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Methods of Analysis by the U.S. Geological SurveyNational Water Quality Laboratory— Determination of DissolvedArsenic, Boron, Lithium, Selenium, Strontium, Thallium, andVanadium Using Inductively Coupled Plasma–Mass Spectrometry

Open-File Report 99–093

U.S. Department of the InteriorU.S. Geological Survey


By John R. Garbarino

U.S. Geological SurveyOpen-File Report 99–093

Denver, Colorado1999

U.S. DEPARTMENT OF THE INTERIORBRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEYCharles G. Groat, Director

The use of brand, firm, and trade names in this report is for identificationpurposes only and does not constitute endorsement by the U.S. Government.

For additional information write to: Copies of this report can be purchased from:

U.S. Geological Survey U.S. Geological SurveyChief, National Water Quality Laboratory Branch of Information ServicesBox 25046, Mail Stop 407 Box 25286Federal Center Federal CenterDenver, CO 80225-0046 Denver, CO 80225-0286

CONTENTS III

CONTENTS

Abstract ........................................................................................................................ 1Introduction ................................................................................................................... 1Analytical method ......................................................................................................... 2

1. Application ........................................................................................................ 22. Summary of method ........................................................................................... 33. Interferences ...................................................................................................... 34. Apparatus, instrumentation, and operating conditions ......................................... 155. Reagents and calibration standards...................................................................... 176. Sample preparation ............................................................................................. 187. Analytical procedure........................................................................................... 188. Calculations ....................................................................................................... 189. Reporting results................................................................................................. 18

Discussion of results ...................................................................................................... 19Results for standard reference material ..................................................................... 19Spike recoveries in natural-water samples ............................................................... 20Comparison of inductively coupled plasma–mass spectrometry to former

methods of analysis ............................................................................................ 22Conclusions ................................................................................................................... 30References cited ............................................................................................................. 30

FIGURES

1–7. Graphs showing relations among selected cations and anions and newelements determined by inductively coupled plasma–mass spectrometry:

1. Arsenic ............................................................................................................... 52. Boron ................................................................................................................. 63. Lithium............................................................................................................... 74. Selenium............................................................................................................. 85. Strontium............................................................................................................ 96. Thallium ............................................................................................................. 107. Vanadium ........................................................................................................... 11

8. Accuracy of inductively coupled plasma–mass spectrometric arsenicdeterminations in the presence of bromide using different correctionequations .................................................................................................................. 14

9. Relation of apparent chromium-52 signal from 40Ar12C+ as a function ofincreasing concentrations of carbon ..................................................................... 16

10. The accuracy of inductively coupled plasma–mass spectrometric chromium-52determinations in the presence of carbon .................................................................. 16

Determination of Dissolved Arsenic, Boron, Lithium, Selenium, Strontium, Thallium, andVanadium Using Inductively Coupled Plasma–Mass Spectrometry

IV

FIGURES— Continued

11–16. Graphs showing statistical results for filtered, acidified, natural-water samples from inductively coupled plasma–mass spectrometry andformer methods of analysis:

11. Arsenic results for 59 samples.......................................................................... 2412. Boron results for 62 samples............................................................................ 2513. Lithium results for 63 samples ......................................................................... 2614. Selenium results for 64 samples ....................................................................... 2715. Strontium results for 63 samples ...................................................................... 2816. Vanadium results for 17 samples ..................................................................... 29

TABLES

1. Inorganic constituents and codes .............................................................................. 32. Former methods and ICP–MS method detection limits and calibration limits

for new elements determined in filtered, acidified natural water ............................... 43. Concomitant concentrations that result in more than a 20-percent ionization

suppression on new elements determined by ICP–MS .............................................. 134. Statistical analysis of long-term ICP–MS results for U.S. Geological Survey

Standard Reference Water Sample T145................................................................... 205. Short-term analytical variability as a function of elemental concentration for

ICP–MS ................................................................................................................... 216. Average percent spike recoveries in reagent-water, surface-water, and ground-

water matrices by inductively coupled plasma–massspectrometry............................................................................................................. 21

7. Chemical characteristics of natural-water samples used to evaluate ICP–MS............ 228. Statistical analysis summary of inductively coupled plasma–mass

spectrometry and former methods of analysis ........................................................... 23

CONVERSION FACTORS, ABBREVIATED WATER-QUALITY UNITS,OTHER ABBREVIATIONS, AND DEFINITIONS

V

CONVERSION FACTORS, ABBREVIATED WATER-QUALITY UNITS, OTHERABBREVIATIONS, AND DEFINITIONS

Multiply By To obtaingram (g) 3.53 x 10-2 ounceliter (L) 2.64 x 10-1 gallonmicrogram (µg) 3.53 x 10-8 ouncemicrometer (µm) 3.94 x 10-5 inchmilligram (mg) 3.53 x 10-5 ouncemilliliter (mL) 2.64 x 10-4 gallon

Degree Celsius (°C) may be converted to degree Fahrenheit (°F) by using the followingequation:

°F = 9/5 (°C) + 32.

Abbreviated water-quality units used in this report are as follows:

mg/L milligram per literµg/L microgram per literµS/cm microsiemens per centimeter at 25°C

Other abbreviations also used in this report:amu atomic mass unitASTM American Society for Testing and MaterialsCC catalyzed colorimetryDCP–AES direct current plasma–atomic emission spectrometryF–AAS flame–atomic absorption spectrophotometryFEP fluorinated ethylene propylene (Teflon)GF–AAS graphite furnace–atomic absorption spectrophotometryHG–AAS hydride generation–atomic absorption spectrophotometryICP–MS inductively coupled plasma–mass spectrometryICP–AES inductively coupled plasma–atomic emission spectrometry, also known as

inductively coupled plasma–optical emission spectrometry (ICP–OES)MDL(s) method detection limit(s)MRL(s) minimum reporting level(s)MPV(s) most probable value(s)NIST National Institute of Standards and TechnologyNWQL National Water Quality LaboratorySRWS(s) U.S. Geological Survey Standard Reference Water Sample(s)SOP standard operating procedureUSGS U.S. Geological Survey> greater than< less than≤ less than or equal to± plus or minus


VI

Definitions:

MDL The method detection limit (MDL) is defined as the minimum concentration of anelement that can be measured and reported with 99-percent confidence that theconcentration is greater than zero and is determined from analysis of a sample in agiven matrix containing the element of interest (U.S. Environmental ProtectionAgency, 1997).

MPV The most probable value (MPV) is equal to the median value for numerousinterlaboratory analyses from multiple analytical methods.

INTRODUCTION 1


By John R. Garbarino

ABSTRACT

The inductively coupled plasma–massspectrometric (ICP–MS) methods have beenexpanded to include the determination ofdissolved arsenic, boron, lithium, selenium,strontium, thallium, and vanadium infiltered, acidified natural water. Methoddetection limits for these elements are now10 to 200 times lower than by former U.S.Geological Survey (USGS) methods, thusproviding lower variability at ambientconcentrations. The bias and variability ofthe method was determined by using resultsfrom spike recoveries, standard referencematerials, and validation samples. Spikerecoveries at 5 to 10 times the methoddetection limit and 75 micrograms per literin reagent-water, surface-water, and ground-water matrices averaged 93 percent forseven replicates, although selected elementalrecoveries in a ground-water matrix with anextremely high iron sulfate concentrationwere negatively biased by 30 percent.Results for standard reference materialswere within 1 standard deviation of the mostprobable value. Statistical analysis of theresults from about 60 filtered, acidifiednatural-water samples indicated that therewas no significant difference betweenICP–MS and former USGS official methodsof analysis.

INTRODUCTION

The U.S. Geological Survey (USGS)National Water Quality Laboratory (NWQL)offers several methods for the determinationof dissolved arsenic, boron, lithium,selenium, strontium, thallium, and vanadiumin filtered, acidified water samples. FormerUSGS methods use single-elementquantification, such as flame– (F–AAS),graphite furnace– (GF–AAS), and hydridegeneration–atomic absorptionspectrophotometry (HG–AAS), directcurrent plasma–atomic emissionspectrometry (DCP–AES), catalyzed colori-metry (CC), or the simultaneousmultielement technique of inductivelycoupled plasma–atomic emissionspectrometry (ICP–AES). This reportprovides data that validates the addition ofthe aforementioned elements to the existinginductively coupled plasma–massspectrometric (ICP–MS) method.

Elements that are being added to theexisting multielement ICP–MS method andtheir corresponding former USGS methodsof analysis are listed in the following table:Dissolved arsenic, boron, lithium, strontium,thallium, and vanadium are validated for usein ICP–MS method I-2477-92 (see Faires,1993; Garbarino and Taylor, 1994).


2

Element CC F–AAS GF–AAS HG–AAS DCP–AES ICP–AESArsenic 4 4Boron 4 4Lithium 4 4Selenium 4 4Strontium 4 4Thallium 4Vanadium 4 4

ICP–MS is compared to one formermethod from the list in the preceding table.In all comparisons, the most current (as ofJanuary 1998) former method is used.Dissolved arsenic and selenium ICP–MSresults are compared to HG–AAS. TheHG–AAS methods require that the samplebe digested to oxidize organocompounds.The former HG–AAS method for dissolvedarsenic is based on an online sulfuricacid/potassium persulfate digestion. Theformer HG–AAS method for dissolvedselenium is based on an offline hydrochloricacid/potassium persulfate digestion. Nosuch digestions are necessary to determinedissolved elements by the ICP–MS methodbecause the high-temperature plasmadissociates organometallic species. TheICP–MS method for dissolved boron,lithium, strontium, and vanadium iscompared to ICP–AES. The ICP–MSthallium method was compared to GF–AAS.

The objectives of this report are asfollows:

• To determine whether ICP–MSresults are biased with respect to formerUSGS methods of analysis for determinationof the new elements in filtered natural water.Multiple strategies are used in the statisticalanalysis of the experimental data to providea practical estimate of the expectedaccuracy.

• To compare the variability of theICP–MS method to former USGS methods ofanalysis.

• To estimate potential effects ofusing results from the ICP–MS method onlong-term water-quality studies.

The revised methods were developed bythe USGS for use at the NWQL. Thesemethods supplement other official USGSinorganic methods (Fishman, 1993; Fishmanand Friedman, 1989). The new elements willbe available in the ICP–MS schedules.

ANALYTICAL METHOD

1. Application

The determination of dissolved arsenic,boron, lithium, selenium, strontium, thallium,and vanadium in filtered, acidified naturalwater has been added to the ICP–MS method(I-2477-92). Table 1 lists the new lab code,parameter code, method code, and reportingunit for every element. A report by Garbarinoand Taylor (1994) describes the determinationof these elements in filtered, acidified naturalwater; therefore, only supplementaryinformation and validation data are providedhere. Filtered, acidified natural water isprocessed as described in Horowitz and others(1994).

ANALYTICAL METHOD 3

Table 1. Inorganic constituents and parametercodes

Metals, Dissolved, Method I-2477-92

Element Lab codeParameter

and methodcodes

Arsenic, µg/L 2503 01000DBoron, µg/L 2504 01020GLithium, µg/L 2505 01130CSelenium, µg/L 2506 01145CStrontium, µg/L 2507 01080IThallium, µg/L 2508 01057IVanadium, µg/L 2509 01085E

The short-term method detection limits(MDLs) and analytical concentration rangesare listed in table 2. Even though the uppercalibration standard is listed as 200 µg/L orless, the elemental linear dynamic range isextended to at least 1 mg/L by calibratingboth the pulse and analog measurementmodes. Short-term MDLs were calculatedby using the U.S. Environmental ProtectionAgency’s (1997) definition and representpooled averages on the basis of four MDLsdetermined on different days over severalweeks. The MDLs have not beenestablished for most former methods,therefore, minimum-reporting levels(MRLs) are listed that are probably within afactor of 5 of the MDL.

2. Summary of Method

The ICP–MS methods have beendescribed previously in Faires (1993) andGarbarino and Taylor (1994). Thefollowing sections only provide additionalinformation specific to the elements that arebeing added to the method.

3. Interferences

Several types of physical and spectralinterference are recognized and documentedfor ICP–MS techniques (see Horlick and

Shao, 1992; Garbarino and Taylor, 1994).Physical interferences are associatedprimarily with sample introduction and areminimized by using the internalstandardization technique. Isotopes measuredin this procedure have been selectedspecifically to minimize spectral interferencesfrom isobaric, doubly charged, and molecularions. Multiple isotopes can be measured forselected elements that have potential isobaricor molecular ion interference. The analystmust be aware of these interferences becausethey might be present with certain types ofsample matrices.

3.1 Physical interferences. The use ofthe ratio of elemental ion intensity to theinternal standard element ion intensity forcalibration reduces the effects of sampletransport, instrumental drift, and matrix-induced fluctuations in plasma characteristics.Accurate results depend on having a constantinternal standard ion intensity throughout theanalysis of all standards and samples. Stableinternal standard ion intensity requires thatany interference associated with the internalstandard element can be corrected and that theinternal standard element is not naturallypresent in the samples being analyzed. Groupsof elements with similar response character-istics or mass-to-charge ratios are oftenreferenced to an internal standard elementwith similar characteristics.

Three internal standard elements areroutinely used to cover the mass range from 6to 240— for example, 72Ge+, 115In+, and 209Bi+

(Garbarino and Taylor, 1994). Alternativeisotopes may be substituted after ensuring thatthere are no spectral interferences associatedwith the new selections.

Memory effects related to sampletransport are negligible for most elements thatare normally present in natural water.Carryover from samples that have arsenic,


4

Table 2. Former methods and ICP–MS method detection limits and calibration limits for new elementsdetermined in filtered, acidified natural water

[All concentrations are in micrograms per liter; MRL, minimum reporting level; MDL, method detection limit;CC, catalyzed colorimetry; DCP–AES, direct current plasma–atomic emission spectrometry; F–AAS, flame–atomic absorption spectrophotometry; GF–AAS, stabilized temperature graphite furnace–atomic absorptionspectrophotometry; HG–AAS, hydride generation–atomic absorption spectrophotometry; ICP–AES, inductivelycoupled plasma–atomic emission spectrometry; ICP–MS, inductively coupled plasma–mass spectrometry]

Former methods ICP–MS

Element Technique MRL

Upperconcentrationlimit (without

dilution)

Short-termMDL

Uppercalibrationstandard1

Arsenic GF–AAS 0.9 50 0.07 200HG–AAS 1 20

Boron ICP–AES 16 10,000 2 0.5 100Lithium F–AAS 10 1,000 0.03 200

ICP–AES 4 100,000Selenium GF–AAS 1 50 0.1 200

HG–AAS 1 20Strontium F–AAS 10 5,000 0.008 200

ICP–AES 1 10,000Thallium GF–AAS 1 9 0.005 200Vanadium CC 1 40 0.08 200

ICP–AES 10 10,0001Upper concentration limit extends to greater than 1 milligram per liter without dilution by

calibrating the analog stage of the detector.2Method detection limit for boron is limited by reagent blank concentration.

boron, lithium, selenium, and strontiumconcentrations less than or equal to 200µg/L is negligible when using the sampleintroduction described in Garbarino andStruzeski (1998). However, thallium andvanadium did not recover to reagent-blankintensity levels within the rinse period;thallium and vanadium intensities were 10and 2 times greater than reagent-blanklevels, respectively. Consequently, theanalyst must review all analytical results toensure that errors from carryover areminimized.

Sample matrix composition could alsoaffect the bias and variability of ICP–MSdeterminations. The use of internalstandardization compensates for most matrixeffects, however, some matrix interferencesremain problematic. Matrix composition

can suppress the ionization efficiency of theplasma and result in negatively biasedelemental concentrations. Aluminumsulfate, calcium sulfate, iron sulfate,manganese sulfate, sodium sulfate, andsodium chloride were evaluated for theirmatrix interference potential. A solutioncontaining about 25 µg/L of arsenic, boron,lithium, selenium, strontium, thallium, andvanadium was prepared in increasingconcentrations of each compound.

The effects of increasing concomitantconcentration are shown in figures 1 through7. For example, the graphs in figure 1 showthe effects of increasing concentrations ofcations (aluminum, calcium, iron,manganese, and sodium) and anions (sulfate

ANALYTICAL METHOD 5

Figure 1. Relations among selected cations and anions and arsenic determined by inductively coupledplasma–mass spectrometry (mg/L, milligrams per liter).

-50 0 50 100 150 200 250 300 350 400 450 500 550 60005

101520253035404550

ALUMINUM CONCENTRATION FROM Al 2(SO 4)3, IN mg/L-50 0 50 100 150 200 250 300 350 400 450 50005

101520253035404550

CALCIUM CONCENTRATION FROM CaSO 4, IN mg/L

0 200 400 600 800 1,000 1,200 1,400 1,600 1,80005

101520253035404550

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IRON CONCENTRATION FROM FeSO 4, IN mg/L

0 200 400 600 800 1,000 1,200 1,400 1,600 1,80005

101520253035404550

MANGANESE CONCENTRATION FROM MnSO 4, IN mg/L

0 500 1,000 1,500 2,000 2,500 3,000 3,50005

101520253035404550

SODIUM CONCENTRATION FROM NaCl, IN mg/L


6

Figure 2. Relations among selected cations and anions and boron determined by inductively coupledplasma–mass spectrometry (mg/L, milligrams per liter).

-50 0 50 100 150 200 250 300 350 400 450 500 550 60005

101520253035404550


101520253035404550


0 200 400 600 800 1,000 1,200 1,400 1,600 1,80005

101520253035404550

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IRON CONCENTRATION FROM FeSO 4, IN mg/L0 200 400 600 800 1,000 1,200 1,400 1,600 1,800

05

101520253035404550


0 500 1,000 1,500 2,000 2,500 3,000 3,50005

101520253035404550


ANALYTICAL METHOD 7

Figure 3. Relations among selected cations and anions and lithium determined by inductively coupledplasma–mass spectrometry (mg/L, milligrams per liter).

-50 0 50 100 150 200 250 300 350 400 450 500 550 6000

50

100

150

200


101520253035404550


0 200 400 600 800 1,000 1,200 1,400 1,600 1,80005

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8

Figure 4. Relations among selected cations and anions and selenium determined by inductively coupledplasma–mass spectrometry (mg/L, milligrams per liter).

-50 0 50 100 150 200 250 300 350 400 450 500 550 60005

101520253035404550


101520253035404550


0 200 400 600 800 1,000 1,200 1,400 1,600 1,80005

101520253035404550

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0 500 1,000 1,500 2,000 2,500 3,000 3,500-150

-125

-100

-75

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-25

0

25

50


ANALYTICAL METHOD 9

Figure 5. Relations among selected cations and anions and strontium determined by inductivelycoupled plasma–mass spectrometry (mg/L, milligrams per liter).

-50 0 50 100 150 200 250 300 350 400 450 500 550 60005

101520253035404550


500

1000

1500

2000


0 200 400 600 800 1,000 1,200 1,400 1,600 1,80005

101520253035404550

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10

Figure 6. Relations among selected cations and anions and thallium determined by inductively coupledplasma–mass spectrometry (mg/L, milligrams per liter).

-50 0 50 100 150 200 250 300 350 400 450 500 550 60005

101520253035404550


101520253035404550


0 200 400 600 800 1,000 1,200 1,400 1,600 1,80005

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ANALYTICAL METHOD 11

Figure 7. Relations among selected cations and anions and vanadium determined by inductivelycoupled plasma–mass spectrometry (mg/L, milligrams per liter).

-50 0 50 100 150 200 250 300 350 400 450 500 550 60005

101520253035404550


101520253035404550


0 200 400 600 800 1,000 1,200 1,400 1,600 1,80005

101520253035404550

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12

and chloride) on the determination ofarsenic. A relation with a slope equal tozero indicates no interference. A slopegreater than zero suggests a positiveinterference or contamination; a slope of lessthan zero indicates signal suppression.Some elemental concentrations, forexample, boron in calcium sulfate (fig. 2),lithium in aluminum sulfate (fig. 3), andstrontium in calcium sulfate (fig. 5),increased with concomitant concentration.Such increases are from contamination(confirmed by using ICP–AES). Signalsuppressions greater than 20 percent areindicated for some elements, depending onthe nature of the concomitant. The resultsindicate that the suppression is not fromsulfate or chloride but rather from theaccompanying metal cation. Sulfur andchloride have low degrees of ionization, 14and 0.9 percent, respectively, and therefore,they should not significantly induce signalsuppression (Douglas, 1992).

In contrast, all the metal cations testedhave degrees of ionization exceeding 90percent. High iron concentrationsignificantly suppressed all elements exceptfor thallium. The response function wassimilar for each suppressed element,reaching a maximum of 75-percentsuppression at about 1,700 mg/L iron.Lighter elements (less than 80 amu) wereaffected to a higher degree than heavierelements; for example, thallium was notaffected. Manganese suppressed boron andvanadium in a similar manner, although to alesser degree than iron. Selenium isseverely suppressed in the presence of 3,200mg/L sodium; the suppression is enhancedby selenium’s 33-percent degree ofionization. In general, light elements areaffected more severely than heavy elements,and heavier concomitant cations cause moresevere suppression (Horlick and Shao,1992). Concomitant concentrations thatproduce more than a 20-percent suppression

are summarized in table 2. For mostconcomitant elements, 99 percent of thesamples submitted for analysis at NWQLhave concentrations that are much less thanthe concentrations listed in table 3.Nevertheless, the analyst must be aware ofpossible suppression whenever any elementis present in a sample at unusually highconcentrations; the sample can be diluted toeliminate the suppression.

3.2 Spectral interferences.Whenever possible, the isotope used forquantitation either has no spectralinterferences or has a small number ofpotential spectral interferences. Spectralinterferences can originate from isobaricions, molecular ions, or doubly chargedions. The analyst must be aware of thesepotential spectral interferences whenreviewing analytical results. Knownspectral interferences for elements beingadded to the ICP–MS method are listed inthe following sections.

3.3 Isobaric interferences. The onlyisobaric interference that affects the newelements is krypton on 82Se+and 78Se+.Krypton can be a minor contaminant foundin argon gas that is used to support theplasma, however, its concentration willremain constant for the standards andsamples, and, therefore, usually does notrequire correction.

3.4 Molecular-ion interferences.There are several known molecular-ioninterferences associated with arsenic,selenium, and vanadium. Chloride-associated molecular-ion interferences havebeen documented on arsenic (75As+) andvanadium (51V+) (see Horlick and Shao,1992; Garbarino and Taylor, 1994). Themolecular ions 40Ar35Cl+ and 35Cl16Ointerfere with 75As+ and 51V+, respectively.The 81BrH+ ion interferes with thedetermination of selenium at 82 amu.


Table 3. Concomitant concentrations that result in more than a 20-percent ionization suppression onnew elements determined by ICP–MS

[ICP–MS, inductively coupled plasma–mass spectrometry; mg/L, milligrams per liter; NWQL 99th percentile, theelemental concentration that is greater than 99 percent of the samples submitted to the National Water QualityLaboratory (NWQL); >, the greater than symbol signifies that the concentration causing 20-percent ionizationsuppression is greater than the stated concentration; nd, not determined because concomitant solution wascontaminated with element]

Element Aluminum,mg/L

Calcium,mg/L

Iron,mg/L

Manganese,mg/L

Sodium,mg/L

NWQL 99th percentile 55 570 19 11 1,800

Arsenic >500 >400 300 >1,700 >3,200Boron >500 nd 300 300 >3,200Lithium nd >400 300 300 >3,200Selenium nd >400 300 >1,700 600Strontium >500 nd 300 >1,700 ndThallium nd >400 >1,700 >1,700 >3,200Vanadium >500 >400 300 300 >3,200

Because arsenic is monoisotopic, andvanadium has only one isotope with suitablenatural abundance, no other interference-free isotopes are available for measurement.Equation 2 can be used to correct chlorideinterference on 75As+; it is derived asfollows:

75As+corr = 75As+ – [40Ar35Cl+/40Ar37Cl+] ×

{40Ar37Cl+ – [77Se/82Se] × 82Se+}. (1)

The portion of the signal at 75 amufrom the 40Ar35Cl+ interference is calculatedby converting the signal at 77 amu from40Ar37Cl+ (by measuring 77Se+) aftersubtracting the contribution from 77Se+ (bymeasuring 82Se+) to its proportion at 75 amu.All the terms identified by corr are thecorrected isotope intensities, and the termsin square brackets are the natural abundanceisotope ratios; all other terms representisotope intensities. After substituting

constant terms that are based on naturalabundance, equation 1 simplifies to

75As+corr = 75As+ – 3.127 × {77Se+ – [0.874] ×82Se+}. (2)

However, if bromide is also present in asample, using equation 2 will give positivelybiased results because of the 81BrH+

interference at 82Se+. Therefore, wheneverbromide is present, the correction must use78Se+ as shown in equation 3.

75As+corr = 75As+ – 3.127 × {77Se+ – [0.322] ×78Se+}. (3)

The positive bias in arsenic results whenusing equation 2 in the presence of bromide isshown in figure 8. The arsenic concentrationwill be 40 percent greater than is actuallypresent with only 200 µg/L bromide in asample. When using equation 3, such bias isnot indicated. Nevertheless, in the absense ofbromide, equation 2 is accurate as shown infigure 1 for up to 5,000 mg/L chloride.


14

Figure 8. Accuracy of inductively coupled plasma–mass spectrometric arsenic determinations in thepresence of bromide using different correction equations. The error bars correspond to one standarddeviation based on three instrumental measurements.

0 200 400 600 800 1,0000.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

75As+-3.1x{77Se+-0.87x 82Se +}

75As+-3.1x{77Se+-0.32x 78Se +}

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BROMIDE CONCENTRATION,

IN MICROGRAMS PER LITER


Equation 5 must be used to correctchloride interference on 51V+; it is derived asfollows:

51V+corr = 51V+ – [35Cl16O/37Cl16O] ×

{37Cl16O – [53Cr/52Cr] × 52Cr+}. (4)

The portion of the signal at 51 amufrom the 35Cl16O + interference is calculatedby converting the signal at 53 amu from37Cl16O + (by measuring 53Cr+) aftersubtracting the contribution from 53Cr+ (bymeasuring 52Cr+) to its proportion at 51 amu.After substituting constant terms that arebased on natural abundance, equation 4simplifies to

51V+corr = 51V+ – 3.127 × {53Cr+ – [0.113] × 52Cr+}. (5)

The accuracy of using equation 5 is shownin figure 7 for chloride concentrations lessthan 5,000 mg/L.

Masses at 52 and 53 amu are effectedby interference from 40Ar12C+ and 40Ar13C+,respectively, that arise from any carbonpresent in the sample. The interferencecorrection is based on the linear relation ofthe response at 52 amu from 40Ar12C+ to theconcentration of carbon as shown in figure 9(a similar linear relation is used for 53Cr+).A water-soluble carbon compound such assodium carbonate can be used to determinethe relation (see Krushevska and others,1998). The corrections are based onequations 6 and 7.

52Cr+corr = 52Cr+ – (52Cr+ / 13C+) × 13C+. (6)

53Cr+corr = 53Cr+ – (53Cr+ / 13C+) × 13C+. (7)

The terms (52Cr+ / 13C+) and (53Cr+ / 13C+) arevariables that equal about 0.2 and 0.002,respectively. These variables depend on theinstrument operating conditions and must bedetermined before every calibration. Whenusing equation 6, figure 10 shows that thecarbon interference can be eliminated. Such a

correction is accurate at carbon concentrationsto 1,000 mg/L. The 40Ar13C+ interference at53Cr+ is less severe than the correspondinginterference at 52Cr+ because of the lower naturalabundance of 13C. Nevertheless, the accuracy ofusing equation 7 is comparable to that shown infigure 10 for 52Cr+.

3.5 Doubly charged ion interferences.A doubly charged ion is created in the plasmafor any element that has a second ionizationpotential less than the ionization potential ofargon. Strontium is the only new element thatis potentially affected by doubly charged ioninterferences. However, lutetium andytterbium rarely are found in environmentalsamples at significant concentrations, andtheir interfering isotopes (176Lu2+ and 176Yb2+)have low natural abundance.

4. Apparatus, Instrumentation, andOperating Conditions

4.1 Labware. Use clean Type A glassvolumetric flasks to prepare all solutions.Store solutions in fluorinated ethylenepropylene (FEP Teflon) bottles to maintainstable elemental concentrations. Regularlyverify the accuracy of all pipets andvolumetric flasks for preparing standardsolutions by using either an analytical balanceor an automatic calibrating spectrophotometer.

4.2 Instrumentation. Instrumentationpreviously described in methods by Faires(1993) and Garbarino and Taylor (1994) hasbeen replaced by a Perkin-Elmer Elan 6000ICP–MS. The Elan 6000 operates under thesame general principles as earlierinstrumentation but has higher sensitivity andlower MDLs. The standard Perkin-Elmercross-flow nebulizer and spray chamber isused to introduce samples. The cross-flownebulizer resists clogging (≤ 0.5 percent totaldissolved solids) and is chemically inert.


16

Figure 9. Relation of apparent chromium-52 signal from 40Ar12C+ as a function of increasing concentrationsof carbon.

Figure 10. The accuracy of inductively coupled plasma–mass spectrometric chromium-52 determinations inthe presence of carbon.

0 5 10 15 20 25

0

50,000

100,000

150,000

200,000

Linear fit

APP

AR

EN

T

52C

r+ IN

TE

NSI

TY

, IN

CO

UN

TS

PER

SE

CO

ND

CARBON CONCENTRATION, IN MILLIGRAMS PER LITER

0 5 10 15 20 25

0

2

4

6

8

10

12

14

Corrected

Uncorrected

52C

r+ CO

NC

EN

TR

AT

ION

, IN

MIC

RO

GR

AM

S PE

R L

ITE

R

CARBON CONCENTRATION, IN MILLIGRAMS PER LITER


Other nebulizer designs can be used butmust be resistant to clogging and capable ofproviding MDLs that are within a factor oftwo of those listed in table 1. Details ofinstrument operation are fully documentedin the NWQL Standard Operating ProcedureIM0011.1 (T.M. Struzeski, U.S. GeologicalSurvey, written commun., 1998) and in thePerkin-Elmer (1997a and 1997b) softwareand hardware manuals.

Primary isotopes used to determinedissolved concentrations for the newelements are as follows:

Arsenic 75 amu Boron 11 amuLithium 7 amu Selenium 78 & 82 amuStrontium 88 amu Thallium 205 amuVanadium 51 amu

5. Reagents and CalibrationStandards

Earlier reports (Faires, 1993;Garbarino and Taylor, 1994) describe thepreparation of calibration standards, internalstandard solution, performance checksolution, and tuning solution. New elementsare calibrated by including them in themultielement standards described in methodI-2477-92 at the same concentrations.Multielement standards are prepared in amatrix that is matched to the acidconcentration of the samples being analyzed.Arsenic, selenium, and vanadium must notbe included in a multielement standard thatcontains bromide or chloride. Theinterference solution has been added toverify the accuracy of interferencecorrections on arsenic and vanadium.

ASTM Type I reagent water (AmericanSociety for Testing and Materials, 1995,p. 122–124), spectroscopic grade commercial

standards, and ultrapure acids must be used toprepare all solutions. All percentagesrepresent volume-to-volume ratios. Allconcentrated acids and commercial standardsmust be verified to contain concentrations ofconcomitant elements that are less than theMDLs after the prescribed dilution. Everysolution must be stored in a designated FEPTeflon bottle.

5.1 Nitric acid (HNO3): Concentrated,specific gravity 1.41.

5.2 Calibration blank: Reagent wateracidified to 0.4 percent HNO3.

5.3 Commercial single-elementstandard solutions, 1.00 mL = 10 mgpreserved in HNO3 for each of the following:As, B (in water), Li, Se, Sr, Tl, and V.

5.4 Commercial standard solutions,chloride: 1.00 mL = 100 mg in water andbromide: 1.00 mL = 1.0 mg in water.

5.5 Carbon standard solution, 1.00mL = 10 mg of carbon. Weigh 88.3 g dryNa2CO3 and transfer quantitatively into a 1-Lvolumetric flask; dissolve salt using 1,000 mLof reagent water.

5.6 Multielement stock solution I,1.00 mL = 0.010 mg of As, B, Li, Se, Sr, Tl,and V: Dilute 1.0 mL of each commercialsingle-element standard to 1,000 mL in avolumetric flask with 1 percent HNO3.

5.7 5.7 Multielement calibrationstandard I, 1.00 mL = 0.025 µg of As, B, Li,Se, Sr, Tl, and V: Include new elements in anexisting calibration standard by diluting 0.250mL of multielement stock solution I in a 100-mL volumetric flask with the calibrationblank.


18

5.8 Multielement calibration standardII, 1.00 mL = 0.100 µg of As, B, Li, Se, Sr,Tl, and V: Include new elements in anexisting calibration standard by diluting 1.0mL of multielement stock solution I in a 100-mL volumetric flask with the calibrationblank.

5.9 Multielement calibrationstandard III, 1.00 mL = 0.200 µg of As, B,Li, Se, Sr, Tl, and V: Include new elementsin an existing calibration standard bydiluting 2.0 mL of multielement stocksolution I in a 100-mL volumetric flask withthe calibration blank.

5.10 Interference check standard,1.00 mL = 0.500 mg Cl, 0.0005 mg Br,0.025 mg C, and 0.005 µg As and V: Dilute5.0 mL of the commercial chloride standardsolution, 0.50 mL of commercial bromidestandard solution, 2.5 mL of carbon standardsolution, and 0.50 mL of multielement stocksolution I in a 1,000-mL volumetric flaskwith the calibration blank.

6. Sample Preparation

Filtered, acidified natural-watersamples analyzed by ICP–MS for dissolvedarsenic, boron, lithium, selenium, strontium,thallium, and zinc and other elements do notrequire additional processing.

7. Analytical Procedure

Refer to Perkin-Elmer (1997a, 1997b)and NWQL Standard Operating ProcedureIM0011.1 (T.M. Struzeski, U.S. GeologicalSurvey, written commun., 1998) for details ofthe analytical procedure. In addition, theaccuracy of interference-correction equations

must be determined by analyzing theinterference check standard (see section 5.10)with every batch of samples. The Elansoftware automatically verifies that the resultsmeet acceptance criteria.

8. Calculations

No additional calculations are requiredin this method.

9. Reporting Results

The number of significant figuresreported varies with element and is a functionof concentration. Whenever the concentrationis less than the MDL for an element, the resultis reported as less than the MDL (< MDL).All other elemental results should be reportedusing the criteria listed below. These criteriaare based on the uncertainty suggested in thefollowing Discussion of Results section.Alternatively, the variability in the meanconcentration could be used to establish theappropriate number of significant figures toreport for each individual sample matrix. Theuse of such a procedure would provide themost accurate estimate of the uncertaintyassociated with each sample.

For arsenic, lithium, and vanadium—

• If the concentration is greater thanthe MDL, but less than 10 µg/L, report resultto two decimal places.

• If the concentration is greater than 10µg/L, but less than 100 µg/L, report result toone decimal place.

• If the concentration is greater than100 µg/L, report result to three significantfigures.

DISCUSSION OF RESULTS 19

For strontium and thallium—

• If the concentration is greater thanthe MDL, but less than 0.5 µg/L, reportresult to three decimal places.

• If the concentration is greater than0.5 µg/L, but less than 10 µg/L, report resultto two decimal places.

• If the concentration is greater than10 µg/L, but less than 100 µg/L, reportresult to one decimal place.


For boron—

• If the concentration is greater thanor equal to the MDL, but less than 100 µg/L,report result to one decimal place.


DISCUSSION OF RESULTS

The bias of the ICP–MS method forthe determination of dissolvedconcentrations is established by comparingresults to former USGS methods of analysis.ICP–MS method variability was determinedfrom replicate analyses over a range ofelemental concentrations prepared in acalibration-blank matrix. Results from theanalysis of standard reference material,spiked samples, and up to 64 filterednatural-water samples are used to verify thatdissolved arsenic, boron, lithium, selenium,strontium, thallium, and vanadium can bedetermined accurately by using ICP–MS.

All former methods are USGS-approvedmethods (Fishman and Friedman, 1989;Fishman, 1993).

Results for Standard ReferenceMaterial

U.S. Geological Survey SRWS T145was analyzed repetitively for 3 weeks todetermine the long-term bias and variability ofthe ICP–MS method (see table 4). Results forall elements are within one standard deviationof the most probable value (MPV). Theaverage long-term variability is 4±2 percentfor elemental concentrations ranging from 10to 200 µg/L. The paired Student t-Test wasused to test the null hypothesis that the ICP–MS method yields mean elementalconcentrations that are not significantlydifferent from the MPVs. The p-values werecalculated for each element to provide a levelof confidence in accepting the null hypothesis.The larger the p-value (the level ofsignificance) the greater the confidence inaccepting the null hypothesis. When the p-value exceeds 0.05, the null hypothesis isacceptable at the 95-percent confidence level.The Student t-Test indicated that only theboron and lithium experimental results are notsignificantly different from the MPV.Nevertheless, the differences between theMPV and the experimental mean for arsenic,selenium, strontium, thallium, and vanadiumare analytically insignificant.

The short-term variability of ICP–MSover an extended concentration range isshown in table 5. The variability is based onthree replicate determinations (an acquisitiontime of about 1 minute) at each elementalconcentration in the calibration blank matrix.The short-term variability was less than orequal to 5 percent at 0.5 µg /L for all elementsexcept for boron which was 5 percent at 1µg/L.


20

Table 4. Statistical analysis of long-term ICP–MS results for U.S. Geological Survey StandardReference Water Sample T145

[ICP–MS, inductively coupled plasma–mass spectrometry; elemental results are in micrograms per liter;MPV, the most probable value; ±, the plus or minus symbol precedes the F-pseudosigma in the MPVcolumn and the standard deviation in the experimental mean column; n, number of replicates used tocalculate the experimental mean; p-value, level of significance; <, less than]

Element MPV Experimentalmean, n=12 t-Test statistic p-value

Arsenic 10 ± 1 10.3 ± 0.2 9.00 <0.0001Boron 46 ± 6 45 ± 3 -0.23 0.8253Lithium 27 ± 2 28 ± 1 1.54 0.1528Selenium 10 ± 1 11.2 ± 0.2 18.63 <0.0001Strontium 203 ± 9 208 ± 4 3.93 0.0024Thallium 15 ± 3 14 ± 1 -3.12 0.0098Vanadium 12 ± 2 10.6 ± 0.5 -7.86 <0.0001

Spike Recoveries in Natural-WaterSamples

Spike recovery percentages listed intable 6 were determined for the newelements in matrices that are representativeof reagent water, surface water, and groundwater. Seven replicate recoveries at 5 to 10times the MDL (the low-level spike) and 75µg/L (the high-level spike) were determinedin each matrix over a period of about 1week. Average recoveries in the reagent-water matrix ranged from 93 to 105 percentfor all elements except boron, whose low-level spike recovery was 77 percent.Recovery variability for the low-level spikeranged from 4 to 11 percent, depending onthe element and matrix.

Recoveries of the high-level spike inthe surface-water matrix were similar tothose of the reagent-water matrix. Ambientconcentrations of lithium, strontium,thallium, and vanadium in the surface water,however, hindered the recovery of the low-level spike. Recovery of low-level arsenic,boron, and selenium ranged from 88 to 126

percent. The variability in the recovery of 1µg/L boron in the presence of 40 µg/L boronwas 52 percent; the variability for arsenicand selenium was less than 6 percent atabout the same spike concentration.

The ground-water matrix used forspike recoveries was selected to examine theeffects of interferent species on elementaldeterminations. The ground water had highiron (340 mg/L) and sulfate (2,300 mg/L)concentrations, which exceed the levelfound in most samples analyzed by NWQL.Ambient concentrations of boron, lithium,and strontium precluded the recovery of thelow-level spike, however, recoveries forarsenic, selenium, thallium, and vanadiumaveraged 70 percent. High-level spikerecoveries averaged 70±4 percent, excludingthallium, which had 97 percent. The lowpercent recoveries for the lighter elementsconfirm the effects from ionizationsuppression (see section 3.1). Only thalliumrecovery, a heavier element, was notaffected.


Table 5. Short-term analytical variability as a function of elemental concentration for ICP–MS

[ICP–MS, inductively coupled plasma–mass spectrometry, the percent relative standard deviations are based onthree sequential determinations in a 0.4-percent solution of concentrated nitric acid in deionized water; µg/L,micrograms per liter; <MDL, less than the method detection limit; nd, not determined]

Percent relative standard deviationElementalconcentration,

in µg/L 0.05 0.1 0.5 1.0 10 50 100 250Arsenic <MDL 30 3 4 2 nd 1 0.6Boron nd 10 nd 5 2 0.4 0.2 ndLithium 0.9 8 2 2 0.7 nd 0.7 0.6Selenium <MDL 60 5 3 2 nd 2 0.7Strontium 0.4 2 2 0.8 0.3 nd 0.7 0.6Thallium 2 2 0.2 0.9 2 nd 0.8 1Vanadium 8 10 2 0.6 0.7 nd 0.5 0.5

Table 6. Average percent spike recoveries in reagent-water, surface-water and ground-water matricesby inductively coupled plasma–mass spectrometry

[µg/L, micrograms per liter; number following the plus or minus symbol (±) is the standard deviation on the basisof seven determinations accrued on separate days; high spike, 75 µg/L for all elements; na, not applicable becausethe difference between the spike concentration and ambient concentration was greater than a factor of 10; <MDL,concentration is less than the method detection limit; %, percent]

Reagent-water matrix Surface-water matrix

ElementLow-spike,in µg/L

Ambientconcen-tration,in µg/L

Low-spikerecovery,

in %

High-spike

recovery,in %

Ambientconcen-tration,in µg/L

Low-spikerecovery,

in %

High-spike

recovery,in %

Arsenic 0.5 <MDL 102±4 101±1 1 112±6 105±2Boron 1 <MDL 77±9 103±2 40 88±52 102±4Lithium 0.2 <MDL 94±8 105±3 30 na 99±7Selenium 0.9 <MDL 110±10 99±2 2 126±7 104±3Strontium 0.2 <MDL 93±4 104±3 390 na 114±8Thallium 0.05 <MDL 97±11 100±6 60 na 103±5Vanadium 0.6 <MDL 98±4 105±4 60 na 104±4

Ground-water matrixArsenic 0.5 1 75±8 68±4Boron 1 350 na 70±20Lithium 0.2 220 na 72±12Selenium 0.9 1 80±10 73±4Strontium 0.2 6,000 na naThallium 0.05 <MDL 60±20 97±5Vanadium 0.6 <MDL 64±20 64±10


22

Comparison of Inductively CoupledPlasma–Mass Spectrometry toFormer Methods of Analysis

The ICP–MS results are compared toformer methods of analysis such as hydridegeneration–atomic absorption spectrophoto-metry (HG–AAS) and inductively coupledplasma–atomic emission spectrometry (ICP–AES). Up to 64 filtered, acidified natural-water samples were selected from thepopulation of such samples submitted to theNWQL. The samples have a wide range ofelemental concentrations and specificconductance. Surface-water and ground-water samples are included in the sampleset; the number of each type is aboutproportional to its fraction of the totalsubmitted for analysis during an averageyear. Other chemical characteristics thatoften influence the performance ofanalytical methods, such as sulfate andchloride concentrations, were alsoconsidered in the selection process (seetable 7 for the chemical characteristics ofthe samples).

Results were evaluated by usingseveral different approaches. Because the

data extend over a wide concentration range,it is inappropriate to use the paired Studentt-test to evaluate the null hypothesis becauseerrors, whether random or systematic, areindependent of the concentration.Consequently, linear regression analysis isused to calculate the slope, y-intercept, andcoefficient of determination (R2) for theequation that describes the relation betweenICP–MS and a former USGS method. Aslope coefficient of one and a y-intercept ofzero indicate exact correlation. Thecorresponding p-values indicate the degreeof confidence in each coefficient. Box plotsare also provided to show the distribution ofthe results from each method. The non-parametric Wilcoxon Signed Rank Test isused as another test to determine whetherthere is a significant difference betweenresults from the ICP–MS and former USGSmethods. Data that were less than thehighest MDL or MRL were omitted fromthe data set prior to statistical analysis.Statistical analysis results are summarized inthe following paragraphs and are listed intable 8. Illustrations for the statistical testsare provided in figures 11 through 16.

Table 7. Chemical characteristics of natural-water samples used to evaluate ICP–MS

[ICP–MS, inductively coupled plasma–mass spectrometry; µg/L, microgram per liter; mg/L, milligram per liter;<MDL, less than the method detection limit; SC, specific conductance]

Element, in µg/L 25thpercentile Median 75th

percentile Maximum

Arsenic 1.7 2.9 7.4 104Boron 52 120 210 1,700Lithium 22 39 69 1,600Selenium 2.9 4.5 8.2 34Strontium 350 630 1,900 20,800Thallium All results were <MDL for former methodsVanadium 1.8 4.7 9.7 1,530ConstituentChloride, in mg/L 9.8 68 501 9,176SC, in µS/cm1 455 944 2,490 52,600Sulfate, in mg/L 108 309 1,306 16,832

1Specific conductance in microsiemens per centimeter at 25 °C (µS/cm).


Results of the linear regressionanalyses show that there is strong correlationbetween ICP–MS and former USGSmethods for the analysis of filtered, acidifiednatural water. The slope coefficients rangefrom 0.90 to 1.1 (see table 8). Thecorresponding p-values indicate with 95-percent confidence that all the slopes are notsignificantly different from 1.0. Thalliumconcentrations were less than the GF–AASand ICP–MS method detection limits in allsamples. With the exception of lithium andvanadium, the p-values for the y-interceptsindicate a significant nonzero intercept thatmight result from differences in sensitivityor from the wide range of sample

concentrations. For example, boron andstrontium concentrations range from theMDLs to about 1,700 and 21,000 µg/L,respectively (see table 7).

The p-values of the Wilcoxon SignedRank Test indicate with 95-percentconfidence that there is no significantdifference between ICP–MS and ICP–AESresults for dissolved boron and vanadium.In general, the descriptive statistics andfigures 11 through 16 indicate thatdifferences among results for the otherelements are analytically insignificantconsidering the range of sampleconcentrations in the data set.

Table 8. Statistical analysis summary of inductively coupled plasma–mass spectrometry and formermethods of analysis

[Coef., either the slope or the y-intercept regression coefficient; p-value, level of significance; R2, coefficient ofdetermination; HG–AAS, hydride generation–atomic absorption spectrophotometry; ICP–AES, inductively coupledplasma–atomic emission spectrometry; GF–AAS, stabilized temperature graphite furnace–atomic absorptionspectrophotometry; nd, not detected; <, less than. All thallium results were less than the method detection limit]

Slope y-intercept

Element Formermethod Coef. p-valuea Coef. p-valueb R2

Wilcoxonsigned

rank testp-valuec

Arsenic HG–AAS 1.0 <0.0001 0.95 <0.0001 0.945 <0.0001Boron ICP–AES 0.90 <0.0001 13 0.0002 0.995 0.3714Lithium ICP–AES 0.92 <0.0001 1.0 0.2197 0.999 <0.0001Selenium HG–AAS 1.1 <0.0001 1.7 0.0002 0.864 <0.0001Strontium ICP–AES 0.94 <0.0001 96 0.0044 0.996 0.0030Thallium GF–AAS nd nd nd nd nd ndVanadium ICP–AES 0.96 <0.0001 0.10 0.9242 0.919 0.1340

aThe null hypothesis: slope is not equal to one.bThe null hypothesis: y-intercept is equal to zero.cThe null hypothesis: the difference in concentration between the new inductively coupled plasma–mass

spectrometric method and the former USGS method is equal to zero.


24

Figure 11. Statistical analysis of arsenic (As) results for 59 filtered, acidified, natural-watersamples from inductively coupled plasma–mass spectrometry and hydride generation–atomicabsorption spectrophotometry.

Percentile

10th25th

50th

75th90th

3.82

4.87

0.63

59

0.20

27.00

0

4.76

4.99

0.65

59

0.93

26.48

0

Mean

Std. Dev.

Std. Error

Count

Minimum

Maximum

# Missing

As by HG-AAS, in µg/L As by ICP-MS, in µg/L

Descriptive Statistics

-5

0

5

10

15

20

25

30

As

by IC

P-M

S, i

n µg

/L

-5 0 5 10 15 20 25 30As by HG-AAS, in µg/L

Regression Plot

-5

0

5

10

15

20

25

30

As by HG-AAS, in µg/L As by ICP-MS, in µg/L

Box Plot

0

1

-6.01

<0.0001

-6.01

<0.0001

# 0 Differences

# Ties

Z-Value

P-Value

Tied Z-Value

Tied P-Value

Wilcoxon Signed Rank Test for As by HG-AAS, in µg/L, As by ICP-MS, in µg/L

53 1681.50 31.73

6 88.50 14.75

Count Sum Ranks Mean Rank

# Ranks < 0

# Ranks > 0

Wilcoxon Rank Info for As by HG-AAS, in µg/L, As by ICP-MS, in µg/L

EXPLANATIONHydride generation-atomic absorption spectrophotometry (HG-AAS);Inductively coupled plasma-mass spectrometry (ICP-MS); microgramsper liter (µg/L); Std. Dev., standard deviation; Std. Error, standard error;<, less than; >, greater than; #, number

Line of regression


Figure 12. Statistical analysis of boron (B) results for 62 filtered, acidified, natural-watersamples from inductively coupled plasma–mass spectrometry and inductively coupled plasma–atomic emission spectrophotometry.

Percentile

10th25th

50th

75th90th

218.74

347.35

44.11

62

9.15

1849.43

0

210.77

315.04

40.01

62

10.31

1721.82

0

Mean

Std. Dev.

Std. Error

Count

Minimum

Maximum

# Missing

B by ICP-AES, in µg/L B by ICP-MS, in µg/L


-200

0

200

400

600

800

1000

1200

1400

1600

1800

2000

B b

y IC

P-M

S, i

n µg

/L

-200 200 600 1000 1400 1800B by ICP-AES, in µg/L

Regression Plot

-200

0

200

400

600

800

1000

1200

1400

1600

1800

2000

B by ICP-A ES, in µg/L B by ICP-MS, in µg/L

Box Plot

0

0

-0.89

0.3714

-0.89

0.3714

# 0 Differences

# Ties

Z-Value

P-Value

Tied Z-Value

Tied P-Value

Wilcoxon Signed Rank Test for B by ICP-AES, in µg/L, B by ICP-MS, in µg/L

34 1104.00 32.47

28 849.00 30.32


# Ranks < 0

# Ranks > 0

Wilcoxon Rank Info for B by ICP-AES, in µg/L, B by ICP-MS, in µg/L

EXPLANATIONInductively coupled plasma-atomic emission spectrometry (ICP-AES);Inductively coupled plasma-mass spectrometry (ICP-MS); microgramsper liter (µg/L); Std. Dev., standard deviation; Std. Error, standard error;<, less than; >, greater than; #, number

Line of regression


26

Figure 13. Statistical analysis of lithium (Li) results for 63 filtered, acidified, natural-watersamples from inductively coupled plasma–mass spectrometry inductively coupled plasma–atomicemission spectrophotometry.

Percentile

10th25th

50th

75th90th

81.33

223.15

28.11

63

5.20

1760.38

0

75.39

204.17

25.72

63

4.01

1615.30

0

Mean

Std. Dev.

Std. Error

Count

Minimum

Maximum

# Missing

Li by ICP-AES, in µg/L Li by ICP-MS, in µg/L


-200

0

200

400

600

800

1000

1200

1400

1600

1800

Li b

y IC

P-M

S, i

n µg

/L

-200 0 200 400 600 800 1000 1200 1400 1600 1800Li by ICP-AES, in µg/L

Regression Plot

-200

0

200

400

600

800

1000

1200

1400

1600

1800

Li by ICP-AES, in µg/L Li by ICP-MS, in µg/L

Box Plot

0

0

-3.90

<0.0001

-3.90

<0.0001

# 0 Differences

# Ties

Z-Value

P-Value

Tied Z-Value

Tied P-Value

Wilcoxon Signed Rank Test for Li by ICP-AES, in µg/L, Li by ICP-MS, in µg/L

19 439.00 23.11

44 1577.00 35.84


# Ranks < 0

# Ranks > 0

Wilcoxon Rank Info for Li by ICP-AES, in µg/L, Li by ICP-MS, in µg/L


Line of regression


Figure 14. Statistical analysis of selenium (Se) results for 64 filtered, acidified, natural-watersamples from inductively coupled plasma–mass spectrometry and hydride generation–atomicabsorption spectrophotometry.

Percentile

10th25th

50th

75th90th

5.07

5.91

0.74

64

0

27.80

0

7.09

6.80

0.85

64

0.22

33.62

0

Mean

Std. Dev.

Std. Error

Count

Minimum

Maximum

# Missing

Se by HG-AAS, in µg/L Se by ICP-MS, in µg/L


-5

0

5

10

15

20

25

30

35

Se

by IC

P-M

S, i

n µg

/L

-5 0 5 10 15 20 25 30 35Se by HG-AAS, in µg/L

Regression Plot

-5

0

5

10

15

20

25

30

35

Se by HG-AAS, in µg/L Se by ICP-MS, in µg/L

Box Plot

0

0

-6.86

<0.0001

-6.86

<0.0001

# 0 Differences

# Ties

Z-Value

P-Value

Tied Z-Value

Tied P-Value

Wilcoxon Signed Rank Test for Se by HG-AAS, in µg/L, Se by ICP-MS, in µg/L

62 2066.00 33.32

2 14.00 7.00


# Ranks < 0

# Ranks > 0

Wilcoxon Rank Info for Se by HG-AAS, in µg/L, Se by ICP-MS, in µg/L

EXPLANATIONHydride generation-atomic absorption spectrophotometry (HG-AAS);Inductively coupled plasma-mass spectrometry (ICP-MS); microgramsper liter (µg/L); Std. Dev., standard deviation; Std. Error, standard error;<, less than; >, greater than; #, number

Line of regression


28

Figure 15. Statistical analysis of strontium (Sr) results for 63 filtered, acidified, natural-watersamples from inductively coupled plasma–mass spectrometry and inductively coupled plasma–atomic emission spectrophotometry.

Percentile

10th25th

50th

75th90th

1891.95

3644.10

459.11

63

31.02

22068.32

1

1870.69

3425.62

431.59

63

30.53

20823.39

1

Mean

Std. Dev.

Std. Error

Count

Minimum

Maximum

# Missing

Sr by ICP-AES, in µg/L Sr by ICP-MS, in µg/L


-5000

0

5000

10000

15000

20000

25000

Sr b

y IC

P-M

S, i

n µg

/L

-5000 0 5000 10000 15000 20000 25000Sr by ICP-A ES, in µg/L

Regression Plot

-5000

0

5000

10000

15000

20000

25000

Sr by ICP-AES, in µg/L Sr by ICP-MS, in µg/L

Box Plot

0

0

-2.96

0.0030

-2.96

0.0030

# 0 Differences

# Ties

Z-Value

P-Value

Tied Z-Value

Tied P-Value

One case w as omitted due to missing values.

Wilcoxon Signed Rank Test for Sr by ICP-AES, in µg/L, Sr by ICP-MS, in µg/L

43 1441.00 33.51

20 575.00 28.75


# Ranks < 0

# Ranks > 0

One case w as omitted due to missing values.

Wilcoxon Rank Info for Sr by ICP-AES, in µg/L, Sr by ICP-MS, in µg/L


Line of regression


Figure 16. Statistical analysis of vanadium (V) results for 17 filtered, acidified, natural-watersamples from inductively coupled plasma–mass spectrometry and inductively coupled plasma–atomic emission spectrophotometry.

Percentile

10th25th

50th

75th90th

11.46

8.23

2.00

17

4.60

37.54

0

11.06

8.21

1.99

17

4.64

34.10

0

Mean

Std. Dev.

Std. Error

Count

Minimum

Maximum

# Missing

V by ICP-A ES, in µg/L V by ICP-MS, in µg/L


-5

0

5

10

15

20

25

30

35

40

V b

y IC

P-M

S, i

n µg

/L

-5 0 5 10 15 20 25 30 35 40V by ICP-A ES, in µg/L

Regression Plot

-5

0

5

10

15

20

25

30

35

40

V by ICP-A ES, in µg/L V by ICP-MS, in µg/L

Box Plot

0

0

-1.49

0.1359

-1.49

0.1359

# 0 Differences

# Ties

Z-Value

P-Value

Tied Z-Value

Tied P-Value

Wilcoxon Signed Rank Test for V by ICP-AES, in µg/L, V by ICP-MS, in µg/L

6 45.00 7.50

11 108.00 9.82


# Ranks < 0

# Ranks > 0

Wilcoxon Rank Info for V by ICP-AES, in µg/L, V by ICP-MS, in µg/L


Line of regression


30

CONCLUSIONS

Results from reference material,spike recoveries, and the analysis ofnatural-water samples were used toevaluate the overall bias and variabilityof the determination of dissolvedarsenic, boron, lithium, selenium,strontium, thallium, and vanadium byinductively coupled plasma–massspectrometry (ICP–MS). All test resultsprovide an accurate estimate of theexpected analytical performance. Thefollowing list outlines the majorconclusions of this report. In addition toanalytical performance comparisons,suggestions are provided for selectingappropriate methodology and thepotential effects of the use of ICP–MSon long-term trend analysis in water-quality studies.

• Method detection limits (MDLs)for ICP–MS are between about 10 and200 times lower than hydridegeneration–atomic absorptionspectrophotometry (HG–AAS) andinductively coupled plasma–atomicemission spectrometry (ICP–AES)methods.

• The short- and long-termaccuracy for the determination of thenew elements by ICP–MS wereacceptable; all the elements were withinone standard deviation of the mostprobable value.

• Data from up to 64 surface- andground-water samples indicated thatthere was no significant method bias forthe determination of dissolved arsenic,boron, lithium, selenium, strontium, andvanadium by ICP–MS.

• Determination of dissolvedarsenic and selenium by ICP–MS does

not require any digestion procedure asdoes the HG–AAS method.

• Matrix interferences from highconcentrations of concomitant metalscan affect the determination of lighterelements (< 60 amu). The concomitantconcentrations that cause significantinterference, however, are generallyabove the 99th percentile of elementalconcentrations in samples submitted tothe National Water Quality Laboratory(NWQL).

• ICP–MS is a state-of-the-artmultielement technique that is moreefficient and cost effective than formerUSGS single-element methods, such asHG–AAS. ICP–MS is the method ofchoice whenever multiple elements mustbe determined or whenever elementalconcentrations are less than 10 µg/L.

• Use of the ICP–MS method forthe determination of arsenic andselenium reduces the amount ofchemical reagents and chemical wastewhen compared to HG–AAS.

• Data from ICP–MS will affectlong-term trends in water-quality studiesbecause of the reduced bias andvariability at elemental concentrationsless than 10 µg/L.

REFERENCES CITED

American Society for Testing andMaterials, 1995, Annual book ofASTM standards, Section 11,Water (D1193, Standardspecification for reagent water):Philadelphia, v. 11.01, p. 122–124.

Douglas, D.J., 1992, Fundamentalaspects on inductively coupledplasma–mass spectrometry inMontaser, Akbar, and Golightly,

REFERENCES CITED 31

D.W., eds., Inductively coupledplasmas in analytical atomicspectrometry (2d ed.): New York,VCH Publishers, Inc., p. 614.

Faires, L.M., 1993, Methods of analysisby the U.S. Geological SurveyNational Water QualityLaboratory— Determination ofmetals in water by inductivelycoupled plasma–mass spectrometry:U.S. Geological Survey Open-FileReport 92-634, 28 p.

Fishman, M.J., and Friedman, L.C., eds.,1989, Methods for determination ofinorganic substances in water andfluvial sediments: U.S. GeologicalSurvey Techniques of Water-Resources Investigations, book 5,chap. A1, 545 p.

Fishman, M.J., ed., 1993, Methods ofanalysis by the U.S. GeologicalSurvey National Water QualityLaboratory— Determination ofinorganic and organic constituentsin water and fluvial sediments: U.S.Geological Survey Open-FileReport 93-125, 217 p.

Garbarino, J.R., and Struzeski, T.M., 1998,Methods of analysis by the U.S.Geological Survey National WaterQuality Laboratory— Determination ofelements in whole-water digests usinginductively coupled plasma–opticalemission spectrometry and inductivelycoupled plasma–mass spectrometry:U.S. Geological Survey Open-FileReport 98-165, 101 p.

Garbarino, J.R., and Taylor, H.E., 1994,Inductively coupled plasma–massspectrometric determination ofdissolved trace elements in naturalwater: U.S. Geological SurveyOpen-File Report 94-358, 28 p.

Horlick, Gary, and Shao, Youbin, 1992,Inductively coupled plasma–massspectrometry for elemental analysis

in Montaser, Akbar, and Golightly,D.W., eds., Inductively coupledplasmas in analytical atomicspectrometry (2d ed.): New York,VCH Publishers, Inc., p. 551.

Horowitz, A.J., Demas, C.R., Fitzgerald,K.K., Miller, T.L., and Rickert,D.A., 1994, U.S. Geological Surveyprotocol for the collection andprocessing of surface-water samplesfor the subsequent determination ofinorganic constituents in filteredwater: U.S. Geological SurveyOpen-File Report 94-539, 57 p.

Krushevska, Antoaneta, Waheed,Shahida, Nóbrega, Joaquim,Amarisiriwardena, Dulasiri, andBarnes, R.M., 1998, Reducingpolyatomic interferences in theICP–MS determination ofchromium and vanadium inbiofluids and tissues: AppliedSpectroscopy, v. 52, no. 2, p. 205–211.

Perkin-Elmer, 1997a, Elan 6000inductively coupled plasma–massspectrometer software guide:Norwalk, Connecticut, Perkin-Elmer part number 0993-8968Rev. F.

_______1997b, Elan 6000 inductivelycoupled plasma–mass spectrometerhardware guide: Norwalk,Connecticut, Perkin-Elmer partnumber 0993-8969 Rev. E.

U.S. Environmental Protection Agency,1997, Guidelines for establishing testprocedures for the analysis ofpollutants (Part 136, Appendix B.Definition and procedure for thedetermination of the method detectionlimit— Revision 1.11): U.S. Code ofFederal Regulations, Title 40, revisedas of July 1, 1997, p. 265–267.

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