3125 METALS BY INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY*

3125 A. Introduction

1. General Discussion

This method is designed to determine trace metals and metalloids in surface, ground, and drinking waters via inductively

coupled plasma mass spectrometry (ICP–MS). Although best

suited for ambient or pristine freshwater matrices, this method

can also be used to analyze wastewater, soils, sediments, sludge,

and biological samples after appropriate digestion followed by

dilution and/or cleanup to reduce matrix effects to a manageable

level.1,2 Various cleanup techniques are available to reduce

matrix interferences and/or concentrate analytes of interest.3–7

For many analytes, the instrument detection limits (IDLs) are

between 1 and 100 ng/L. The quadrupole-based ICP–MS may

include collision cell technology (CCT) and/or dynamic reaction

cell (DRC), which remove the need for mathematical interference correction for many elements. Additional data (Tables

3125:I and 3125:II) demonstrate the performance for elements

seriously affected by polyatomic interferences.8,9

The method is intended to be performance-based, so the elemental analyte list can be extended, new “clean” preparation

techniques can be implemented, and other appropriate modifications can be made as technology evolves. Any modifications to

the base method must be validated via suitable quality control

standards. More sources of information on quality assurance and

other aspects of ICP–MS analysis of metals are available.10–12

Ideally, the analysts who use this method will have experience

using ICP–MS, interpreting spectral and matrix interferences,

and implementing corrective procedures. Before generating data,

analysts should demonstrate their proficiency in this method by

successfully analyzing a performance evaluation sample for each

matrix type.

2. References

1. MONTASER, A. & D.W. GOLIGHTLY, EDS. 1992. Inductively Coupled

Plasmas in Analytical Atomic Spectrometry, 2nd ed. VCH Publishers, Inc., New York, N.Y.

2. DATE, A.R. & A.L. GRAY. 1989. Applications of Inductively Coupled Plasma Mass Spectrometry. Blackie & Son, Ltd., Glasgow,

U.K.

3. MCLAREN, J.W., A.P. MYKYTIUK, S.N. WILLIE & S.S. BERMAN. 1985.

Determination of trace metals in seawater by inductively coupled

* Approved by Standard Methods Committee, 2009.

Joint Task Group: Robert Henry (chair), Christopher J. Baggett, Alois F. Clary,

William R. Kammin, Gregg Oelker, Cindy A. Ziernicki.

TABLE 3125:I. METHOD PERFORMANCE WITH CALIBRATION VERIFICATION

STANDARDS*†

Element Mass

Mean

Recovery

%

Mean

ng/mL

SD

ng/mL

RSD

%

V 51 100.00 200.0 9.25 4.62

V CCT 51 100.44 200.9 7.30 3.63

Cr 52 99.56 199.1 9.39 4.72

Cr CCT 52 100.56 201.1 7.16 3.56

Mn 55 101.36 1014 54.09 5.34

Mn CCT 55 98.28 982.8 31.95 3.25

Fe 56 101.84 10184 566.43 5.56

Fe CCT 56 100.25 10025 336.29 3.35

Co 59 101.58 20.32 0.89 4.38

Co CCT 59 101.67 20.33 0.64 3.15

Ni 60 101.08 202.2 9.06 4.48

Ni CCT 60 101.90 203.8 6.61 3.24

Cu 63 101.76 1018 62.46 6.14

Cu CCT 63 105.63 1056 36.79 3.48

Cu 65 102.12 1021 56.06 5.49

Zn 66 100.52 1005 47.63 4.74

Zn CCT 66 99.51 995.1 33.94 3.41

As 75 99.54 19.91 1.02 5.14

As CCT 75 98.24 19.65 0.78 3.99

Se 78 102.77 20.55 1.04 5.07

Se CCT 78 104.78 20.96 1.73 8.26

* Single-laboratory, single-operator data acquired 12-2005 using Thermo Electron

X Series in Standard and CCT-KED modes.

† Continuing Calibration Verification Standard (N  28)

TABLE 3125:II. METHOD PERFORMANCE WITH STANDARD REFERENCE

WATER*†

Element Mass

Mean

Recovery

%

Mean

ng/mL

SD

ng/mL

RSD

%

V 51 95.49 12.40 0.62 5.01

V CCT 51 101.60 13.20 0.32 2.39

Cr 52 93.34 36.03 0.80 2.21

Cr CCT 52 95.31 36.79 0.89 2.42

Mn 55 99.09 120.40 2.25 1.87

Mn CCT 55 96.99 117.8 2.18 1.85

Fe 56 110.45 37.89 4.32 11.41

Fe CCT 56 102.20 35.06 1.78 5.09

Co 59 98.43 19.96 0.32 1.62

Co CCT 59 99.22 20.12 0.36 1.79

Ni 60 98.78 27.06 0.68 2.51

Ni CCT 60 100.69 27.59 0.50 1.80

Cu† 63 115.36 98.29 3.68 3.74

Cu CCT‡ 63 122.91 104.7 2.81 2.68

Cu‡ 65 118.83 101.2 2.84 2.80

Zn 66 95.88 51.01 1.36 2.67

Zn CCT 66 97.64 51.95 1.91 3.68

As 75 98.61 26.30 0.48 1.81

As CCT 75 98.38 26.24 0.59 2.26

Se 78 95.83 21.04 0.45 2.15

Se CCT 78 97.58 21.43 1.86 8.66

* Single-laboratory, single-operator data acquired 12-2005 using Thermo Electron

X Series in Standard and CCT-KED modes.

† NIST 1640 (N  7)

‡ Copper contamination

1
plasma mass spectrometry with preconcentration on silica-immobilized 8-hydroxyquinoline. Anal. Chem. 57:2907.

4. BURBA, P. & P.G. WILLMER. 1987. Multielement preconcentration

for atomic spectroscopy by sorption of dithiocarbamate metal complexes (e.g., HMDC) on cellulose collectors. Fresenius Z. Anal.

Chem. 329:539.

5. WANG, X. & R.M. BARNES. 1989. Chelating resins for on-line flow

injection preconcentration with inductively coupled plasma atomic

emission spectroscopy. J. Anal. Atom. Spectrom. 4:509.

6. SIRIRAKS, A., H.M. KINGSTON & J.M. RIVIELLO. 1990. Chelation ion

chromatography as a method for trace elemental analysis in complex environmental and biological samples. Anal. Chem. 62:1185.

7. PUGET SOUND WATER QUALITY AUTHORITY. 1996. Recommended

Guidelines for Measuring Metals in Puget Sound Marine Water,

Sediment and Tissue Samples. Appendix D: Alternate Methods for

the Analysis of Marine Water Samples. Puget Sound Water Quality

Authority, Olympia, Wash.

8. NELMS, S., ED. 2005. Inductively Coupled Plasma Mass Spectrometry Handbook. Blackwell Science, Cambridge, Mass.

9. BECKER, J.S. 2008. Inorganic Mass Spectrometry: Principles and

Applications. Wiley Interscience, Malden, Mass.

10. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1994. Determination of

trace elements in waters and wastes by inductively coupled plasma–

mass spectrometry, Method 200.8. U.S. Environmental Protection

Agency, Environmental Monitoring Systems Laboratory, Cincinnati, Ohio.

11. LONGBOTTOM, J.E., T.D. MARTIN, K.W. EDGELL, S.E. LONG, M.R.

PLANTZ & B.E. WARDEN. 1994. Determination of trace elements in

water by inductively coupled plasma–mass spectrometry: collaborative study. J. AOAC Internat. 77:1004.

12. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1995. Method 1638: Determination of trace elements in ambient waters by inductively

coupled plasma–mass spectrometry. U.S. Environmental Protection

Agency, Off. Water, Washington, D.C.

3125 B. Inductively Coupled Plasma Mass Spectrometry (ICP–MS) Method

1. General Discussion

a. Principle: In this method, analysts introduce sample material to an argon-based, high-temperature radio-frequency

plasma, usually via pneumatic nebulization. As energy transfers

from the plasma to the sample stream, the target element desolves, atomizes, and ionizes. The resulting ions are extracted

from the plasma through a differential vacuum interface and

separated based on their mass-to-charge (m/z) ratio by a mass

spectrometer. Typically, either a quadrupole (with or without

CCT or DRC) or magnetic sector (high-resolution) mass spectrometer is used. An electron multiplier detector counts the

separated ions, and a computer-based data-management system

processes the resulting information.

b. Applicable elements and analytical limits: This method has

been demonstrated to be suitable for aluminum, antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper,

lead, manganese, molybdenum, nickel, selenium, silver, strontium, thallium, uranium, vanadium, and zinc (see Table 3125:

III). It is also acceptable for other elemental analytes as long as

the same quality assurance practices are followed, with documented acceptance limits.

Before implementing the method, determine the instrument

detection limit (IDL) and method detection limit (MDL) for all

analytes (see Section 1030C). Typical IDLs for method analytes

are presented in Table 3125:III.1,2

Determining the MDL for each element is critical when analyzing complex matrices, such as seawater, brines, and industrial

effluents. In these cases, the MDL typically will be higher than

the IDL because of background levels of analytes introduced

during sample preparation, as well as laboratory-derived contamination and matrix-based interferences. Determine both IDL

and MDL when first implementing this method, and then repeat

yearly or whenever the instrument configuration changes or

major maintenance occurs, whichever happens first.

Also, determine the linear dynamic range (LDR) for all

method analytes, including multi-element mixtures (to account

for possible inter-element effects). LDR is the maximum analyte

concentration above the highest calibration point at which the

analyte response is within 10% of its theoretical response.

When determining LDRs, avoid using unnecessarily high analyte

concentrations because they might damage the detector. Determine LDR when first implementing this method, and then repeat

yearly.

c. Interferences: ICP–MS is subject to several types of interferences, including the following:

1) Isobars: Isobars are isotopes of different elements that form

ions with the same nominal atomic mass units/charge number

(m/z) ratio that cannot be resolved by a quadrupole or highresolution mass spectrometer. Typically, ICP–MS operating

software includes all known isobaric interferences and will perform the necessary calculations automatically (see Table 3125:

IV.) Monitor 83Kr, 99Ru, 118Sn, and 125Te to correct for isobaric

interference caused by 82Kr on 82Se, by 98Ru on 98Mo, by 114Sn

on 114Cd, by 115Sn on 115In, and by 123Te on 123Sb. Monitor ArCl

at mass 77 to estimate chloride interferences. Verify that all

elemental and molecular correction equations used in this

method are correct and appropriate for the mass spectrometer

used and the sample matrix.

2) Abundance sensitivity: Abundance sensitivity is the possibility that the low and high “wings” of any abundant mass peak

will contribute to or obscure adjacent masses. Adjust the mass

spectrometer resolution and quadrupole pole bias to minimize

these interferences.

3) Polyatomics: Polyatomics are (molecular) ion interferences

caused by ions with more than one atom that have the same

nominal m/z ratio as the isotope of interest. Most of the common

molecular ion interferences have been identified (see Table

3125:V). Because of the severity of chloride ion interference on

important analytes, particularly vanadium and arsenic, hydrochloric acid is not recommended for use in ICP–MS sample

preparation. Because most environmental samples contain some

chloride ion, analysts must use chloride-correction equations for

affected masses. Collision cell technology and dynamic reaction

INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY (3125)/ICP–MS Method

2
cell effectively reduce most polyatomic species to analytically

negligible levels in quadrupole-based ICP–MS systems, sometimes removing the need for complex correction equations. A

high-resolution ICP–MS resolves many—but not all—interferences caused by polyatomic ions. Polyatomic interferences are

strongly influenced by instrument design and plasma operating

conditions; they sometimes can be reduced by carefully adjusting nebulizer gas flow and other instrument operating parameters.

4) Doubly-charged: Some elements (e.g., barium and strontium) form significant levels of M2 ions under normal plasma

conditions. The M2 ions occur in the mass spectrum at M/2 and,

in the case of Ba and Sr, will interfere with some isotopes of zinc

and calcium, respectively.

5) Physical interferences: These include differences in viscosity, surface tension, and dissolved solids between samples and

calibration standards. To minimize these effects, analytical samples should not contain more than 0.5% of dissolved solids.

Dilute water and wastewater samples with higher dissolved

solids levels before analyzing them. Use internal standards to

correct for physical interferences, provided that their analytical

behavior is comparable to the elements being determined.

Table 3125:IV.B includes information on each isotope and

cautions on using correction equations. Quadrupole ICP–MS

with CCT or DRC is recommended for all analytes, except

perhaps beryllium (due to lower sensitivity), because it eliminates many of the equations typically needed in the standard

operating mode.

6) Memory interferences: These occur when analytes from a

previous sample or standard are measured in the current sample.

Use a long enough rinse (flush) between samples to minimize

such interferences. Persistent memory interferences may indicate

problems in the sample-introduction system. Severe memory

interferences may require analysts to disassemble and clean the

entire sample-introduction system, including the plasma torch

and the sampler and skimmer cones.

7) Ionization interferences: These result when moderate (0.1%

to 1%) amounts of a matrix ion change the analyte signal. This

TABLE 3125:III. RECOMMENDED ANALYTE MASSES, INSTRUMENT

DETECTION LIMITS (IDL), AND INTERNAL STANDARDS

Element

Analytical

Mass

IDL*

g/L

Recommended

Internal

Standard

Be 9 0.025 Li

Al 27 0.03 Sc

V 51 0.02 Sc

Cr 52 0.04 Sc

Cr 53 0.03 Sc

Mn 55 0.002 Sc

Co 59 0.002 Sc

Ni 60 0.004 Sc

Ni 62 0.025 Sc

Cu 63 0.003 Sc

Cu 65 0.004 Sc

Zn 66 0.017 Ge

Zn 68 0.020 Ge

As 75 0.025 Ge

Se 77 0.093 Ge

Se 82 0.064 Ge

Ag 107 0.003 In

Ag 109 0.002 In

Cd 111 0.006 In

Cd 114 0.003 In

Sb 121 0.07 In

Sb 123 0.07 In

Tl 203 0.03 Th

Tl 205 0.03 Th

Pb 208 0.005 Th

U 235 0.032 Th

U 238 0.001 Th

Mo 98 0.003† In

Ba 135 0.008† In

Sr 88 0.001‡ In

* IDLs were determined on a Perkin Elmer Elan 6000 ICP–MS using seven

replicate analyses of a 1% nitric acid solution, at Manchester Environmental

Laboratory, July 1996.

† From EPA Method 200.8 for the Analysis of Drinking Waters—Application

Note, Order No. ENVA-300A, The Perkin Elmer Corporation, 1996.

‡ From Perkin Elmer Technical Summary TSMS-12.

TABLE 3125:IV.A. ELEMENTAL ABUNDANCE EQUATIONS AND COMMON

MOLECULAR ION CORRECTION EQUATIONS

Elemental and Molecular Equations*†

Li 6  C 6

Be 9  C 9

Al 27  C 27

Sc 45  C 45

V 51  C 51 – (3.127)[(C 53) – (0.113  C 52)]

Cr 52  C 52

Cr 53  C 53

Mn 55  C 55

Co 59  C 59

Ni 60  C 60

Ni 62  C 62

Cu 63  C 63

Cu 65  C 65

Zn 66  C 66

Zn 68  C 69

As 75  C 75 – (3.127)[(C 77) – (0.815  C 82)]

Se 77  C 77

Se 82  C 82 – (1.008696  C 83)

Sr 88  C 88

Mo 98  C 98 – (0.110588  C 101)

Rh 103  C 103

Ag 107  C 107

Ag 109  C 109

Cd 111  C 111 – (1.073)[(C 108) – (0.712  C 106)]

Cd 114  C 114 – (0.02686  C 119)

Sb 121  C 121

Sb 123  C 123 – (0.127189  C 125)

Ba 135  C 135

Ho 165  C 165

Tl 203  C 203

Tl 205  C 205

Pb 208  C 208  (1  C 206)  (1  C 207)

Th 232  C 232

U 238  C 238

* C  calibration blank-corrected counts at indicated masses.

† From EPA Method 200.8 for the Analysis of Drinking Water—Application

Note, Order No. ENVA-300A, The Perkin Elmer Corporation, 1996.

INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY (3125)/ICP–MS Method

3
TABLE 3125:IV.B. ELEMENTS, MASSES, ABUNDANCES, AND CORRECTION EQUATIONS (UPDATED 2008)

Isotope Mass Abundance

Interference Calculation

(if required)* Comments

Li (IS)† 6 7.52 C 6 - 0.08131  Li7 Corrects for natural lithium in samples (I)‡

Be 9 100 C 9

C 13 1.108 C 13 For ArC52 correction

Al 27 100 C 27

Ca 43 0.13 C 43 - 0.0004  Sr88 Corrects for Sr2§

Sc (IS) 45 100 C 45

V 51 99.76 C 51 - 3.1270  ClO53 Corrects for variable chloride matrix (I) [not normally required in CCT

Mode]

Cr 52 83.76 C 52 - 0.0900  C13 Corrects for variable carbon content (P) [not normally required in CCT

Mode]

ClO (Cr) 53 C 53 - 0.1140  Cr52 CLO interference corrected for chromium (I)

Mn 55 100 C 55

Fe 56 91.52 C 56 CCT Mode only (ArO and CaO interferences in Standard Mode)

Co 59 100 C 59

Ni 60 26.16 C 60 - 0.00150  Ca43 Corrects for CaO (P) [not normally required in CCT Mode]

Ni 62 3.66 C 62

Cu 63 69.09 C 63 Possibility of ArNa (P) in high sodium matrix (sea or brackish waters)

Cu 65 30.91 C 65

Zn 66 27.81 C 66

Zn 68 18.56 C 68 - 0.0153  Ba135 Ba2 interferes with Zn68

Ge (IS) 72 27.43 C 72 Possibility of FeO (P) interference in high iron matrix

As 75 100 C 75 - 3.1270  ArCl77 Corrects for variable chloride matrix (I) Correction not required in CCT

Mode (I)

ArCl (Se 77) 77 C 77 - 0.8484  Se82 ArCl interference corrected for Se.

Se 78 23.61 C 78 CCT Mode only due to ArAr78 in Standard Mode

Se 82 8.84 C 82 - 1.0009  Kr83 Corrects for krypton in argon (I)

Kr 83 11.55 C 83 Variable levels in argon

Sr 88 82.56 C 88 Forms Sr2 easily

Mo 95 14.78 C 95 No isobaric correction required (unlike Mo98)

Mo 98 24 C 98 - 0.1307  Ru101 Corrects for variable ruthenium content (I)

Ru 101 16.98 C 101 For Mo98 correction

Rh (IS) 103 100 C 103

Ag 107 51.35 C 107

Ag 109 48.65 C 109

Cd 111 12.86 C 111 - 0.0017  Mo95 Corrects for MoO (P) [not normally required in CCT Mode]

Cd 114 28.81 C 114 - [0.0271  Sn118] -

[0.0028  Mo95]

Correction for MoO (P) and Sn (I) [MoO correction not normally

required in CCT Mode]

In (IS) 115 95.84 C 115 - 0.0142  Sn118 Corrects for variable tin content (I)

Sn 118 24.01 C 118 For Cd114 and In115 correction

Sb 121 57.25 C 121

Sb 123 42.75 C 123 - 0.0449  Te125 Corrects for variable tellurium content (I)

Te 125 6.99 C 125 For Sb123 correction

Ba 135 6.59 C 135

Tb (IS) 159 100 C 159

Ho (IS) 165 100 C 165 Ho2 may interfere with Se82

Tl 203 29.5 C 203

Tl 205 70.5 C 205

Pb 206 25.15 C 206

Pb 207 21.11 C 207

Pb 208 52.38 C 208  1  Pb206  1 

Pb207

Correction for variable lead abundance (I)

Bi (IS) 209 100 C 209

U 235 0.715 C 235

U 238 99.28 C 238

* All corrections should be verified and corrected for mass bias.

† IS denotes an internal standard element.

‡ I denotes an isobaric correction.

§ Doubly charged corrections may vary with plasma tuning conditions and should be verified by the user before use.

 P denotes correction that may vary with plasma tuning conditions and should be verified by the user before use.

INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY (3125)/ICP–MS Method

4
effect, which usually reduces the analyte signal, also is known as

suppression. Correct for suppression by using internal standardization techniques.

2. Apparatus

a. Inductively coupled plasma–mass spectrometer: Available

from several manufacturers, this instrument includes a mass

spectrometer, detector, an ICP source, mass flow controllers for

regulating ICP gas flows, a peristaltic pump for introducing

samples, and a computerized data acquisition and instrument

control system. An x-y autosampler also may be used with

appropriate control software.

b. Laboratory ware: Use pre-cleaned plastic laboratory ware

for standard and sample preparation. Teflon*—either tetrafluoroethylene hexafluoropropylene-copolymer (FEP), polytetrafluoroethylene (PTFE), or perfluoroalkoxy PTFE (PFA)—is preferred for standard preparation and sample digestion, while highdensity polyethylene (HDPE) and other dense, metal-free

plastics may be acceptable for internal standards, known-addition solutions, etc. Check each new lot of autosampler tubes for

suitability, and pre-clean autosampler tubes and pipet tips if

certificate of analysis is unavailable (see Section 3010C.2).

c. Air-displacement pipets, sized 10 to 100 L, 100 to 1000

L, and 1 to 10 mL.

d. Analytical balance, accurate to 0.1 mg.

e. Sample-preparation apparatus, such as hot plates, microwave digesters, and heated sand baths. Any sample-preparation

device could introduce trace levels of target analytes to the

sample.

f. Clean hood (optional), Class 100 (certified to contain less

than 100 particles/m3

), for sample preparation and manipulation.

If possible, perform all sample manipulations, digestions, dilutions, etc. in a certified Class 100 environment. Alternatively,

handle samples in glove boxes, plastic fume hoods, or other

environments that minimize random contamination by trace metals.

3. Reagents

a. Acids: Use ultra-high-purity grade (or equivalent) acids to

prepare standards and process samples. Redistilled acids are

acceptable if each batch is demonstrated to be uncontaminated

by target analytes. Use extreme care when handling acids in the

laboratory to avoid contaminating them with trace levels of

metals.

1) Nitric acid (HNO3) conc (specific gravity 1.41).

2) Nitric acid, 1  1: Add 500 mL conc HNO3 to 500 mL

reagent water.

3) Nitric acid, (v/v) 2%: Add 20 mL conc HNO3 to 100 mL

reagent water; dilute to 1000 mL.

4) Nitric acid, (v/v) 1%: Add 10 mL conc HNO3 to 100 mL

reagent water; dilute to 1000 mL.

b. Reagent water: When preparing blanks, standards, and

samples, use metal-free water that was prepared using the methods listed in Section 1080, described below, or provided elsewhere (if they have been proven effective). Use only high-purity

* Or equivalent.

TABLE 3125:V. COMMON MOLECULAR ION INTERFERENCES IN ICP–MS1

Molecular Ion Mass

Element Measurement

Affected by Interference

Background molecular ions:

NH 15 —

OH 17 —

OH2

 18 —

C2

 24 Mg

CN 26 Mg

CO 28 Si

N2

 28 Si

N2H 29 Si

NO 30 —

NOH 31 P

O2

 32 S

O2H 33 — 36ArH 37 Cl 38ArH 39 K 40ArH 41 —

CO2

 44 Ca

CO2

H 45 Sc

ArC, ArO 52 Cr

ArN 54 Cr

ArNH 55 Mn

ArO 56 Fe

ArH 57 Fe 40Ar36Ar 76 Se 40Ar38Ar 78 Se 40Ar2

 80 Se

Matrix molecular ions:

Bromide: 81BrH 82 Se 79BrO 95 Mo 81BrO 97 Mo 81BrOH 98 Mo

Ar81Br 121 Sb

Chloride: 35ClO 51 V 35ClOH 52 Cr 37ClO 53 Cr 37ClOH 54 Cr

Ar35Cl 75 As

Ar37Cl 77 Se

Sulfate: 32SO 48 Ti 32SOH 49 — 34SO 50 V, Cr 34SOH 51 V

SO2

, S2 64 Zn

Ar32S 72 Ge

Ar34S 74 Ge

Phosphate:

PO 47 Ti

POH 48 Ti

PO2

 63 Cu

ArP 71 Ga

Group I & II metals:

ArNa 63 Cu

ArK 79 Br

ArCa 80 Se

Matrix Oxides*

TiO 62-66 Ni, Cu, Zn

ZrO 106-112 Ag, Cd

MoO 108-116 Cd

NbO 109 Ag

* Oxide interferences normally will be tiny and will affect method elements only

when oxide-producing elements are present at relatively high concentrations, or

when the instrument is improperly tuned or maintained. Preferably monitor Ti and

Zr isotopes in soil, sediment, or solid waste samples, because these samples could

contain high levels of such interfering elements.

INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY (3125)/ICP–MS Method

5
water to prepare samples and standards. Reagent water containing trace amounts of analyte elements will skew results.

Analysts can produce suitable reagent water using a multistage system that includes a softener/reverse osmosis unit, a

dual-column, strong acid/strong base ion exchange unit; an activated carbon filter; and a UV sterilization process. This will

produce a polished water free of metals and organics.

c. Stock, standard, and other required solutions: See Sections

3111B, 3111D, 3114B, and 3120B.3d on preparing standard

stock solutions from elemental materials (pure metals, salts).

Preferably, purchase high-purity, commercially prepared stock

solutions and dilute to required concentrations. Single- or multielement stock solutions (1000 mg/L) of the following elements

are required: aluminum, antimony, arsenic, barium, beryllium,

cadmium, cerium, chromium, cobalt, copper, germanium, indium, lead, magnesium, manganese, molybdenum, nickel, rhodium, scandium, selenium, silver, strontium, terbium, thallium,

thorium, uranium, vanadium, and zinc. Prepare internal-standard

and target-element stock solutions separately; they could be

incompatible and cause precipitation or other solution instability.

1) Internal standard stock solution: Germanium, indium, lithium, scandium, and thorium are suggested as internal standards.

The following masses are monitored: 72Ge, 115In, 6

Li, 45Sc, and 232Th. Add enough internal standard to all samples, standards,

and quality control (QC) samples to give a suitable counts/

second (cps) signal and stability (100 000 to 300 000 cps for

most internal standards). Minimize dilution-related errors by

using an appropriately high concentration of internal standard

mix solution. Maintain volume ratio for all internal standard

additions.

Prepare internal standard mix as follows: Prepare a nominal

50-mg/L solution of 6

Li by dissolving 0.15 g 6Li2CO3 (isotopically pure, i.e., 95% or greater purity†) in a minimal amount of

1:1 HNO3. Pipet 5.0 mL 1000-mg/L germanium, indium, scandium, and thorium standards into the lithium solution, dilute

resulting solution to 500.0 mL, and mix thoroughly. The resultant concentration of Ge, In, Sc, and Th will be 10 mg/L.

Determine the internal standard concentrations required to

achieve acceptable levels of precision, and dilute the internal

standard stock accordingly.

Other internal standards, such as bismuth, holmium, rhodium,

terbium, and yttrium, may also be used in this method. Ensure

that the internal standard mix used is stable and that there are no

undesired interactions among elements.

All new sample matrices should be screened for internal

standard elements before analysis. Analyzing a few representative samples for internal standards should be sufficient. Analyze

samples “as received” or “as digested” (before adding internal

standard), then add internal standard mix and re-analyze. Monitor counts at the internal standard masses. If the “as received” or

“as digested” samples show appreciable detector counts (10% or

higher of samples with added internal standard), dilute sample or

use another internal standard. If the response of a sample containing the internal standard is not within 70% to 125% of the

response for a calibration blank with the internal standard, either

dilute the sample before analysis or use another internal standard. During actual analysis, monitor internal standard masses

and note all internal standard recoveries that are more than 125%

of the internal standard response in the calibration blank. Interpret results for these samples with caution.

The internal standard mix may be added to blanks, standards,

and samples via a “Y” connector after the peristaltic pump to

mix it with the sample stream during sample introduction.

2) Instrument optimization/tuning solution, containing beryllium, cadmium, cobalt, copper, germanium, indium, rhodium,

scandium, terbium, thallium, (for sensitivity and stability evaluation), barium (for doubly-charged evaluation), cerium (for

oxide evaluation), magnesium (mass calibration check), and lead

(mass calibration check). Prepare this solution in 2% HNO3. This

mix includes all common elements used to optimize and tune

various ICP–MS operating parameters. It may be possible to use

fewer elements in this solution, depending on the instrument

manufacturer’s recommendations.

3) Calibration standards: A five standard calibration is recommended, from 0 to 100 g/L.‡ Other calibration regimes are

acceptable if the full suite of quality assurance samples and

standards is run to validate any method changes. Fewer standards

may be used, and a two-point blank/mid-range calibration technique commonly used in ICP optical methods should also produce acceptable results. Calibrate all analytes using the selected

concentrations. Prepare all calibration standards and blanks in a

matrix of 2% nitric acid. Add internal standard mix to all

calibration standards to provide appropriate count rates for interference correction. NOTE: Add the same ratio of internal

standard mix to all standards and blanks used in this method.

4) Method blank (MB): A method blank (also known as

reagent blank) is a portion of reagent water (¶ 3b) treated exactly

as a sample, including exposure to all equipment, glassware,

procedures, and reagents. It is used to assess whether analytes or

interferences are present in the analytical process or system. No

MB should contain a warning level of any analyte of interest

(based on the end user’s requirements). Undertake immediate

corrective action for MB measurements above the minimum

reporting level (see Section 3020). Include at least one MB with

each batch of samples prepared. For dissolved samples, take

reagent water through same filtration and preservation processes

used for samples. For samples requiring digestion, process reagent water with the same digestion techniques as samples. Add

internal standard mix to method blank.

5) Calibration verification standard: Prepare a mid-range

standard using a different source than that used for the calibration standards, in 2% HNO3, with equivalent addition of internal

standard.

6) Calibration verification blank: Use 2% HNO3, the same

solution as the zero calibration standard.

7) Laboratory-fortified blank (LFB): The laboratory-fortified

blank (also known as a blank spike) is a method blank that has

been fortified with a known concentration of analyte. It is used

to evaluate ongoing laboratory performance and analyte recovery in a clean matrix. Prepare fortified concentrations approximating the midpoint of the calibration curve (50 ng/mL) or lower

with stock solutions prepared from a different source than that

used to develop working standards. Calculate percent recovery,

plot control charts, and determine control limits for these meas-

† Cambridge Isotope Laboratories or equivalent. ‡ Performance data for the method were obtained with these concentrations.

INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY (3125)/ICP–MS Method

6
urements. Ensure that the LFB meets the method’s performance

criteria when such criteria are specified. Establish corrective

actions to be taken in case the LFB does not satisfy acceptance

criteria. Include at least one LFB with each batch of samples

prepared. This standard, sometimes also called a laboratory

control sample (LCS), is used to validate digestion techniques

and known-addition levels.

8) Reference materials: Use externally prepared reference

material, preferably from National Institute of Standards and

Technology§ (NIST) 1643 series or equivalent.

9) Known-addition solution for samples: Add stock standard

to sample so the volume change is less than 5%. In the absence

of information on analyte levels in the sample, prepare known

additions at around 50 g/L or lower. If analyte concentration

levels are known, add at 50% to 200% of the sample levels. For

samples undergoing digestion, make additions before digestion.

For dissolved metals determinations, make additions after filtration, preferably immediately before analysis.

10) Low-level standards: Use both a 0.3- and a 1.0-g/L

standard when expected analyte concentration is less than 5

g/L. Prepare both standards in 2% nitric acid.

Prepare volumetrically a mixed standard containing the

method analytes at desired concentration(s) (0.30 g/L, 1.0

g/L, or both). Prepare weekly in 100-mL quantities.

d. Argon: Use a prepurified grade of argon unless it can be

demonstrated that other grades can be used successfully. Prepurified argon is usually necessary because technical argon often

contains significant levels of impurities (e.g., carbon and krypton). 82Kr interferes with the determination of 82Se. Monitor 83Kr

at all times.

4. Procedures

a. Sample preparation: See Sections 3010 and 3020 for general guidance and additional specific requirements on sampling

and quality control. See Section 3030E for the recommended

sample-digestion technique for all analytes except silver and

antimony. Use the method in Section 3030F for silver and

antimony, paying special attention to chloride ion interferences

and using all applicable elemental corrections. Alternative digestion techniques and more guidance on sample preparation are

available.3,4

Ideally, use a “clean” environment when handling, manipulating, or preparing samples. Preferably perform all sample manipulations in a Class 100 clean hood or room to minimize potential

contamination artifacts in digested or filtered samples.

b. Instrument operating conditions: Follow manufacturer’s

standard operating procedures for initialization, mass calibration,

gas flow optimization, and other instrument operating conditions. Maintain complete, detailed information on the instrument’s operating status whenever it is used.

c. Analytical run sequence: Table 3125:VI outlines a suggested analytical run sequence, including tuning and optimizing

the instrument, checking reagent blanks, calibrating the instrument, verifying the calibration, analyzing samples, and analyzing

quality control samples and blanks.

d. Instrument tuning and optimization: Follow manufacturer’s

instructions for optimizing instrument performance. The most

important optimization criteria include nebulizer gas flows, detector and lens voltages, radio-frequency forward power, and

mass calibration. Periodically check mass calibration and instrument resolution. Ideally, optimize the instrument to minimize the

formation of oxide and doubly charged species. Measure CeO:

Ce and Ba2:Ba ratios to monitor the formation of oxide and

doubly charged species, respectively. Both ratios should meet

the manufacturer’s criteria before instrument calibration. Monitor background counts at mass 220 after optimization, and compare with manufacturer’s criteria. (See Table 3125:VII for a

summary of method performance criteria related to optimization

and tuning, calibration, and analytical performance.)

e. Instrument calibration: After optimization and tuning, cal-

§ National Institute of Standards and Technology, www.nist.gov ibrate the ICP–MS using an appropriate range of calibration

TABLE 3125:VI. SUGGESTED ANALYTICAL RUN SEQUENCE

Sample Type Comments

Tuning/optimization standard Check mass calibration and

resolution

Tuning/optimization standard Optimize instrument for

maximum rhodium counts

while keeping oxides,

double-charged ions, and

background within

instrument specifications

Rinse —

Reagent blank Check for contamination

Reagent blank Calibration standard blank

5-g/L standard —

10-g/L standard —

20-g/L standard —

50-g/L standard —

100-g/L standard —

Rinse Check for memory

Initial calibration

verification, 50 g/L

Check for calibration

accuracy

Initial calibration blank Check for memory

0.30-g/L standard Low-level calibration

verification

1.0-g/L standard Low-level calibration

verification

External reference material NIST 1643c or equivalent

Continuing calibration

verification

Check for calibration stability

Continuing blank calibration Check for memory

Project sample method blank Check for contamination

Project sample laboratoryfortified blankCheck for spike recovery

Project sample 1-4 Check for possible

interferences

Project sample 5 —

Project sample 5 with known

addition

Check for spike recovery

Project sample 5 duplicate

with known addition

Check for reproducibility

Continuing calibration

verification (every 10

samples)

Check for calibration stability

Continuing calibration blank

(every 10 samples)

Check for memory

INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY (3125)/ICP–MS Method

7
standards. Use appropriate regression techniques to determine

calibration responses for each analyte. For acceptable calibrations, the regression curves’ correlation coefficients are ideally

0.995 or greater.

Immediately after calibration, run initial calibration verification standard, ¶ 3c5); acceptance criteria are 10% of known

analyte concentration. Next, run initial calibration verification

blank, ¶ 3c6); acceptance criteria are ideally the absolute value

of the instrument detection limit for each analyte, but in practice,

the absolute value of the laboratory reporting limit or the

laboratory method detection limit for each analyte is acceptable.

If analyte concentrations are less than 5 g/L, verify low-level

calibration by using a standard at 40% to 50% of the highest

low-level standard.

f. Sample analysis: Ensure that all vessels and reagents are

uncontaminated. During the analytical run, include quality-control analyses according to schedule of Table 3125:VIII or follow

project-specific QA/QC protocols.

Internal standard recoveries must be between 70% and 125%

of internal standard response in the laboratory-fortified blank;

otherwise, dilute sample, add internal standard mix, and reanalyze.

Make known-addition analyses for each matrix in a digestion

or filtration batch.

5. Calculations and Corrections

Configure instrument software to report internal-standardcorrected results. Water sample results should be reported in

micrograms per liter. Report appropriate number of significant

figures.

a. Correction for dilutions and solids: Correct all dilution

results, and raise the related reporting limits accordingly:

RCorr RUncorr  V

Vs/V

where:

RCorr  diluted corrected results, g/L,

RUncorr  uncorrected elemental results, g/L,

V  volume of digestate (after digestion), L, and

Vs  volume of undiluted sample, L.

Use Method 2540B to determine total solids in solid samples,

and report results as micrograms per kilogram, dry weight.

Correct all results for solids content of solid samples. Use the

following equation to correct solid or sediment sample results for

dilution during digestion and moisture content:

Rcorr Runcorr  V

W  % TS/100

where:

Rcorr  corrected result, g/kg,

Runcorr  uncorrected elemental result, g/L,

V  volume of digestate (after digestion), L,

W  mass of the wet sample, kg, and

% TS  percent total solids determined in the sample.

TABLE 3125:VII. SUMMARY OF PERFORMANCE CRITERIA

Performance

Characteristic Criteria

Mass resolution Manufacturer’s specification

Mass calibration Manufacturer’s specification

Ba2/Ba Manufacturer’s specification

CeO/Ce Manufacturer’s specification

Background counts at

mass 220

Manufacturer’s specification

Correlation coefficient  0.995

Calibration blanks  Reporting limit

Calibration verification

standards

10% of true value

Laboratory fortified blank

(control sample)

30% of true value

Precision 20% relative percent difference

for lab duplicates

Known-addition recovery 75-125%

0.3 and 1.0 g/L

standards

Dependent on data quality

objectives

Reference materials Dependent on data quality

objectives

Internal standard response 70-125% of response in

calibration blank with known

addition

TABLE 3125:VIII. QUALITY CONTROL ANALYSES FOR ICP–MS METHOD

Analysis Frequency Acceptance Criteria

Reference material [¶ 3c9] Greater of: once per sample batch, or 5% Dependent on data quality objectives

Preparatory/method blank [¶ 3c4)] Greater of: once per sample batch, or 5% Absolute value of instrument

detection limit; absolute value of

laboratory reporting limit or MDL

is acceptable

Laboratory fortified blank [¶ 3c7)] Greater of: once per sample batch, or 5% 30% of true value

Duplicate known-addition samples Greater of: once per sample batch, or 5% 20% relative percent difference

Continuing calibration verification

standards [¶ 3c5)]

10% 10% of known concentration

Continuing calibration verification

blank [¶ 3c6)]

10% Absolute value of instrument

detection limit; absolute value of

laboratory reporting limit or MDL

is acceptable

INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY (3125)/ICP–MS Method

8
b. Compensation for interferences: Use instrument software to

correct for previously listed interferences. See Table 3125:V for

a list of the most common molecular ion interferences.

c. Data reporting: Establish appropriate reporting limits for

method analytes based on IDLs and the laboratory blank. For

regulatory programs, ensure that reporting limits for method

analytes are a factor of three below relevant regulatory criteria

whenever possible.

If method blank contamination is typically random, sporadic,

or otherwise not statistically controllable, do not correct results

for the method blank. Consider correcting blank results only if it

can be demonstrated that the blank’s analyte concentration is

within statistical control over a period of months. Report all

method blank data explicitly in a manner identical to samplereporting procedures.

d. Documentation: Maintain documentation for the following

(where applicable): instrument tuning, mass calibration, calibration verification, analyses of blanks (method, field, calibration,

and equipment blanks), IDL and MDL studies, analyses of

samples and duplicates with known additions, laboratory and

field duplicate information, serial dilutions, internal standard

recoveries, and any relevant quality control charts.

Also, maintain all raw data generated in support of the

method, and keep them available for review.5

6. Method Performance

Table 3125:III presents IDL data generated by this method;

this represents optimal instrument detection capabilities, not

recommended method detection or reporting limits. Tables

3125:I, II, and IX through XI contain single-laboratory, singleoperator, single-instrument performance data generated by this

method for calibration verification standards, low-level standards, and known-addition recoveries for freshwater matrices, as

well as data showing standard and CCT performance data for

elements usually affected by polyatomic interferences (Tables

3125:I and II). Section 3125 performance data for some analytes

are currently unavailable; however, performance data for similar

ICP-MS methods are available in the literature.1,4

7. References

1. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1994. Determination of

trace elements in waters and wastes by inductively coupled plasmamass spectrometry, Method 200.8. U.S. Environmental Protection

Agency, Environmental Monitoring Systems Lab., Cincinnati, Ohio.

2. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1984. Definition and procedure for the determination of the method detection limit, revision

1.11. 40 CFR 136, Appendix B.

TABLE 3125:IX. METHOD PERFORMANCE WITH CALIBRATION VERIFICATION STANDARDS*

Continuing Calibration Verification Standard (N  44) Initial Calibration Verification Standard (N  12)

Element Mass

Mean

Recovery

%

Mean

g/L

Standard

Deviation

g/L

Relative Standard

Deviation

%

Mean

Recovery

%

Mean

g/L

Standard

Deviation

g/L

Relative Standard

Deviation

%

Be 9 98.71 49.35 3.43 6.94 100.06 50.03 1.90 3.80

Al 27 99.62 49.81 2.99 6.01 98.42 49.21 1.69 3.44

V 51 100.97 50.48 1.36 2.68 99.91 49.96 1.23 2.47

Cr 52 101.39 50.70 1.86 3.66 99.94 49.97 1.47 2.95

Cr 53 100.68 50.34 1.91 3.79 99.13 49.56 1.44 2.90

Mn 55 101.20 50.60 1.98 3.91 99.48 49.74 1.40 2.82

Co 59 101.67 50.83 2.44 4.79 99.44 49.72 1.61 3.24

Ni 60 99.97 49.99 2.14 4.28 97.98 48.99 1.70 3.47

Ni 62 99.79 49.89 2.09 4.18 97.57 48.79 1.32 2.71

Cu 63 100.51 50.25 2.19 4.36 97.87 48.93 1.63 3.33

Cu 65 100.39 50.19 2.26 4.51 98.34 49.17 1.58 3.20

Zn 66 101.07 50.53 1.93 3.82 98.75 49.38 0.87 1.76

Zn 68 100.42 50.21 1.89 3.77 97.75 48.87 0.50 1.02

As 75 100.76 50.38 1.15 2.28 98.83 49.41 0.89 1.80

Se 77 101.71 50.85 1.43 2.81 99.54 49.77 1.01 2.03

Se 82 101.97 50.98 1.50 2.95 99.76 49.88 0.94 1.89

Ag 107 101.50 50.75 1.68 3.30 99.27 49.63 1.17 2.36

Ag 109 101.65 50.83 1.68 3.31 99.66 49.83 1.54 3.08

Cd 111 100.92 50.46 1.94 3.84 98.61 49.30 1.36 2.77

Cd 114 100.90 50.45 2.07 4.10 99.20 49.60 1.41 2.84

Sb 121 100.14 50.07 2.39 4.77 99.38 49.69 1.38 2.78

Sb 123 99.98 49.99 2.48 4.97 99.09 49.54 1.34 2.71

Tl 203 101.36 50.68 1.64 3.23 100.05 50.02 1.01 2.01

Tl 205 102.40 51.20 1.93 3.78 101.23 50.62 1.45 2.87

Pb 208 101.21 50.61 1.65 3.25 99.33 49.67 0.84 1.69

U 238 101.54 50.77 1.93 3.80 99.80 49.90 1.36 2.72

* Single-laboratory, single-operator, single-instrument data, determined using a 50-g/L standard prepared from sources independent of calibration standard source. Data

acquired January–November 1996 during actual sample determinations. Performance of continuing calibration verification standards at different levels may vary.

Perkin-Elmer Elan 6000 ICP–MS used for determination.

INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY (3125)/ICP–MS Method

9
TABLE 3125:X. METHOD PERFORMANCE FOR RECOVERY OF KNOWN ADDITION IN NATURAL WATERS*

Total Recoverable Metals† Dissolved Metals‡

Element Mass

Mean Recovery

%

Relative Standard Deviation

%

Mean Recovery

%

Relative Standard Deviation

%

Be 9 89.09 5.77 — —

V 51 87.00 8.82 — —

Cr 52 87.33 8.42 88.38 6.43

Cr 53 86.93 7.90 88.52 5.95

Mn 55 91.81 10.12 — —

Co 59 87.67 8.92 — —

Ni 60 85.07 8.42 89.31 5.70

Ni 62 84.67 8.21 89.00 5.82

Cu 63 84.13 8.46 88.55 8.33

Cu 65 84.37 8.05 88.26 7.80

Zn 66 86.14 23.01 95.59 13.81

Zn 68 81.95 20.31 91.94 13.27

As 75 90.43 4.46 97.30 8.84

Se 77 83.09 4.76 105.36 10.80

Se 82 83.42 4.73 105.36 10.75

Ag 107 — — 91.98 5.06

Ag 109 — — 92.25 4.96

Cd 111 91.37 5.47 96.91 6.03

Cd 114 91.47 6.04 97.03 5.42

Sb 121 94.40 5.24 — —

Sb 123 94.56 5.36 — —

Tl 203 97.24 5.42 — —

Tl 205 98.14 6.21 — —

Pb 208 96.09 7.08 100.69 7.28

* Single-laboratory, single-operator, single-instrument data. Samples were Washington State surface waters from various locations. Data acquired January–November 1996

during actual sample determinations. Performance of known additions at different levels may vary. Perkin-Elmer Elan 6000 ICP–MS used for determination.

† Known-addition level 20 g/L. Additions made before preparation according to Section 3030E (modified by clean-hood digestion in TFE beakers). N  20.

‡ Known-addition level for Cd and Pb 1 g/L; for other analytes 10 g/L. Additions made after filtration through 1:1 HNO3 precleaned 0.45-m filters. N  28.

TABLE 3125:XI. METHOD PERFORMANCE WITH CALIBRATION VERIFICATION STANDARDS*

1.0-g/L Standard 0.3-g/L Standard

Element Mass

Mean

Recovery

%

Mean

g/L

Standard Deviation

g/L

Relative Standard

Deviation

%

Mean

Recovery

%

Mean

g/L

Standard Deviation

g/L

Relative Standard

Deviation

%

Be 9 97 0.97 0.06 6.24 95 0.284 0.03 12.11

Al 27 121 1.21 0.32 26.49 196 0.588 0.44 74.30

V 51 104 1.04 0.06 5.83 111 0.332 0.10 28.96

Cr 52 119 1.19 0.34 28.62 163 0.490 0.37 75.90

Cr 53 102 1.02 0.36 35.54 113 0.338 0.32 93.70

Mn 55 103 1.03 0.07 6.55 110 0.329 0.08 25.64

Co 59 103 1.03 0.07 6.42 102 0.307 0.04 12.53

Ni 60 101 1.01 0.05 5.24 107 0.321 0.05 14.14

Ni 62 102 1.02 0.06 5.42 109 0.326 0.05 15.94

Cu 63 107 1.07 0.09 8.78 118 0.355 0.06 18.29

Cu 65 107 1.07 0.10 9.05 117 0.352 0.06 17.69

Zn 66 117 1.17 0.51 43.52 182 0.547 0.68 124.13

Zn 68 116 1.16 0.50 42.90 179 0.537 0.66 122.12

As 75 97 0.97 0.05 5.23 101 0.302 0.06 18.29

Se 77 89 0.89 0.08 8.72 88 0.265 0.08 29.07

Se 82 92 0.92 0.14 15.50 106 0.317 0.14 43.91

Ag 107 101 1.01 0.05 4.53 94 0.282 0.04 15.74

Ag 109 103 1.03 0.07 6.57 92 0.277 0.04 13.68

Cd 111 98 0.98 0.04 3.80 96 0.288 0.03 8.74

Cd 114 100 1.00 0.03 3.39 98 0.293 0.03 8.70

Sb 121 94 0.94 0.05 5.28 93 0.280 0.06 21.89

Sb 123 94 0.94 0.05 5.36 93 0.278 0.06 22.39

Tl 203 101 1.01 0.04 3.57 98 0.294 0.03 11.89

Tl 205 104 1.04 0.05 5.15 100 0.300 0.03 10.43

Pb 208 104 1.04 0.04 3.65 104 0.312 0.03 11.13

U 238 106 1.06 0.05 4.64 102 0.307 0.03 9.92

* Single-laboratory, single-operator, single-instrument data. N  24 for both standards.

INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY (3125)/ICP–MS Method

10
3. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1991. Methods for the

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D.C.

4. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1995. Method 1638: Determination of trace elements in ambient waters by inductively coupled plasma mass spectrometry. U.S. Environmental Protection

Agency, Off. Water, Washington, D.C.

5. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1995. Guidance on the

Documentation and Evaluation of Trace Metals Data Collected for

Clean Water Act Compliance Monitoring. U.S. Environmental Protection Agency, Off. Water, Washington, D.C.

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