ICP Methods for Consistent Trace Elemental Data

WEALA Technical Workshop - 25-Apr-2013

The Analytical Process

Sampling

Bulk Prep

Analytical Prep

Analysis

Calculations and

Reporting

Scope

WEALA• Focus on soil• Environmental slant (presenter’s bias)

“Consistency” relates to ‘variability’• Event-to-event consistency relies on

controlling systematic variability

Sampling

“Sampling involves the selection from the total population of a subset of individuals upon which measurements will be made; the measurements made on this subset (or sample) will then be used to estimate the properties (or parameters) of the total population.”

Carter and Gregorich, pg. 1, first sentence

Sampling - Design

• Total concentrations in the whole?• Extent and boundaries of a spill?

What question is to be answered?

• Account for heterogeneity/stratificationRepresentative of the site

• Precautions against contamination• Appropriate containers• Appropriate storage conditions and time

Protect sample integrity

Field sampling design and execution is the first consideration for consistency of data for a source or site

Bulk Prep

Preparation of field samples for laboratory use and storage

Drying

Field moistAir dry (60°C)

Oven dry (105°C)

Sieving

< 6mm < 2mm

< 63 um

Splitting/Subsampling

Appropriate type of subsample;

Appropriate amount of subsample;Representative

subsample

Grinding

Break up clumps;Pulverize to defined

size

Archiving

As-received;Prepped

Particle Size Fractionsfor Metals Analysis

Terrestrial Soils(< 2 mm)

Aquatic Sediment(< 63 um)

Particle size chart from Carter, 2008; Figure 55.1

Analytical Prep

Extract/Dissolve Analytes from Solid Phase into Aqueous

Solution

Salinity Trace Metals

Special Techniques

Salinity

• Defined in Carter, Handbook 60 and SSSA• Well-defined procedure:• Add water to saturation• Stand > 4 hr• Vacuum-filter

• Saturation % = 100 * Wwater / Wsoil• Analytical results conventionally reported as mg/L

Saturation Extract

• 1:1 or 1:5 most commonly listed • When unable to prepare a Saturation Extract (type

or amount of sample)• Report as mg/kg(?)

Other Extraction

Ratios

Tessier Model -Metals Fractions in Sediments

• MgCl2, pH 7.0Exchangeable

• NaOAC/HOAc, pH 5.0Bound to Carbonates• NH2OH·H2O in 25% HOAc,

pH~2Bound to Fe-Mn Oxides

• H2O2/HNO3, pH~2 (+NH4OAc)Bound to Organic Matter

• HF and HClO4ResidualAnalytical Chemistry 51(7) June 1979

Tessier Model – Extension to Terrestrial Soils(?)

• WaterSoluble

• MgCl2, pH 7.0Exchangeable

• NaOAC/HOAc, pH 5.0Bound to Carbonates• NH2OH·H2O in 25% HOAc,

pH~2Bound to Fe-Mn Oxides

• H2O2/HNO3, pH~2(+NH4OAc)Bound to Organic Matter

• HF and HClO4Residual

Metals Fractionsof Environmental Significance

Soluble

Exchangeable

Bound to Carbonates

Bound to Fe-Mn Oxides

Bound to Organic Matter

Residual

‘Environmentally Available’

or‘Total Recoverable’

metals

Standard Analytical Prep Procedures for Trace Metals

EPA 200.2 EPA 3050B ICPMS/GFAA

EPA 3050B ICPAES/FLAA

EPA 3050B* ICPAES/FLAA

Nominal Sample Mass (g - dry wt.) 1 1 1 1

1st Acid Addition

HNO3 Concentration 1 + 1 1 + 1   ConcHNO3 Volume (mL) 4 10   2.5HCl Concentration 1 + 4   Conc Conc

HCl Volume (mL) 10   10 10Temperature (°C) ~95 95 ± 5 95 ± 5 95 ± 51st Heating Time (min)   10 to 15    2nd Acid Addition

HNO3 Concentration   Conc    HNO3 Volume (mL)   5    

Reflux Time (min) 30'heat and add acid until no change in

appearance'15 15

Evaporate to (mL)   ~5    

Peroxide Addition

30% H2O2 (mL)   3    DIW Volume (mL)   2    

Additional 30% H2O2  1-mL increments until

minimal effervescence, to maximum 10 mL total

   

Evaporate to (mL)   ~5    

First Filter Treatment

Hot Conc HCl Rinse (mL)       ≤ 5Hot DIW Rinse (mL)       20

Conc. HCl Digest Acid (mL)       5

Reflux Time (min)       "until the filter paper dissolves"

Final Acid Addition

Condition       "If a precipitate forms"

Conc. HCl (mL)       10Final Digest Volume (mL) 100 100 100 100Remove solids (if present) after or before Final Volume adjustment? after before before before

* EPA 3050B states "may be used to improve the solubilities and recoveries of antimony, barium, lead, and silver when necessary. These steps are optional and are not required on a routine basis"

Applicable MetalsEPA 200.2 EPA 3050B

ICPMS/GFAAEPA 3050B

ICPAES/FLAAAluminum (Al) X XAntimony (Sb) X XArsenic (As) X X Boron (B) X Barium (Ba) X XBeryllium (Be) X X XCadmium (Cd) X X XCalcium (Ca) X XChromium (Cr) X X XCobalt (Co) X X XCopper (Cu) X XIron (Fe) X X XLead (Pb) X X XLithium (Li) X Magnesium (Mg) X XManganese (Mn) X XMercury (Hg) X Molybdenum (Mo) X X XNickel (Ni) X XPhosphorus (P) X Potassium (K) X XSelenium (Se) X X Silica (SiO2) Silver (Ag) X XSodium (Na) X XStrontium (Sr) X Thallium (Tl) X X XThorium (Th) X Tin (Sn) X Uranium (U) X Vanadium (V) X XZinc (Zn) X X

Analytical Prep

Special Techniques

Hot-Water Extraction

Boron

Solvent Extraction

Sulphur

Fusion

Barite-Barium Silicon

Analysis

• Multi-element• Wide dynamic range

• % to sub-ppt• Manageable interferences

Why ICP?

• ICP = Inductively-Coupled Plasma• For metals analysis, typically an argon plasma

sustained in a radio-frequency electromagnetic field• Serves to atomize and ionize the sample• Detection of the atoms/ions define the technique

(AES or OES; MS or CRC-MS; HR-MS)

What is

“ICP”?

Anatomy of a Plasma

1Spectroscopy 16(6) June 2001

Plasma Formation1

Plasma Temperature Zones1

2Perkin-Elmer, 1997

Droplet Conversion in the ICP Source2

Basic ICP Instrument Architecture

Perkin-Elmer, 1997

Emission Spectroscopy

Light Spectra

White Light (sunlight)

Emission Spectrum of Iron (Fe)

Grating Equation

Palmer, C. and Loewen, E., Diffraction Grating Handbook, sixth edition; Newport Corporation, 2005.

mλ = d (sinα + sinβ), where m is the spectral order (an integer) and λ is the wavelength

Rowland Circle Polychromator

Perkin-Elmer, 1997

Spectral Overlap

Palmer, C. and Loewen, E., Diffraction Grating Handbook, sixth edition; Newport Corporation, 2005.

Echelle Optical Mount

Perkin-Elmer, 1997

2-D Spectral Array

Perkin-Elmer, 1997

Atomic Mass Spectra

40 91.22

ZrZirconium

Relative Abundance of the Natural Isotopes

Spectroscopy 17(10) October 2002; Perkin-Elmer

Relative Abundance of the Natural Isotopes

Spectroscopy 17(10) October 2002; Perkin-Elmer

Elements analyzed by ICPMS

http://www.perkinelmer.ca/EN-CA/CMSResources/Images/44-74849tch_icpmsthirtyminuteguide.pdf

Plasma – Mass Spectrometer Interface

Spectroscopy 16(7) July 2001

Quadrupole Spectrometer

Spectroscopy 16(10) October 2001

Conventional ICPMS

Spectroscopy 17(2) February 2002 (edited graphic)

Collision/Reaction Cell ICPMS

Spectroscopy 17(2) February 2002

Collision/Reaction Cell ICPMS

Spectroscopy 17(2) February 2002

Hi-Resolution ICPMS

Spectroscopy 16(11) November 2001

Hi-Resolution ICPMS

Name Symbol MassAbundance

(%) Name Symbol MassAbundance

(%)

Oxygen 16O 15.994915 99.757 Iron 54Fe 53.939615 5.84517O 16.999132 0.038 56Fe 55.934942 91.75418O 17.99916 0.205 57Fe 56.93540 2.119

58Fe 57.93328 0.282Chlorine 35Cl 34.968853 75.78

37Cl 36.96590 24.22 Arsenic 75As 74.92160 100

Argon 36Ar 35.967546 0.336538Ar 37.962732 0.063240Ar 39.962383 99.6003

40Ar16O 55.957298 40Ar16O - 56Fe = 0.022356

40Ar35Cl 74.931236 40Ar35Cl - 75As = 0.009640

Hi-Resolution ICPMS

Spectroscopy 16(11) November 2001

Interferences in ICP

• Affects how much of the sample gets to the plasma or spectrometer

• Differences in viscosity, surface tension, TDS yield variations in solution transport and nebulization

Physical

• Affects the nature of the plasma or the analytes in the plasma

• Molecular compound formation; ionization; solute vaporization

Chemical

• Affects the intensity of the analyte signal reaching the detector

• Background shifts• Overlapping wavelengths/masses

Spectral

Addressing Interferences

• Matrix-matching of standards with samples• Use of internal standard(s)• Water-saturated nebulization gas (prevent salt

build-up)• Dilution

Physical

• (Tend not to be prevalent in ICP owing to high energy)

• Optimization of operating conditions• Matrix-matching of standards with samples• Use of internal standard(s)• Dilution

Chemical

Physical Effects in ICPMS -Space-Charge Interference

Spectroscopy 16(9) September 2001

Addressing Interferences

Spectral – AES

• Matrix-matching• Off-peak correctionBackground

• Alternate wavelength• Inter-element correctionOverlap

Correcting AES Spectral Interference

Off-peak background correction Inter-element correction

where,

determined previously from standards

http://inorganicventures.com/tech/icp-operations/spectral-interference-correction/correction-icp-oes

Types of MS Spectral Overlap

• Two isotopes of same mass• e.g. 40Ar on 40CaIsobaric

• Molecular ions formed in the plasma• e.g. 40Ar35Cl on 75AsPolyatomic

• Mass discrimination based on m/z• e.g. 136Ba++ on 68Zn+

Double-charged

ions

Addressing Interferences

Spectral – MS

• Matrix-matchingBackground

• Alternate mass• Mass equation• Collision/Reaction• Increase resolution

Overlap

Correcting MS Spectral Interference

Alternate Mass Mass equation

where,

Spectroscopy 17(10) October 2002; Perkin-Elmer

Correcting MS Spectral Interference

Collision/Reaction Increase Resolution

http://inorganicventures.com/tech/icp-operations/spectral-interference-correction/correction-icp-oes

Calculation and Reporting

• Dilutions• Alternate lines• Correction equations

Method modifications

• mg/L to mg/kg (as required)Units

• As-received• Air-dried basis• Oven-dried basis

Moisture-basis

Summary

The final result of an analytical measurement depends on a series of numerous decisions for processing the sample, each of which has an influence on the magnitude of that result

Consistency of results within or between sampling events depends on the consistency of the processes applied to the sample

ICP-based analytical techniques are valuable tools for measuring metals in soils but require understanding of their behaviour in the given sample matrix

References• Boss, C.B. and Fredeen, K.J.; Concepts, Instrumentation and Techniques in

Inductively Coupled Plasma Optical Emission Spectrometry, Second Edition; The Perkin-Elmer Corporation, 1997.

• Carter, M.R. and Gregorich, E.G., eds., Soil Sampling and Methods of Analysis, Second Edition; Canadian Society of Soil Science, 2008.

• Palmer C. and Loewen, E., Diffraction Grating Handbook, sixth edition; Newport Corporation, 2005.

• Spectroscopy magazine and www.spectroscopyonline.com• Richards, L.A., ed., Agriculture Handbook No. 60: Diagnosis and

Improvement of Saline and Alkali Soils; USDA, 1954.• Tessier, A., Campbell, P.G.C., Bisson, B., Sequential Extraction Procedure for

the Speciation of Particulate Trace Metals; Analytical Chemistry: 15(7), June 1979, pp. 844-851.

Questions

?