normalized to 136Ce:142Ce ~0.016 88 using the exponential law and corrections were madefor isobaric interferences from CeO, 138LaO, 142NdO and141PrO. A 138Ce:142Ce ratio of 0.022 580 0¡0.000 001 9 (2s)was measured for Johnson-Matthey JMC304 (high purity CeOreagent).

An ID-TIMS procedure based on stepwise dissolution gaveprecise and concordant U–Pb age data for metamict and highlydiscordant zircon crystals in agreement with SIMS data.263 Itwas possible to date three out of four zircons studied. A Pb–Pbevaporation procedure systematically gave dates slightlyyounger than those obtained by the other methods whichwere considered to be minimum 207Pb : 206Pb apparent ages.The discrepancy was attributed to the indiscriminate nature ofstep heating which released Pb from a reaction front.

Methods for the determination of ultratrace amounts of 231Pain sea-water should have a low sample requirement and shortmeasurement times, together with a low procedural blank,high S/N and high instrumental sensitivity. Traditional a- andb-counting techniques typically require several thousand litresof sea-water and counting times of up to months. An ID-TIMSprocedure has been modified264 to extend the technique to verylow concentrations of 50–2000 ag (1 ag ~ 1610218 g). Atten-tion was paid to reducing procedural blanks and improvingionization efficiency. An ionization efficiency of 0.3–0.5% wasachieved by loading Pa in 1 M HNO3–0.4 M HF followedby one drop of cleaned colloidal graphite on a single spot and5–7 layers of graphite (y0.2 mg). In comparison with previousstudies, the increased ion beam current improved the S/N by afactor of 2–3. The procedural blank was 16¡15 ag (2s) and themeasurement time v1 h. The amount of 231Pa needed toproduce data with an uncertainty of ¡4–12% was 100–1000 ag(36105–36106 atoms). The minimum sample size was 2 l of

surface sea-water for analysis of suspended particulate matteror v1 l for filtered (v0.4 mm) sea-water.

The paper of Richter and Goldberg265 was notable for thedepth and clarity of detail given for their improved procedurefor the determination of U isotopes in nuclear materials with highaccuracy. In a modification of the conventional total evapora-tion technique, not only were data acquired until the wholesample had been consumed, but corrections were also made forthe peak tails from 235U and 238U contributing to the measured234U and 236U ion beams, respectively. This background wasshown to be sample specific and so had to be measured andcorrected on-line regularly during an analysis. Intensities weremeasured at m/z 233.5, 234.5, 235.5 and 236.5 and thebackground at m/z 234 and 236 calculated. In addition, theion beam was refocused and recentred, thereby transformingthe procedure closer to that in which measurement of blocks ofratios is made. The correction for the background introducedan interruption to the total evaporation procedure so it wasnecessary to introduce an external calibration using a RManalysed in a manner analogous to the samples. The newevaporation procedure was used with a dynamic MC measure-ment routine that further improved accuracy. During theswitching from one set of isotopes to another, a slight change ofion beam dispersion occurred so it was necessary to change thedispersion within a fraction of a second. A new instrumentequipped with a dispersion quadrupole was used to achieve thisfast switching of the ion beam dispersion. Each mass cycle ofthe procedure required a unique setting of the dispersionquadrupole voltage. The 234U:235U and 236U:235U ratios in RMU500 were measured over a three year period with repeat-abilities of 0.002 and 0.015% (1s), respectively.

A procedure for the determination of DU was used todemonstrate the significant presence of DU isotopes in bodyfluids nine years after exposure by inhalation.266 The DU wasrecovered from urine samples, using ion exchange resins, forboth ID and isotopic composition measurements. The Ufractions were loaded on to single rhenium filaments using asilica gel–H3PO4 procedure and the UO2

1 ion beam measuredusing a secondary electron multiplier in peak jumping mode.The mean uncertainty was 0.74%, making it possible to detectdeviations of w2% from the natural U isotopic composition.

9.4 Sample preparation

A procedure for the determination of Li isotopic composition insea-water involved reduction of the Na content by passing thesample through a column of hydrated Sb2O5 which is an ionexchanger specific for removal of Na.267 No further separationwas carried out as any ion exchange procedure would induceisotopic fractionation. The samples (40 ng Li as LiOH) wereloaded directly onto single rhenium filaments using a silica gelprocedure. A stable 7Li ion intensity of 1610210 A could bemaintained for 4 h. A 6Li:7Li ratio of 0.082 60¡0.000 03 (n ~5) was measured for IAEA RM L-SVEC (Li2CO3).

The use of mixed cation exchange (H1 form) and anionexchange (OH2 form) resins in addition to a boron-specificresin (Amberlite IRA 743) improved separation for therecovery of B from samples with low concentrations.268 Noisotopic fractionation was introduced by the procedure. Theisotopic composition of B in foraminifera was determined usinga positive ionization procedure with measurement of theCs2BO2

1 ions.A modified digestion procedure for preparation of relatively

large (w0.2 g) oil samples for the ID-TIMS determination of Scontent was based on the use of semi-open vessels inside apressurized autoclave.269 The method was intended to combinethe advantages of both open and closed vessel digestion, suchas low acid consumption, the use of HNO3 alone rather thanmixed with H2SO4, comparatively large sample amounts andsimple vessel design. Although reaction gases could escape


from the reaction vessels into the autoclave, no losses ofelements by volatilization were observed because of the highpressure of N2. After digestion, sulfate was reduced to H2S andS precipitated as As2S3 for TIMS analysis. The authorsadmitted that the new procedure held no advantage overconventional closed vessel digestion other than that of moreconvenient handling. Improvements to the MS procedurewere, however, still required to enable low concentrations of S(v10 mg g21) to be measured.

The measurement of 226Ra by TIMS has the advantages ofgreater precision, smaller sample requirement and fasterdetermination when compared with conventional radioactivecounting methods. Isolation of Ra from samples is, however,problematical because of the high Ca : Ra and Sr : Ra ratios inmany environmental samples, and consequently few datahave been reported. A chemical separation procedure for theefficient isolation of Ba and Ra from carbonate samples wasbased on co-precipitation with MnO2, followed by separationof Ba and Ra on Sr-spec resin.270 The ability to extract Ra fromcarbonate samples or sea-water will allow it to be used as aHolocene chronometer or as a tracer of sea-water mixing.

Good isolation of Ra, Th and U, essential for use of U-seriesdisequilibria in studies of geological processes, has beenachieved through use of a series of steps using element-specificand other resins.271 Recovery yields of y90% were obtainedfor all three elements. Analytical reproducibilities were 0.7, 0.2,0.4, 0.1 and 0.6% for 230Th:232Th, 234U:238U, Th content, Ucontent and 226Ra content, respectively, for the GeologicalSociety of Japan RM JB-3.

10. Other methods

10.1. Electrospray mass spectrometry (ESMS)

Three excellent reviews highlighted the role of ESMS in specia-tion analysis, in particular for providing molecule-specificdetection to confirm the identity of species eluted fromchromatographic systems. Rosenberg272 considered in detailthe use of MS with atmospheric pressure ionization techniques(ES and chemical ionization), with emphasis on the literaturepublished since 1998. The principles of the ionizationtechniques were covered in some depth but the major part ofthe 270-reference review discussed recent applications includ-ing the elucidation of metal–complex structures and thecharacterization and quantification of small organometallicspecies in environmental, health and food studies. Particularattention was given to the characterization of biomolecules andmetalloproteins. McSheehy and Mester outlined the develop-ments of ESMS for and applications to the analysis of bio-logical tissues in a two-part review. The first part273 coveredbiosynthesised species of As and Se, whereas the second274

dealt with bio-induced ligands (metallothioneins and phyto-chelatins) and environmental contaminants (Hg, Pb, Sn). Afeature of all the reviews was the substantial tables, whichsummarized the analytical procedures (extraction, separationand detection) and the metal species detected in the applica-tions considered.

Analytical methods for determining As species are becomingincreasingly important. The ability of ESMS to provide bothelemental and molecular information is useful for the identi-fication of As compounds because As does not have a uniqueisotopic signature. Kuehnelt et al.275 observed, however, thatthe generation of As1 ions was critically dependent on theamount of O2 present in the N2 drying gas. At concentrationsw0.1% O2, no As1 signal was observed. As the O2 concen-tration was decreased, the ion intensity at m/z 75 increased toabout 3% of the total ion current.

Measurement of urinary I is an efficient way to revealdeficiencies in the nutritional supply of this essential traceelement. In an investigation of ion-exchange chromatography

hyphenated with MS for the determination of I2 in urine,satisfactory chromatographic isolation of I2 could be achievedon a hydrophilic polymer-based anion-exchange column with35 mM NaOH (0.45 ml min21) as the mobile phase.276 Asuppressor and flow splitter (split ratio 1 : 10) was requiredbetween column and ES interface in order to convert themobile phase into water and make the flow rate compatiblewith the interface. The ESMS LOD of 15 mg l21, obtained witha flow rate into the interface of 0.03 ml min21, was two or threeorders of magnitude poorer than that achievable using ICP-MS. In addition, the reproducibility (7%) was poorer than that(0.55%) of magnetic sector ICP-MS. Although ICP-MS wasseen as the preferred method for the determination of I2, themolecular detection capability of ESMS was considered anadvantage when differentiation of I2 from co-eluting specieswas required.

Triiodobenzoic acid derivatives used in X-ray contrast mediaare detectable in the aquatic environment. Negative ionizationESMS, which would allow I2 to be detected, has not previouslybeen used for the analysis of these compounds because of thevery low ion yield. Deliberate in-source fragmentation throughapplication of a high cone voltage (w70 V) was useful for theESMS analysis of organic iodine compounds with detection ofI2 at m/z 127.277 A series of iodobenzene derivatives could bedetected even in complex matrices but the MS signal wasdependent on the compound structure and on the compositionof the LC eluent. This analysis could routinely be undertakenusing ICP-MS but the ESMS procedure was considered analternative tool for those without access to ICP-MS. Anadvantage of the new procedure was that reduction of the conevoltage provided both elemental detection and speciationinformation.

Although the specificity of ESMS is a great advantage, thepoor sensitivity limits more widespread application. The use ofHPLC for the separation of methylmercury species rather thanthe more commonly used GC methods was seen to hold theprincipal advantage of not requiring the use of derivatizationprocedures.278 Although the LOD (39 mg l21) of the HPLC-ESMS procedure was sufficient for investigating the equili-brium distribution of methylmercury in aqueous solution, itstill exceeded that of other detectors. Consequently, ESMS waslimited to the analysis of samples with high methylmercuryconcentrations such as fish samples. In addition, the HPLCseparation required the use of an ion-pairing reagent whichconverted the majority of the methylmercury cations touncharged complexes undetectable by MS. Only a minorfraction of the ionic methylmercury species was actuallydetectable by ESMS. The specificity of ESMS allowed theexistence of several polynuclear mercury species in aqueoussolutions to be established for the first time.

The applications of ESMS are numerous and continue toexpand into new fields of speciation analysis. The role of ESMSin metal speciation analysis is predominantly that of confirmingthe identity of species detected by ICP-MS in hyphenatedsystems. The Ni-containing peak in the ESMS spectrum of a Nicomplex from a hyperaccumulating plant was identified as achelate with a tricarboxylic amino acid ligand.279 The secondmost abundant As metabolite in sheeps’ urine after adminis-tration of arsenosugars was identified as dimethylarsinoyl-acetate.280 Arsenosugars in seaweed were metabolized byhumans to dimethylarsinic acid, which was more toxic thanthe parent compound.281 Although ESMS could be used toidentify water-soluble Se-containing proteins in yeast, thedrawbacks of poor sensitivity and the formation of doubly-charged ions of selenopeptides led to consideration ofMALDI-TOFMS as an alternative detection technique.282,283

In the procedure subsequently developed, chromatographicpeaks identified by ICP-MS as containing Se were analysedby MALDI-TOFMS to select target ions for CID-ESMSsequencing of the detected peptides. The characterization of


metallothionein isoforms in liver284 and brain285 was madeusing ESMS, which was also used to study the interaction of Ascompounds with metallothionein.286

10.2. Gas chromatography–electron ionization massspectrometry (GC–EIMS)

In this section, GC-EIMS is defined as the classical coupling ofGC with EIMS, associated primarily the with structure deter-mination of organic analytes.

The majority of GC-EIMS reports concern the samplepreparation and derivatization procedures required for thedetermination of organometallic compounds. These proce-dures can be tedious and time consuming. A dual-arm roboticsample preparation system was used to automate fully the in-vial derivatization and headspace SPME of aqueous samplesfor EI-GCMS.287 A key feature of the system was that therobotic arms were reprogrammed with a new software packagewhich prevented collisions between the arms and allowedsimultaneous rather than sequential operations to be carriedout. The use of tetrahydrofuran (THF) as solvent kept thederivatizing agents (alkylborates) stable for 2 or 4 weeks whenstored at 27 or 5 uC, respectively. The automated procedureoffered improved precision (1.4, 3.2 and 1.7% RSD for Hg, Pband Sn, respectively) in comparison with manual procedures(5.3–8.2% RSD).

The potential of GC-EIMS for the structural analysis oforganometallic species of As, Sb, Sn and Te in environmentalsamples has been investigated.288 Ionic species from compostsamples were derivatized with NaBH4 and introduced into themass spectrometer via a P&T system. Of seven As speciesidentified by typical fragmentation patterns, three (diarsine,dimethylarsenomercaptane and monomethyl diarsine) werereported for the first time in environmental samples. Asdimethylarsenomercaptane was also identified in the volatilefraction of hydrothermal gases, sampled directly on SPMEfibres without derivatization, the presence of this species wasnot an artefact of the derivatization procedure. AlthoughEIMS was less sensitive than ICP-MS, it was considered tohave the advantage of providing essential molecule-specificinformation and should therefore be used as a techniquecomplementary to ICP-MS.

The determination of organotin compounds has continuedto dominate the development of GC-EIMS procedures. Afeature of a fast method for the determination of eightcompounds in water, sediments and mussels was the use of low-pressure GC coupled to tandem MS.289 A vacuum was appliedto the GC column (0.53 mm diameter) to increase the optimumlinear velocity of the carrier gas from 15 to 80–100 cm s21,thereby permitting low volume injection to be used. In com-parison with conventional capillary GC, the overall analysistime was halved. The ionic Sn compounds were complexed withsodium diethyldithiocarbamate (NaDDC), extracted andderivatized by a Grignard reagent (n-pentylmagnesium bro-mide). Whereas SPME was used for water samples, extractioninto toluene was preferred for sediment and mussel samples.Improvements to previously used procedures included theapplication of a clean-up step before injection of the extractsand the use of matrix-matched calibrants to avoid matrixeffects. The LOD were in the ranges 1.1–9.6 ng l21 and 0.03–6.1 mg kg21 in water samples and other matrices, respectively.

A new method for the speciation of butyltin compounds inmarine sediments was based on the use of a continuous FIsystem for preconcentration and elution.290 Extracts (10 ml,pH 5.3) were introduced into the continuous flow system andthe butyltin compounds were complexed with NaDDC andadsorbed on C60 fullerene. The high adsorption capacity of thefullerene allowed it to be used even in the presence of metalions. The compounds were retained as neutral chelates, whichwere subsequently eluted and simultaneously derivatized using

sodium tetra-n-propylborate in ethyl acetate. The procedurewas linear over the range 0.2–35 ng g21 and the LOD were 0.07,0.09 and 0.10 ng g21 (as Sn) for mono-, di-, and tribuytltin,respectively.

The use of hydride generation combined with headspaceSPME has been proposed for the determination of butyltincompounds in sediments and sea-water291 and of arsenicmetabolites in human urine.292 Metals were converted to theirhydride forms using NaBH4 and the hydrides adsorbed on toa polydimethylsiloxane (PDMS)-coated fibre (100 mm filmthickness). The optimum extraction time in both studies was20 min. After extraction, the fibre was inserted immediatelyinto the GC injection port for thermal desorption. The methodwas solvent free, simple and fast in comparison with alternativeprocedures. The LOD for the butyltin compounds were in therange 0.1–0.5 pg. The LOD and precision for the As com-pounds were in the ranges 0.1–0.8 mg l21 and 7–11%,respectively, except for AsV, for which they were 6.8 mg l21

and 52%, respectively.Some used transformer oils containing about 60% poly-

chlorinated biphenyls (PCB) also contain tetraphenyltin as astabilizing agent. As the PCB-containing waste oils must bedestroyed without creating a new pollution of Sn compounds,methods are required to identify those wastes which contain Sn.A GC-EIMS method was the most sensitive and reproducibleof several methods considered for the determination ofphenyltin compounds in PCB-based transformer oil samples.293

Whereas tetraphenyltin could be determined following injec-tion of a hexane extract of the oil directly on to the GC column,the mono-, di-, and triphenyltins needed to be derivatized withGrignard reagent (n-propylmagnesium bromide in THF). TheLOD were 30, 9.8, 5.5 and 0.60 ng ml21 for mono-, di-, tri- andtetraphenyltin, respectively. Although the selectivity of MS wasslightly poorer than for the other detectors, interference fromthe PCB matrix was insignificant.

Conflicting conclusions in the literature about the suitabilityof alkyl chloroformates as derivatization reagents for selenoa-mino acids prompted Haberhauer-Troyer et al.294 to carry out adetailed study. All selenoamino acids except selenocystinecould be identified by detection of their molecular ion andtypical fragment ions. Selenocystine could not be identifiedeven at high concentrations. Although the reaction had a high(w90%) extraction and derivatization efficiency, the massspectra were highly irreproducible due to decompositionreactions. These reactions were dependent not only on tem-perature but also the state of the mass spectrometer. The use ofthe most inert materials was therefore recommended. Use of asuitable internal standard (for example, selenoethionine forselenomethionine) compensated for the variations in condi-tions and the overall method repeatability was good at 1–2%RSD. Of three reagents tested, methyl chloroformate was thepreferred derivatization reagent.

A proposed scheme for the conversion of iodide into 4-iodo-N,N-dimethylaniline by reaction with 4-N,N-dimethylanilinewas rapid (v1 min) and insensitive to the many ionic com-pounds present in salts, pharmaceuticals or foodstuffs.295 Thederivative was suitable for extraction by SPME using PDMS-or polyacrylate-coated fibres or by single drop microextraction(SDME). In the latter technique, a drop (1 ml) of isooctane atthe end of a 10 ml syringe needle was immersed for 15 min in thesolution of derivatized sample, drawn back into the syringe andimmediately injected into the GC-EIMS system for analysis.The calibration was linear over the range 0.1–104 mg l21. TheLOD using SPME or SDME were 25 and 10 ng l21,respectively. The recovery of peroxydisulfate oxidation wasbetter than that of reaction with KOH for the mineralization oforganic I in vegetables and milk powder. Organic matter inpharmaceutical samples was removed by solid phase extractionon a C18 cartridge. Iodate in salt samples was reduced to iodidewith ascorbic acid.


The 2.1% precision of a species-specific ID procedure for thedetermination of methylmercury in fish was a three-foldimprovement over that obtainable using the standard additionmethod with ethylmercury as internal standard.296 Sampleswere digested with methanolic KOH and a 198Hg-enrichedmethylmercury spike was added. The propylated derivative ofmethylmercury (formed by reaction with sodium tetrapropyl-borate) was sampled using SPME with PDMS-coated fused-silica fibres. Selective ion monitoring of the ion currents at m/z256 and 260 gave the 198Hg 202Hg ratio. A limitation on thewider applicability of this new approach is the need for species-specific enriched spikes for the compounds of interest. Themethod LOD of 37 ng g21 for a 250 mg sample was the same asthat (40 ng g21 for a 500 mg sample) obtained by Chen et al.297

using microwave-assisted digestion of fish tissue with tetra-methylammonium hydroxide, addition of Cu21 to promoterelease of methylmercury from amino acid complexes, deriva-tization with sodium tetrapropylborate and extraction inton-heptane. Recovery from three fish tissue RM was in the range89–95%. The concentrations of methylmercury in the NationalResearch Council Canada RM DORM-2 (fish tissue), mea-sured by the two methods, were identical at 4.34¡0.09 and4.34¡0.24 mg g21, respectively, and compared well with thecertified value of 4.47¡0.32 mg g21.

10.3. New methodologies

There is considerable interest in cosmic dust, the fine-grainedmatter that pervades the solar system and beyond. A novelcosmic dust mass spectrometer called the Dustbuster has beendesigned to determine the elemental composition of cosmicdust particles by impact ionization and TOFMS.298 The per-formance of the Dustbuster was tested with mm-sized particlesof copper and iron accelerated to 2–20 km s21 in a 2 MV van deGraaff accelerator. Ions were produced by impact with thetantalum target plate and accelerated through a reflectron on toa microchannel plate. The Dustbuster was considerably smallerand lighter than previous dust analysers and its novel ion opticsgave good mass resolution (typically 150–350, m/Dm, for Cuand Fe) combined with a large sampling area. The accuracy ofthe analysis was limited by the complicated ionizationmechanism in impacts involving charged microparticles.

The standard method for the determination of 15Nabundance in total dissolved nitrogen (TDN) in aqueoussamples, based on Kjeldahl digestion and either distillation ordiffusion to isolate ammonium, is both time-consuming andlaborious. A new approach to the determination of both Ncontent and 15N abundance of TDN in aqueous samplesinvolved a TOC analyser coupled to a quadrupole EIMSinstrument.299 Several fundamental requirements had to be met.The carrier gas (He–O2) had to be completely free of N2 andwith the O2 content at the minimum needed for quantitativeoxidation of the sample. There had to be no conversion ordilution of the product NO leaving the TOC analyser and lowformation of NO in the ion source. The method was rapid(15 min analysis time) and acceptably precise, but suffered froma number of limitations. Fluctuations in the O2 content duringcombustion caused variable background signals in the massspectrometer. This was overcome by coupling the TOCanalyser to the mass spectrometer through a trap and flushinterface. A severe memory effect could only be overcome byrunning five replicates in series and rejecting the first two orthree results. A strong dependence of the 15N measurement onthe N concentration in the sample, in particular at ¡10 mg N,had to be corrected for using an exponential relationship.The precision was sufficient to determine 15N at 0.5 atom% if¢50 mg of N were available per analysis. Correspondinglyless N was required for higher 15N abundances. The LOD was2 mg per analysis, corresponding to 0.7 mg l21 in a maximumsample volume of 3 ml. The whole procedure was controlled

manually and further development was required to produce anautomated system suitable for routine analysis.

A novel spark combustion reactor was designed for thequantitative determination of 13C in highly-enriched hydro-carbons.300 The reactor was loaded with the gaseous sampleand O2 using a steel gas manifold and a high voltage appliedusing a platinum-electrode spark plug. The hydrocarbons wereconverted rapidly into CO2, which was analysed by quadrupoleEIMS. The procedure as presented was limited in performancebecause of the formation of CO2 at natural abundance withinthe reactor itself. The source of this CO2 was suspected to becarbon contained within the interior walls of the reactor andon the spark plug surfaces. Although a 13C abundance of99.6 atom% could be measured for a methane sample with99.9 atom% 13C, it was necessary to repeat the combustioncycle several times to reach this value.

Appendix

Glossary of terms

AMS, Accelerator Mass SpectrometryASV, Anodic Stripping VoltammetryaTOF LMMS, Aerosol Time of Flight Laser Microprobe

Mass SpectrometryBCR, Bureau Communautaire de ReferenceCE, Capillary ElectrophoresisCF, Continuous FlowCHARISMA, Chicago-Argonne Resonant Laser Ionization

Mass AnalyserCID, Collision Induced DissociationCMOS, Complementary Metal-Oxide-Semiconductorcps, counts per secondCRM, Certified Reference MaterialCSIA, Compound Specific Isotope AnalysisCW, Continuous WaveDBT, Dibutyltindc, direct currentDIC, Dissolved Inorganic CarbonDIHEN, Direct Injection High Efficiency NebulizerDIN, Direct Injection NebulizerDNA, Deoxyribonucleic AcidDRC, Dynamic Reaction CellDU, Depleted UraniumEA, Elemental AnalyserEC, Electron CaptureEcHG, Electrochemical Hydride GenerationEI, Electron IonizationEIMS, Electron Ionization Mass SpectrometryEMB, Effusive Molecular BeamEQEL, Electrostatic Quadrupole Extraction LensES, ElectrosprayESA, Electrostatic AnalyserESMS, Electrospray Mass SpectrometryETV, Electrothermal VaporizationFI, Flow InjectionFWHM, Full Width at Half MaximumGC, Gas ChromatographyGC-EIMS, Gas Chromatography Electron Ionization Mass

SpectrometryGC-GD-TOFMS, Gas Chromatography-Glow Discharge

Time of Flight Mass SpectrometryGCIB, Gas-Cluster Ion BeamGD, Glow DischargeGDMS, Glow Discharge Mass SpectrometryGD-OES, Glow Discharge Optical Emission SpectrometryGD-TOFMS, Glow Discharge Time of Flight Mass

Spectrometry


GPL, Generalized Power LawHET, High-Efficiency TorchHG, Hydride GenerationHPLC, High-performance Liquid ChromatographyHR, High ResolutionIAA, IsoamylamineIC, Ion ChromatographyICP, Inductively Coupled PlasmaICP-MS, Inductively Coupled Plasma Mass SpectrometryICP-TOFMS, Inductively Coupled Plasma Time of Flight

Mass Spectrometryid, internal diameterID, Isotope DilutionIMF, Instrumental Mass FractionationIR, InfraredIUPAC, International Union of Pure and Applied

ChemistryKE, Kinetic EnergyLA, Laser AblationLA-ICPMS, Laser Ablation-Inductively Coupled Plasma

Mass SpectrometryLAMPAS, Laser Mass Analyser for Particles in the

Airborne StateLC, Liquid ChromatographyLEIS, Low Energy Ion ScatteringLIMS, Laser Ionization Mass SpectrometryLMMS, Laser Microprobe Mass SpectrometryLOD, Limit of DetectionLOQ, Limit of QuantificationLPI, Laser Plasma IonizationMALDI, Matrix-Assisted Laser-Desorption IonizationMC, Multiple CollectorMCN, Microconcentric NebulizerMIP, Microwave-induced PlasmaMIP-TOFMS, Microwave-Induced Plasma Time of Flight

Mass SpectrometryMS, Mass SpectrometryMT, Metallothioneinm/z, mass-to charge ratioNa-DDC, Sodium DiethyldithiocarbamateNEG, Non-Evaporable GetterNIST, National Institute of Standards and TechnologyNRCC, National Research Council CanadaP&T, Purge and TrapPCA, Principal Components AnalysisPCB, Polychlorinated BiphenylsPDMS, PolydimethylsiloxanePN, Pneumatic Nebulizationppb, parts per billionppm, parts per millionPSIA, Position-specific Isotope AnalysisRBS, Rutherford Backscattering SpectrometryREE, Rare Earth Elementsrf, radiofrequencyRIMS, Resonance Ionization Mass SpectrometryRM, Reference Materialrms, root mean squareRSD, Relative Standard DeviationRSF, Relative Sensitivity FactorSAM, Self-Assembled MonolayerS/B, Signal-to-Background RatioSD, Standard DeviationSDME, Single Drop MicroextractionSEM, Scanning Electron MicroscopySHRIMP, Sensitive High-Resolution Ion MicroprobeSIMS, Secondary Ion Mass SpectrometrySIRMS, Stable Isotope Ratio Mass SpectrometryS/N, Signal-to-Noise RatioSNMS, Sputtered Neutral Mass SpectrometrySOI, Silicon-on-Insulator

SPME, Solid Phase MicroextractionSRM, Standard Reference MaterialS-SIMS, Static Secondary Ion Mass SpectrometrySSJ, Supersonic JetTBT, TributyltinTEM, Transmission Electron MicroscopyTIMS, Thermal Ionization Mass SpectrometryTOC, Total Organic CarbonTOF, Time of FlightTOFMS, Time of Flight Mass SpectrometryTWMHC, Thin-Walled Metal Hollow CathodeUV, UltravioletVOC, Volatile Organic CompoundsXPS, X-ray Photoelectron SpectroscopyYAG, Yttrium Aluminium Garnet3-D, Three Dimensional3MTP, 3-Methylthiopropylaminel, Wavelengtht, Pulse Duration

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