application of infrared laser desorption vacuum-uv single-photon ionization mass spectrometry for...

Application of Infrared Laser DesorptionVacuum-UV Single-Photon Ionization MassSpectrometry for Analysis of Organic Compoundsfrom Particulate Matter Filter Samples

T. Ferge,†,‡ F. Mu1hlberger,† and R. Zimmermann*,†,‡,§

GSF Forschungszentrum, Institut fur Okologische Chemie, Ingolstadter Landstr. 1, 85764 Neuherberg, Germany,Analytische Chemie, Lehrstuhl fur Festkorperphysik, Institut fur Physik, Universitat Augsburg, Universitatsstrasse 1,86159 Augsburg, Germany, and BifA - Bayerisches Institut fur Umweltforschung und -technik,Abteilung Umwelt- und Prozesschemie, Am Mittleren Moos 46, 86167 Augsburg, Germany

A new built instrument suitable for laser desorption-singlephoton ionization time-of-flight mass spectrometry (LD-SPI-TOFMS) with use of Vacuum-UV photons with awavelength of 118 nm was used for the analysis of organiccompounds. Fragmentation-free analysis of a variety ofsubstances was achieved for desorption experiments withpure compounds desorbed from quartz glass filters ap-plying low desorption energies. It was further found thatthe rate of fragmentation is strongly dependent on thedesorption energy. Matrix effects were investigated bydesorption experiments utilizing soot spiked with severalorganic compounds.The characteristics of the desorptionprocess are assessed in more detail and the impact onthe analysis of ambient particulate matter (PM) sampleson filters are discussed. First results obtained from theapplication of the technique to the analysis of organiccompounds from ambient PM are presented. Further-more, possibilities of future developments of the method,in particular for analysis of ambient PM, are discussed.

Several epidemiological studies to date show that there is asignificant relevance of ambient particles in health effects.1

Particles with diameters of less than 2.5 µm (referred to as PM2.5)are strongly associated with mortality and other consequencessuch as cardio-pulmonary diseases.2-4 Recent studies suggest thatultrafine particles (DP < 100 nm) are more toxic than PM10.5

However, the number of ultrafine particles is often poorly

correlated with PM2.5 or PM10, even though there is epidemio-logical evidence for the health effects of these extremely smallparticles.6

Up to now it has not been possible to establish a relationshipbetween particle-related health effects and single components,although intensive research has been carried out in the pastdecade. So far, several investigations suggest a significant con-tribution of organic components to toxicity of particulate matter.7,8

The organic fraction of urban aerosols constitutes a complexmixture of a multitude of different compounds. For PM2.5 theoverall concentration amounts to 1-12 µg/m3, which makes uproughly up to 50% of the total particle mass.9,10 Major compoundclasses in urban aerosols are for example aliphatic hydrocarbons,organic acids, and polycyclic aromatic hydrocarbons.11 For indi-vidual compounds the concentration is usually in the range from0.1 to 10 ng/m3.12 The identification and analysis of organiccompounds requires long sampling times, extensive samplepreparation and clean up, and analysis time in the laboratory.

For time-series studies aimed at the influence of organicaerosols on human health, a compilation of data of severalcompounds or compound classes is necessary at least on a dailybasis. Various chromatographic methods such as gas chroma-tography/mass spectrometry (GC/MS)11,13 and comprehensiveGC (GCxGC)14-16 are used for resolving these analytical chal-lenges. However, the necessity for tedious sample preparation and

* Corresponding author. E-mail: [email protected].† GSF Forschungszentrum.‡ Universitat Augsburg.§ BifA.

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4528 Analytical Chemistry, Vol. 77, No. 14, July 15, 2005 10.1021/ac050296x CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 05/27/2005

the collection of relatively large quantities renders the analysisof aerosol from sites with low concentrations (or when only limitedtime for sampling is available) an elaborate procedure. Hence,additional methods are desirable, which circumvent the time-consuming sample preparation and enable the researcher to easilyanalyze smaller sample quantities. In the recent past, two-step laserdesorption photoionization (LD-PI) mass spectrometry17-24 hasproven to be a suitable candidate for such a purpose as it allowschemical analysis within minutes and without any sample prepara-tion. LD-PI uses a first laser pulse for laser desorption (LD) ofintact neutral molecules from the sample surface and a secondlaser pulse for ionization (PI) of desorbed species. Typically fordesorption a pulsed CO2-laser (10.6 µm) or pulsed YAG-lasers (Er:YAG: λ ) 2.94 µm; Nd:YAG: λ ) 1.064 µm) are used; pulsedUV-lasers with fixed frequency wavelengths of 193 or 266 nm orOPO systems with tunable wavelengths (e.g., 273 and 290 nm24)are applied as ionization lasers. In this UV wavelength range, theresonance-enhanced multiphoton ionization (REMPI) is a selectiveand soft means of ionization, offering the detection of only thosemolecules with appropriate electronic transitions, thus represent-ing an ideal method for analysis of aromatic trace compounds incomplex samples.24-29 In the case of LD-REMPI, for polycyclicaromatic hydrocarbons quantitation in the low-nanogram topicogram range was achieved21,24,30 whereas limits of detectioneven in the attomole range have been reported.31,32 Also, theapplication of LD-REMPI to on-line single-particle analysis wasaccomplished for the analysis of organic model particles as wellas aerosols from wood and cigarette combustion.33

However, REMPI, while providing high sensitivity for aromaticcompounds, is not useful for analysis of aliphatic compounds,which are representing the vast majority of the organic mass inambient PM. An alternative to multiphoton ionization is the single-

photon ionization (SPI) with VUV photons.34,35 Due to mediumselectivity by virtue of the ionization threshold, it is suitable forboth aromatic and aliphatic organic compound classes. Thetypically used radiation (118 nm, 10.5 eV) is just above theionization threshold of most organic compounds, making it a softionization method which produces little or no fragmentation. SPItime-of-flight mass spectrometry (SPI-TOFMS) has proven to bea method which can be applied in various analytical fields withlimits of detection for single compounds down to the low ppbrange.28,29,35-38 The combination of laser desorption with single-photon ionization (LD-SPI) therefore offers the possibility for theanalysis of organic compounds in aerosol samples. So far, SPIwas employed for real-time detection of molecules desorbed fromindividual particles as well as depth profiling of organic compoundsin single particles.39 Also characterization of thermo-desorbedorganic compounds from particles impacted on a heated probehas been reported.40 However, with the currently used SPI-MStechnologies individual particles with diameters less than 1 µmare difficult to analyze due to vanishingly small signal levels. Foranalysis of ambient aerosols this constitutes a problem as mostorganics are present in particles smaller than 1 µm.9,41,42 Thereare several approaches dealing with fast characterization of organiccompounds in “bulk” aerosol samples. In each case particles aresampled through an aerodynamical lens and impacted on a heatedprobe where semivolatile components are vaporized and thenionized by 70-eV electron impact ionization (EI).43,44 With thesetechniques a very good sensitivity is achieved by integrating thesignal over a large number of particles. However, due to the useof EI, mass spectra show extensive fragmentation so that adetection of molecular ions is difficult if possible at all. Recently,SPI was used for analysis of organic aerosols with laser desorptionutilizing radiation from an Nd:YAG laser at 1064 nm.45 Aerosolparticles are sampled through an aerodynamical lens on adeposition probe, laser-desorbed, and then ionized by VUVradiation. Here, a sensitivity of 50-500 ng/m3 was achieved forindividual compounds in an aerosol sample. However, somedegree of fragmentation still occurs in the mass spectra, mostprobably due to the desorption process utilizing 1064 nm photons.Even this rather low rate of fragmentation turns out to be aproblem for compound identification of molecular peaks in themass spectra of more complex samples.

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570.(23) Elsila, J. E.; de Leon, N. P.; Zare, R. N. Anal. Chem. 2004, 76, 2430-2437.(24) Hauler, T. E.; Boesl, U.; Kaesdorf, S.; Zimmermann, R. J. Chromatogr. A

2004, 1058, 39-49.(25) Lubman, D. M., Ed. Lasers and Mass Spectrometry; Oxford University

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Appl. Spectrosc. 2001, 3, 181-206.(28) Cao, L.; Muhlberger, F.; Adam, T.; Streibel, T.; Wang, H. Z.; Kettrup, A.;

Zimmermann, R. Anal. Chem. 2003, 75, 5639-5645.(29) Dorfner, R.; Ferge, T.; Yeretzian, C.; Kettrup, A.; Zimmermann, R. Anal.

Chem. 2004, 76, 1368-1402.(30) Emmenegger, C.; Kalberer, M.; Morrical, B. D.; Zenobi, R. Anal. Chem.

2003, 75, 4508-4513.(31) Clemett, S. J.; Zare, R. N. In Molecules in Astrophysic: Probes and Processes;

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Analytical Chemistry, Vol. 77, No. 14, July 15, 2005 4529

In the present work first results from the application of LD-SPI for analysis of organic compounds from filter samples isdescribed. Individual compounds are desorbed from glass fiberfilters by a CO2 laser (10.6 µm) and vaporized molecules areionized with VUV radiation at 118 nm (10.5 eV). The influence ofthe desorption laser fluence as well as the influence of the samplematrix on the spectra are discussed. Furthermore, first resultsfrom measurements on real-world particulate matter samples arepresented.

EXPERIMENTAL SECTIONInstrumental Setup. The LD-SPI-TOFMS instrument is

shown schematically in Figure 1. The mass spectrometer consistsof a hybrid laser desorption/molecular beam ion source, a vacuumultraviolet frequency tripling cell, and a tunable OPO-laser systemfor application of various laser wavelengths and a reflectron massspectrometer. The instrument was developed as a mobile systemsuitable for laboratory as well as field applications and is describedin detail in a recent publication.24 The first configuration of thelaser system described in there consisted of a µ-TEA CO2-laserfor desorption and an Nd:YAG based OPO-system for ionization.The CO2-laser (µ-TEA, Laser Science Inc., Franklin, MA) gener-ates 10.6 µm pulses with a peak energy of approximately 15 mJand peak width of 100 ns. The energy of the laser can be adjustedby a variable focal aperture located directly in front of the focusingoptics. The ionization laser system is based on an Nd:YAG-laser(Quanta Ray INDI 50-10, Spectra Physics, Mountain View, CA),which includes devices for generation of the third and fourthharmonic frequencies (355 and 266 nm). The 266 nm pulses canbe directly used for REMPI, whereas the 355 nm radiation is usedfor pumping an optical parametric oscillator (OPO) with secondharmonic generation generating tunable UV laser radiation in therange from 220 to 350 nm. The tunability renders the systemconvenient for a wide variety of Ld-REMPI-TOFMS applications.

Generation of VUV Laser Radiation. For the generation ofVacuum-UV laser radiation a conversion cell was added to theinstrumental setup, which is mounted directly onto the ion sourceof the mass spectrometer. The rare gas cell (xenon; purity 4.0;total pressure 30 mbar) was designed similar to correspondingcells in previously described instruments.37,38 Due to the original

design of the instrument as a mobile device, the cell is relativelyshort. In this case phase-matching conditions in a rare gas mixturedo not enhance the efficiency of the conversion too effectively.For this reason we did not implement rare gas mixing in this work.The 355 nm beam generated by the pump laser can be directedto the VUV cell by exchanging only one mirror in the optical setupof the laser system. The 355 nm pulses (∼30 mJ/pulse) arefocused with a quartz lens (f ) 100 mm) through a quartz windowinto a 180 mm long stainless steel tube. Calculation of the beamparameters and beam paths was performed according to litera-ture.46 Separation of the remaining 355 nm and the generated 118nm radiation was done by off-axis irradiation of the 355/118 nmbeams onto a MgF2 lens.34,47 This is necessary to avoid fragmenta-tion of the formed ions of the analyte molecules by the highintensity of the 355 nm fundamental beam.38 The resulting VUV-laser pulse is focused on the center of the ion source. The VUVpulse energy is not measured, but is expected to be on the orderof 0.3 µJ based on an approximate conversion efficiency of 10-5

reported in the literature.48

Laser Desorption/Ionization. The sample to be investigatedis mounted on a probe tip (L ) 6 mm), which is introduced intothe ion source via an airlock. The probe tip is located on levelwith the repeller electrode of the TOFMS.24 The desorption laseris focused on a spot with a diameter of 1.0 mm on the samplesurface. Typically, the desorption energy is adjusted to 1 mJ/pulse,resulting in a power density of 1.2 × 106 W/cm2. The laser spothits the target 1.5 mm off the center. The probe is rotated by asmall stepper motor in order to partly refresh the LD-target surfacefor each laser shot. The plume of desorbed molecules expandsinto the center region of the ion source and is crossed by thebeam path of the ionization laser (VUV). The distance betweensample surface and the VUV laser beam was 2.5 mm, resulting inan optimal delay time between desorption laser pulse and VUVlaser pulse of 12 µs (measured for oleic acid desorbed from glassfiber filter). This value was found optimal for the experimentsmade with oleic acid. Although this value is dependent on thematerial to be desorbed, this delay time was kept for all experi-ments, which were carried out in the frame of this work (seeResults and Discussion section below). The ions generated bythe VUV pulse are analyzed with a reflectron time-of-flight massspectrometer. A more detailed description of the ion source, thedesorption probe, and the mass spectrometer can be foundelsewhere.24

Sample Preparation. In the experiments performed, we useddifferent organic compounds, which are representing typicalorganic compound classes found in ambient aerosols:

Oleic acid is present in tropospheric particles as a product ofvegetative burn with an average concentration of 25 ng/m3 underambient conditions.11 Cholesterol is found at approximately 2 ng/m3 in ambient air and can be used as a tracer for meat cooking.49,50

Pyrene and 9,10-phenanthroquinone are representing the poly-cyclic aromatic hydrocarbons (PAH) and oxygenated PAH (Oxy-

(46) Maker, P. D.; Terhune, R. W. Phys. Rev. 1965, 137 (No. 3A), 801-818.(47) Steenvoorden, R. J. J. M.; Hage, E. R. E.; Boon, J. J.; Kistemaker, P. G.;

Weeding, T. L. Org. Mass Spectrom. 1994, 29, 78-84.(48) Kung, A. H. Opt. Lett. 1983, 8, 24-26.(49) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A.; Cass, G. R. Environ. Sci.

Technol. 1991, 25, 1112-1125.(50) Nolte, C. G.; Schauer, J. J.; Cass, G. R.; Simoneit, B. R. T. Environ. Sci.

Technol. 1999, 33, 3313-3316.

Figure 1. Schematic representation of the instrument. For desorp-tion the beam of a pulsed CO2-laser is focused on the sample surface.After a time delay, the ionization laser is fired (here 118 nm generatedin a frequency tripling cell). Formed ions are subsequently detectedin a time-of-flight mass spectrometer.

4530 Analytical Chemistry, Vol. 77, No. 14, July 15, 2005

PAH) respectively present in urban aerosols.51 These compoundclasses are known air toxics due to their highly carcinogenicactivity52 and their capability to induce oxidative stress in cells.53,54

6-Nitrochrysene was chosen as a member of the highly toxicsubstance group of nitropolycyclic aromatic hydrocarbons, whichare released into ambient air through direct emission or due tosecondary reactions in the atmosphere.55 Long-chain alkanes suchas triacontane (C30H62) are present in ambient aerosols in relativelyhigh individual concentrations of up to 20 ng/m3 12 and originatefrom unburnt fuel (diesel, heating fuel, and vehicle emissions)and vaporized lubricants.56,57 Polar organic compounds have beenanalyzed in ambient particulate samples with n-alkanoic acids suchas triacontanoic acid being present in high individual concentra-tions (1.5-6.5 ng/m3).58,59

All chemicals were purchased from Sigma Aldrich (SigmaAldrich, Taufkirchen, Germany) and used without further treat-ment. The individual compounds were dissolved in organicsolvents (cholesterol, pyrene, 9,10-phenanthrenedione, and oleicacid in ethanol, triacontane and triacontanoic acid in hexane, and6-nitrochrysene in dichloromethane - 5 µg/mL respectively) and5 µL of the solutions were applied to pieces of glass fiber filters(Glass Microfiber Filters, Whatman, Brentford, UK). In addition,samples of elemental carbon particles (spark generated soot -GFG 1000, Palas GmbH, Karlsruhe, Germany) were spiked withthe prepared solutions. For this purpose, 2 µL of the respectivesolution was added per milligram of soot to the particles, allowingthe solvent to dry overnight. For analysis the filter samples werefixed on the probe tip with double-sided adhesive tape, and thesamples of spiked soot were fixed in the same way using theadhesive tape with approximately 1 mg of sample on the probetip. For determination of the exact amount of soot adhered to thetip, the probe tip was weighed before and after the application ofthe soot.

RESULTS AND DISCUSSIONThe primary mechanism for vaporization of analyte molecules

by a desorption laser pulse is thought to be fast heating of theprobe surface and subsequent heat transfer to the materialdeposited on the substrate, which then is desorbed and releasedinto the gas phase.

Figure 2 shows LD-SPI mass spectra of the seven purecompounds investigated in this study as well as those of the blankfilter. The spectrum of oleic acid (Figure 2a) is dominated by thepeak of the molecular ion at 282 m/z. The fragment ion peaksthat originate from loss of H2O and OH (264 and 265 m/z) arevisible. A very small peak at 222 m/z probably is due to loss of

C3H6O, a peak also observed in two-step desorption/ionizationspectra of oleic acid by other researchers.45 Figure 2b shows themass spectrum of cholesterol. Here also the molecular ion is thedominating species of the spectrum together with some small highm/z fragments at 368, 353, and 275, which are known fragmentsin EI-mass spectra of cholesterol. All other spectra (Figure 2c-g) show the same fundamental characteristic of the molecular ionsbeing by far the dominant if not only species visible. The onlyfragmentation that occurs are the mentioned small signals offragments in the spectrum of cholesterol, the water loss from thecarboxylic group in oleic acid, and the loss of CO in 9,10-phenanthroquinone (208 m/z and 180 m/z, Figure 2d). Even thelong-chain aliphatic hydrocarbons as triacontane and triacontanoicacid do not show any fragmentation pattern. Astonishingly,triacontanoic acid does not even show fragmentation due to lossof OH and/or H2O as is the case for oleic acid. Furthermore, it ispossible to detect even 6-nitrochrysene as molecular ion withoutany fragmentation. This is a very interesting aspect as thedetection of nitroaromatic compounds usually is hampered byseveral factors such as the loss of NO and/or NO2 as well as theneed of other UV wavelengths for detection of minor molecularpeaks among typical fragment patterns (e.g., 213 nm60) or theapplication of negative ion mass spectrometry.61 Recently, we couldalso detect molecular ions of 2,4- and 2,6-dinitroanilin withoutfragmentation (data not shown here).

For the analysis of complex sample matrixes the model of purecompounds on filters is certainly not appropriate. In urban aerosolsthe fraction of black carbonaceous matter can account for up to20-50%.11,62 A more convenient sample matrix therefore wouldbe carbonaceous material. To exclude interfering effects of othercompounds, we used the spark-generated soot as stated in theExperimental Section. This soot is known to be composed mainlyof elemental carbon (EC/TC ) 89.9% with EC: elemental carbon,TC: total carbon including organic content).63 Consequently,samples of spiked soot were investigated with LD-SPI. Figure 3shows the resulting mass spectra. In principle, the spectra do notshow major differences when compared to the spectra recordedby desorbing directly from filter material. Only some minor peaks,which obviously stem from the soot matrix, are additionally visibleand marked with asterisks. That these peaks originate from thesoot is shown in Figure 3h, which depicts a spectrum of puresoot substrate obtained under the same experimental conditions.Yet the nature of these peaks remains to be unraveled. In general,with the soot matrix, higher signal intensity is observed for allcompounds in the LD-SPI spectra, which supports the assumptionthat black carbon increases the yield of desorbed material due tothe higher incoupling rate of the laser energy (see also thediscussion below). Differences are observed in the spectra of oleicacid, triacontane andsto a lesser extentsof triacontanoic acid.Here, an apparent number of peaks with the typical fragmentpattern of aliphatic hydrocarbons suggest some rate of fragmenta-tion of the compounds in the LD-SPI process.

(51) Schnelle-Kreis, J.; Gebefugi, I.; Welzl, G.; Jansch, T.; Kettrup, A. Atmos.Environ. 2001, 35, S71-S81.

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The fragmentation we observe in the spectra of aliphatichydrocarbons is influenced by the desorption process as isillustrated in Figure 4. Mass spectra of oleic acid on soot wererecorded with varying desorption energy, applying 1.0, 1.5, and

2.0 mJ, respectively. These pulse energies correspond to laserfluences of approximately 1.2 × 106, 1.9 × 106, and 2.5 × 106

W/cm2 (Figure 4a, b, and c, respectively). In the same way thepeaks stemming from substrate material show stronger signals

Figure 2. LD-SPI mass spectra of (a) oleic acid, (b) cholesterol, (c) pyrene, (d) 9,10-phenathroquinone, (e) triacontane, (f) triacontanoic acid,and (g) 6-nitrochrysene on glass fiber filter; 1 mJ CO2-laser energy, 30 mJ/pulse @ 355 nm for VUV generation. All spectra show molecular ionsas predominant species. No fragmentation occurs, not even with ionization of long-chain aliphatic compounds.


as the energy used for desorption is raised and the overall signalintensity of desorbed material is higher in the spectra with

increased desorption energy. Thus, we deal with two effects uponthe increase of the desorption energy: (1) higher signal intensities

Figure 3. LD-SPI mass spectra of (a) oleic acid, (b) cholesterol, (c) pyrene, (d) 9,10-phenathroquinone, (e) triacontane, (f) triacontanoic acid,and (g) 6-nitrochrysene on soot (derived from a spark generator - see text); 1 mJ CO2-laser energy, 30 mJ/pulse @ 355 nm for VUV generation.(h) LD-SPI mass spectrum of the soot itself. Typical peaks stemming from the soot underground are marked with an asterisk. Oleic acid,triacontane, and to a much lower extent also triacontanoic acid show some fragmentation.


(i.e., more material is desorbed per laser shot) and (2) increasingrate of fragmentation of long-chain organic hydrocarbons. Thehigher power density in the desorption laser spot consequentlyboosts the deposition of laser energy in the sample matrix,resulting in a higher yield of desorbed material. In the same waydesorbed molecules gather excess thermal energy, which forsome species leads to fragmentation of the desorbed neutralmolecules upon ionization. With higher CO2 laser fluence, thedesorbed molecules have a higher amount of thermal excessenergy, thus the formed ions have enough energy to dissociateto produce fragment ions.64

To gain some more insight into the desorption process, wemeasured the signal intensity of the signal of the molecular ionof oleic acid as a function of the delay time between desorptionand ionization laser pulse. The plume of molecules desorbed from

the probe surface expands from the desorption laser spot towardthe center of the ion source and crosses the ionization laser beamfocus 2.5 mm above the target surface. Figures 5a and 5b showthe peak area, which is a measure of the number of moleculesionized, of the molecular ion peak (m/z 282) as a function ofapplied delay time when desorption takes place from a glass fiberfilter and soot, respectively. In the case of the glass fiber filterthe peak area reaches a maximum at a delay time of 12 µs,whereas with soot as substrate the optimal delay time is consider-ably shorter with approximately 7 µs. With the desorption-ionization geometry of the setup these delay times can betranslated into molecular velocities of the desorbed species. Figure5c,d shows the peak areas as a function of the molecular velocity.Support for the assumption of a thermal desorption process isgiven by the fact that the velocity distribution in Figure 5c (glassfiber filter) roughly fits a Maxwell-Boltzmann distribution witha translational temperature of 650 ( 50 K. These results, whichindicate a thermal process, are in principle agreement with recentreports on the vaporization of molecules by laser desorption witha pulsed Nd:YAG laser.45 Figure 5d shows the velocity distributionof oleic acid molecules desorbed from soot. Here, the fit to aMaxwell-Boltzmann distribution is not possible any longer forthe whole velocity range. Only the increasing part of the velocitydistribution can be fitted with a distribution according to atranslational temperature of 1200 ( 50 K, whereas the decliningpart shows higher values in the measurement than expected fromthe pure thermal Maxwell-Boltzmann distribution. The consider-able amount of molecules, which are ejected with very highvelocities (and thus kinetic energies), suggest an explosive processrather than a thermal vaporization.

Thus, with the current setup the desorption mechanism canbe mainly explained by thermal desorption but in the case of thesoot matrix an overlay with explosive vaporization occurs. Sincethe pulse length of the CO2 laser (100 ns) is much shorter thanthe time for thermal equilibration (approximately 10 µs), thedesorption process occurs within the limit of thermal confine-ment,65,66 resulting in a heating of the surface well beyond itsregular boiling temperature. The evaporation mechanism dependson the laser fluence and changes from thermal desorption fromthe surface to an explosive ablation of the overheated surfaceabove the threshold fluence. Both mechanisms are known in theliterature, the threshold value for the transition from thermal toexplosive desorption being in the range of 1.0 × 106 W/cm2.67

The here applied pulse energy of 1 mJ/pulse, which results in alaser fluence of 1.2 × 106 W/cm2, is in the range of this thresholdvalue, suggesting that with the current setup we are close to thistransition. However, this threshold fluence depends on a numberof factors such as the identity of the material from which thecompounds of interest are desorbed. The favored thermal de-sorption requires uniform heating and low laser fluence. Oneprerequisite for uniform heating is that the material is opticallythin, meaning that the product of the material thickness (penetra-tion depth of the laser light) d and the absorption coefficient κ

multiplied by the material density F meet the following inequality,dκF , 1. Under these circumstances the laser beam is notnoticeably attenuated, allowing all points in the sample layer to

(64) Woods, E.; Smith, G. D.; Dessiaterik, Y.; Baer, T.; Miller, R. E. Anal. Chem.2001, 73, 2317-2322.

(65) Zhigilei, L. V.; Garrison, B. J. Appl. Phys. Lett. 1999, 74, 1341-1343.(66) Zhigilei, L. V.; Garrison, B. J. J. Appl. Phys. 2000, 88, 1281-1298.(67) Woods, E.; Miller, R. E.; Baer, T. J. Phys. Chem. A 2003, 107, 2119-2125.

Figure 4. LD-SPI mass spectra of oleic acid on soot with differentdesorption laser energies. With increasing desorption laser energy,a higher rate of fragmentation occurs. Higher desorption laser powerleads to higher energy transfer to desorbed molecules and presum-ably to higher desorption temperatures and thus rather high excessenergy.


experience the same laser fluence. In comparison to pure organiccompounds and glass fiber filter material, soot certainly is anoptically thicker material; therefore, the threshold value for thelaser fluence above which explosive evaporation occurs is lower.This explains the observed explosive characteristics of thedesorption process from the soot matrix. These characteristicscertainly have an impact on the analysis of ambient aerosolsamples (see discussion below).

For estimation of limits of detection (LOD) of the compoundson the soot matrix, the known amounts of the pure standardcompounds, which we applied as described in the ExperimentalSection in a concentration of 2 µL/mg to the soot, were taken asbasis. By assuming an even distribution of compounds on the sootparticles after evaporation of the solvent and with the applicationof a fixed amount of spiked soot (1 mg) to the probe tip, we canestimate the quantity of adsorbed substance to be 10 ng persample. The irradiated surface (IR laser desorption) was used asa basis for further calculation. The laser is focused 1.5 mm off-center to a spot with a diameter of 1 mm on the probe tip. Thus,a ring surface representing 33.3% of the entire probe surface isirradiated by the IR laser. Therefore, 3.33 ng of the respectivecompounds are available for laser desorption. The 200 LD-SPI TOFmass spectra for the measurement of a sample (corresponding

to one full turn of the probe tip) were averaged and the LOD fora signal-to-noise (S/N) ratio of 2 was calculated according to theformula68

where c is the absolute amount of irradiated molecules (3.33 ng),σ the standard deviation of the noise, p the signal height, and mthe baseline level. The LODs are in the low pg range for theinvestigated compounds when desorbed from the pure sootmatrix. In detail the following LODs (S/N ) 2) were deter-mined: Oleic acid 5 pg; cholesterol 15 pg; pyrene 7 pg; 9,10-phenathroquinone 11 pg; triacontane 5 pg; triacontanoic acid 12pg; 6-nitrochrysene 10 pg. These absolute values compare wellwith those reported from other LD-SPI experiments.45 The linearityof the LD-PI method was tested and shown in a recent publica-tion.24

The obtained LODs are in principle sufficient for detection oforganic compounds in ambient samples. A recent study12 dealing

(68) Williams, B. A.; Tanada, T. N.; Cool, T. A. In 24th Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, 1992; pp 1587-1596.

Figure 5. (a) Signal intensity vs delay time for LD-SPI measurements of oleic acid desorbed from glass fiber filter. (b) Signal intensity vs delaytime for LD-SPI measurements of oleic acid desorbed from soot. (c) Approximation of signal intensity distribution (glass fiber) with a Maxwell-Boltzmann velocity distribution. For a distance of 2.5 mm between desorption plate and ionization focus, a maximum in molecule velocity of 187m/s corresponding to a temperature of 650 K can be approximated. (d) Approximation of signal intensity distribution (soot) with a Maxwell-Boltzmann velocity distribution. Only the increasing part of the distribution can be approximated relatively well with a distribution correspondingto a temperature of 1200 K (maximum in velocity distribution of 265 m/s). The high fraction of very fast molecules leads to the assumption oftwo conquering desorption mechanisms: thermal desorption and explosive evaporation.

LODS/N-2 ) 2cσ(p-m)


with analysis of organic compounds in ambient PM in Augsburg,Germany, on a daily basis revealed concentrations of pyrene of0.5-1.8 ng/m3 as well as of triacontane of 3.1-7.3 ng/m3. Thefilter stripes (2.1 × 27 mm) analyzed in this work by direct thermaldesorption GC-TOFMS represented 1 m3 of sampled air. With theabove-mentioned characteristics of the desorption laser spot theirradiated sample surface with these filter samples correspondsto 0.166 m3 air. By assuming an even distribution of compoundson the filter stripe, this accounts in the case of pyrene for a totalaccessible amount of 83-298 pg and in the case of triacontanefor 514-1211 pg. With the above-discussed LOD, the analysis ofthese compounds should be possible by application of LD-SPI.

Figure 6 shows a LD-SPI TOF mass spectrum of such a PMfilter sample of ambient air collected in Augsburg, Germany (1mJ CO2-laser pulse energy, 30 mJ per pulse 355 nm radiation).Panel a shows the low mass region up to m/z ) 150 whereas

panel b shows the whole measured mass region up to m/z ) 500.The spectrum is dominated by several large peaks in the low massregion and exhibits a large number of peaks in the medium massregion ranging from m/z ) 140 to m/z ) 350. In the high massregion of the spectrum some peaks noticeably jut out of thespectral noise. The dominant peaks at m/z ) 17 and 30 are dueto NH3 and NO stemming from ammoniumnitrate (NH4NO3)present in high amounts in ambient aerosols. The peak groupsemerging around m/z ) 43, 57, 71, 85, 99, and 113 can be assignedto alkyl-fragments from aliphatic compounds, which are frag-mented during the desorption/ionization process (see above). Thespectra of oleic acid show the same peak groups (see also Figure4c), when higher laser power is applied. It has to be assumedthat there are several fragmentation pathways accessible for thedesorbed molecules (which carry high excess energy) whenionized by the VUV laser beam. However, some prominent masssignals can be assigned to organic molecules present in thesample. In detail, organic acids are the prevailing species withacetic acid (m/z ) 60), propionic acid (m/z ) 73/74), maleicanhydride (from maleic acid, m/z ) 98), and benzoic acid (m/z) 122) showing the most prominent signals. Even some strongersignals in the medium and high mass range can be tentativelyassigned to aliphatic organic acids. A detailed peak assignmentis given in Table 1. Even numbered hydrocarbon acids and theiresters are usually strongly present in aerosol samples when theseare influenced by biogenic sources.59,69-71 The here investigatedaerosol sample was an urban background aerosol, which is alsoinfluenced by biogenic emissions from plants in the vicinity ofthe sampling site. Although the mass assignment is tentative, thehere given interpretation with aliphatic hydrocarbon acids (evennumber of carbons) and esters (uneven number of carbons) isalso backed by conventional gas chromatographic analysis of thesame sample, which showed these compounds being present inrelatively high concentrations.72 In addition to the assignment ofsingle outstanding peaks, representing compounds present in highconcentrations, it should also be possible to compare different

(69) Simoneit, B. R. T.; Mazurek, M. A. Atmos. Environ. 1982, 16, 2139-2159.(70) Simoneit, B. R. T. Appl. Geochem. 2002, 17, 129-162.(71) Bin Abas, M. R.; Rahman, N.; Omar, N. Y. M. J.; Maah, M. J.; Samah, A. A.;

Oros, D. R.; Otto, A.; Simoneit, B. R. T. Atmos. Environ. 2004, 38, 4223-4241.

(72) Schnelle-Kreis, J. Personal communication, 2005.

Figure 6. LD-SPI mass spectrum of an ambient aerosol sample.The low mass region of the spectrum is dominated by signals fromfragmented aliphatic hydrocarbons and some peaks, which can beassigned to aliphatic hydrocarbon acids. Even a tentative assignmentto long-chain aliphatic acids and/or esters is possible in the high massrange. The inset exemplarily shows the mass region from 220 to 285m/z in an expanded view.

Table 1. Peak Assignment of Ambient Aerosol Sample(Figure 6); Aliphatic Hydrocarbon Acids Are thePrevailing Species in the Mass Spectrum

m/z species

17 NH3 (from NH4NO3)30 NO (from NH4NO3)43/57/71/85/99/113

(peak groups)alkyl fragments

60 acetic acid74 propionic acid98 maleic anhydride122 benzoic acid198 naphthalic anhydride144/228/368/396/424/452 long-chain aliphatic hydrocarbon acids

(even number of C-atoms)270/298 long-chain aliphatic esters

(uneven number of C-atoms)


homologue rows of aliphatic hydrocarbons as alkanes and alde-hydes, alkenes and cycloalkanes, alkynes and dienes, and alcohols,esters, and acids from the spectra. This is obvious from the insetin Figure 6, where the mass region from 220 to 285 m/z isenlarged. One can easily see that the peaks present in the spectraare only present on even numbered mass-to-charge ratios as it isexpected for hydrocarbon compounds. For example, in thisenlarged mass region the peaks with m/z ) 226, 240, 254, 268,and 282 can be assigned as a sum value to the homologue row ofalkanes and aldehydes, the signals at m/z ) 224, 238, 252, 266,and 280 to the row of alkenes and cycloalkanes, the signals atm/z ) 222, 236, 250, 264, and 278 to alkynes or dienes, and finallythe signals at m/z ) 228, 242, 256, 270, and 284 to the row ofalcohols, esters, and acids. Even if with this assignment noinformation on a molecular level is available for sure, it can givean overview of the relative amount of different organic compoundclasses present in the samples.

With the current setup single-species investigation seemsdifficult as only major components are clearly visible in thespectrum. However, in addition to species identification andcomparison of compound classes via the homologue rows, LD-SPI mass spectra can give information on the relative organiccarbon content of filter samples when one takes advantage of thefragmentation, which is induced by the desorption process. Forthis, we applied 1.5 mJ CO2-laser pulse energy for desorption ofambient air filter samples (30 mJ per pulse 355 nm radiation waskept constant as in previous experiments). Figure 7 shows thelow mass region of two mass spectra from two ambient aerosol

samples on glass fiber filters sampled on two different days(PM2.5). The samples were taken near a street in northernMunich with heavy traffic during workdays. As expected, intensefragmentation of hydrocarbon compounds occurs, which is obvi-ous due to the typical peak pattern and prominent peak groupsat m/z ) 43, 57, 71, 85, and 99, corresponding to alkyl-fragments,which are dominating both spectra. In the higher m/z region thereis no molecular information available for both cases under theapplied LD conditions. As is indicated in Figure 7, the sampleswere taken on a Friday and Saturday, respectively. The maindifference between these 2 days is the completely changed trafficflow on Saturday. Traffic-related emissions are known to containa high fraction of organics and this difference is visible in thespectra, as the overall amount of detected compounds (fragments)is strongly reduced for the Saturday sample when car traffic wasnegligible compared to Friday. Even though, due to the strongfragmentation, no identification of compounds is possible, LD-SPI gives information on the overall content of aliphatic hydro-carbons (which make up almost all organic carbon matter) whenthe integrated signal strength of the alkyl fragment peaks is takeninto account. Even though this does not yield absolute values interms of organic carbon content (like conventional EC/OCmeasurements), LD-SPI allows the direct comparison of severalsamples without extensive sample preparation by use of thisinduced fragmentation.

Considering these results, it becomes obvious that LD-SPI-TOFMS is a valuable analytical technique for analysis of aerosolsamples in two ways:

First, it is possible to identify several aliphatic organiccompounds present on filter samples (Figure 6) as well as tocompare the relative amounts of certain organic compoundclasses. Second, with the application of higher desorption laserpower, a fast, comparative analysis of the relative OC content ispossible by the integration over the alkyl-fragment peaks. Incombination with LD-REMPI, which is also possible with the newinstrument, one can gain insights into the composition of theorganic fraction of the aerosol sample regarding both the aliphaticand aromatic hydrocarbons.

Thus, LD-SPI is a feasible analytical tool for identification ofcompounds and compound classes present in ambient aerosols.However, some considerations about the two main factors, whichdetermine the efficiency of the LD-PI method for ambientmeasurements (the desorption and the ionization process), remainto be taken into account.

One has to take into account that ambient aerosols arecomposed of a high degree of inorganic matter (e.g., alkali salts,NH4NO3, which is also present in the spectra). First experimentswith NaSO4 as substrate, which was spiked with small amountsof cholesterol, did not show detectable signals. Recent studiesconcerning the one-step LDI process for organic species haveshown that desorption/ionization efficiency strongly depends onthe sample matrix. For example, it is not possible to detect PAHsin an LDI process from fly ash particles, which consist of mainlyinorganic matter. No PAH signals were obtained even from spikedfly ash samples containing up to 1% of a single PAH species(Pyrene).73 In contrast, PAH adsorbed on black carbon (e.g., soot

(73) Zimmermann, R.; Van Vaeck, L.; Davidovic, M.; Beckmann, M.; Adams, A.Environ. Sci. Technol. 2000, 34, 4780-4788.

Figure 7. Two LD-SPI mass spectra of ambient aerosol sampleson glass fiber filters with induced fragmentation (see text). The Fridaysample with a high load of aliphatic hydrocarbons shows a noticeablehigher amount of fragments from aliphatic hydrocarbons than theSaturday sample. This can be explained with the differing trafficsituation. Even without molecular information accessible, the overallamount of aliphatic hydrocarbons can be estimated via the fragmentpeaks.


particles) can be detected efficiently by LDI mass spectrometry.73,74

This phenomenon was interpreted by the high coupling efficiencyof the laser energy due to the photon absorption of black carbon.A similar effect is known as graphite-assisted MALDI (matrix-assisted laser desorption/ionization).75 Therefore, it seems reason-able that, also in the case of LD-PI, one has to deal with twoconcurrent processes: (1) enhancement of the desorption due toblack carbon, which is accompanied by an increasing rate offragmentation, and (2) a suppression of an effective desorptiondue to inorganic content of the sample matrix, both influencingthe achievable LOD of compounds and thus mass spectralinformation. Increasing the laser power for desorption is impededby the fact that one then changes the desorption from a ratherthermal process to an explosive ablation, leading to increasedfragmentation of the formed ions. However, for analysis of ambientaerosols with low concentrations or when shorter sampling timesare required, the sensitivity has to be increased. In principle, thiscan be accomplished by desorbing more material from the samplesurface. One possible way of keeping the laser fluence at thesample surface constant and at the same time increasing theaccessible amount of material would be to broaden the incidentCO2-laser beam with increased pulse power. However, this causesalso a broader desorption plume and therefore also requires amodification of the ionization beam path (see below). Anotherpromising desorption technique is the Laser Induced AcousticDesorption (LIAD).76 With this technique long-chain aliphatichydrocarbons also can be desorbed as neutrals into the gas phaseand subsequently ionized by several techniques. A titanium foilholding the analytes is fired upon from the back with a pulsedNd:YAG laser at 532 nm. The pulses induce ultrasonic acousticwaves that propagate through the foil, resulting in desorption ofthe molecules from the opposite side. Nevertheless, this requiresmajor rearrangements of the ion source and laser setup of thecurrent instrument.

As stated above, the broadening of the desorption laser beamrequires also a rearrangement of the UV-laser setup to generate118 nm laser radiation. Currently, we are using pure xenon inthe conversion cell and do not apply phase-matching conditionsin a gas mixture. As a result, the incident 355 nm beam has to betightly focused into the conversion cell. Simultaneously, thisaccounts for a small beam waist in the ionization region. However,when the spot size of the desorption laser is expanded, the densityof desorbed molecules (when an even distribution is assumed)in the ionization beam waist will stay constant. This means thatby only expanding the desorption spot and thus desorbing morematerial the achievable LOD will not improve. To again increasethe sensitivity, the ionization focus has to be broadened likewise.This can be done as the ionization efficiency of the SPI-processs

in contrast to multiphoton processessdepends only on theabsolute number of photons and not on the photon density. Onerather simple method to achieve this goal is to relocate theionization focus by moving the focal point in the conversion cell.If even higher photon densities are required, also phase-matching

conditions in the conversion cell can be applied as this is necessarywhen higher energies of the 355 nm laser beam are used to avoidgas breakthrough.

First work on these topics is currently carried out at the GSFresearch center.

CONCLUSION

LD-SPI-TOFMS allows the fragmentation free desorption/ionization of a great variety of organic compounds. This includesaliphatic hydrocarbons such as long-chain alkanes and alkanoicacids as well as aromatic hydrocarbons, oxygenated PAH, andnitroaromatic compounds. Limits of detection for single com-pounds in the range of concentrations comparable to ambientaerosols were achieved in experiments with spiked soot. However,it has to be noted that these LODs are valid only for purecompounds desorbed from a pure soot matrix and cannot betranslated one-to-one to ambient aerosols because of interferingeffects of the inorganic content of the aerosol.

The observed matrix effects on the desorption process stronglyaffect the resulting mass spectra. Soot as optically thick materialhas very high absorption efficiency and therefore features highrates of energy transfer to adsorbed species. This results in thetransition of the evaporation mechanism from the favorablethermal vaporization to an explosive one, which results inmolecules carrying high thermal excess energy, favoring frag-mentation upon ionization. On the other hand, a high amount ofinorganic material seems to suppress efficient desorption. Theseconcurring properties of the matrixes are currently the maindrawback when analyzing real world samples. Additionally,information on the overall amount of organic hydrocarbons isavailable by virtue of the total ion current in the mass spectrawhen one makes use of the induced fragmentation due to highdesorption laser power.

In conclusion, LD-SPI-TOFMS is a promising tool for fast andsensitive analysis of a wide variety of organic compounds. Itrequires no tedious sample preparation and cleanup and consti-tutes an easy method for obtaining a general view on samplecomposition and in combination with LD-REMPI can give anoverview of the organic composition including aliphatic andaromatic organic compounds.

ACKNOWLEDGMENT

This work was carried out within the scope of the GSF-Focus“Health relevance of aerosols” which coordinates aerosol-relatedresearch within the GSF Research Center. The authors like tothank T. Streibel, T. Adam, S. Mitschke, W. Welthagen, M. Bente,M. Sklorz, and J. Schnelle-Kreis for valuable discussions as wellas support with the sampling of ambient aerosols and E. Karg forproviding the soot particles used in this study. Funding of theDFG (Grant Number ZI 764 1-1) are gratefully acknowledged.

Received for review February 17, 2005. Accepted April 27,2005.

AC050296X

(74) Zimmermann, R.; Ferge, T.; Galli, M.; Karlsson, R. Rapid Commun. MassSpectrom. 2003, 17, 851-859.

(75) Dale, M. J.; Knochenmuss, R.; Zenobi, R. Anal. Chem. 1996, 68, 3321-3329.

(76) Campbell, J. L.; Crawford, K. E.; Kenttamaa, H. I. Anal. Chem. 2004, 76,959-963.


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