METHODS OF ANALYSIS BY THE U.S. GEOLOGICAL SURVEY
METHODS OF ANALYSIS BY THE U.S. GEOLOGICAL SURVEYNATIONAL WATER QUALITY LABORATORY-DETERMINATIONOF ANTIMONY BY AUTOMATED-HYDRIDE ATOMICABSORPTION SPECTROPHOTOMETRYBy Glenda E. Brown and Betty J. McLainU.S. GEOLOGICAL SURVEYOpen-File Report 93-664mi1Denver, Colorado1994
U.S. DEPARTMENT OF THE INTERIORBRUCE BABBITT, SecretaryU.S. GEOLOGICAL SURVEYRobert M. Hirsch, Acting DirectorFor additional information write to:Copies of this report can be purchased from:Chief, National Water Quality LaboratoryU.S. Geological SurveyU.S. Geological SurveyBooks and Open-File Reports SectionBox 25046, Mail Stop 407Box 25425Federal CenterFederal CenterDenver, CO 80225-0425Denver, CO 80225-0425
CONTENTSPageAbstract. 1Introduction. 1Analytical method. 31. Application. 32. Summary of method. 33. Interferences. 34. Apparatus. 45. Reagents. 46. Procedure. 87. Calculations. 98. Reporting of results. 99. Precision. 9Discussion of results . 10Precision and bias . 10Spike recovery. 13Detection limit . 16Conclusions. 17References. 17FIGURESFigure 1.2.3.4.Diagram showing gas-liquid separator and quartztube furnace.Schematic showing antimony manifold.Graph showing relation between antimony mean concentrationsdetermined by automated-hydride atomic absorptionspectrophotometry and interlaboratory means for severalStandard Reference Water Samples.Graph showing antimony precision.561215TABLESTable 1.2.3.4.Precision and bias of antimony method. 11Recovery of antimony in surface-water samples. 14Sample replication listing mean, standard deviation, andrelative standard deviation for four separate analyses. 14Determination of the antimony method detection limit. 16111
CONVERSION FACTORS AND ABBREVIATIONSMultiplyBycentimeter (cm)gram (g)liter (L)meter (m)microgram ( ig)micrometer (}im)milliliter (mL)millimeter (mm)nanometer (nm)pound per square inch 10.0016.895To obtainmeterounce, micrometerkilopascalDegree Celsius ( C) may be converted to degree Fahrenheit ( F) by using thefollowing equation: F 9/5 ( C) 32The following units of measurement and terms also are used in this report:gram per litermicrogram per litermilliliter per in)(mV)(M)Other abbreviations are as follows:AASASTMICPMDLMPVNPDESNWQLSRWSUSGSatomic absorption spectrophotometryAmerican Society for Testing and Materialsinductively coupled plasmamethod detection limitmost probable valueNational Pollutant Discharge Elimination SystemNational Water Quality LaboratoryStandard Reference Water SampleU.S. Geological SurveyIV
METHODS OF ANALYSIS BY THE U.S. GEOLOGICAL SURVEYNATIONAL WATER QUALITY LABORATORY-DETERMINATIONOF ANTIMONY BY AUTOMATED-HYDRIDEATOMIC ABSORPTION SPECTROPHOTOMETRYBy Glenda E. Brown and Betty J. McLainABSTRACTThe analysis of natural-water samples for antimony by automated-hydrideatomic absorption spectrophotometry is described. Samples are prepared foranalysis by addition of potassium persulfate and hydrochloric acid followed by anautoclave digestion. After the digestion, potassium iodide and sodiumborohydride are added automatically. Antimony hydride (stibine) gas isgenerated, then swept into a heated quartz cell for determination by atomicabsorption spectrophotometry.Precision and accuracy data are presented. Results obtained on StandardReference Water Samples agree with means established by interlaboratorystudies. Spike recoveries for actual samples range from 90 to 114 percent.Replicate analyses of water samples of varying matrices give relative standarddeviations from 3 to 10 percent.INTRODUCTIONAntimony is a metallic element occasionally found in low concentrations innatural water. It is often used in the semiconductor industry and can serve as atracer element in hydrogeologic systems for gold deposits, or as a pollutionindicator. Determination of antimony also is required by the U.S. EnvironmentalProtection Agency for National Pollutant Discharge Elimination System(NPDES) permits.Determination of antimony by conventional air-acetylene flame atomicabsorption spectrophotometry (AAS) is particularly difficult. The resonancewavelength of antimony is in the low ultraviolet region of the spectrum, whereabsorbance from the acetylene in the flame can occur. Analysis of stibine(antimony hydride or H3Sb) in a heated quartz cell, after hydride generationAAS, removes these interferences and provides lower detection limits than eitherflame AAS or inductively coupled plasma atomic emission spectrophotometry.
In hydride generation A AS, the gaseous hydride is chemically produced byadding sodium borohydride to the sample (Andreae and others, 1981, p. 17661771). The gas is carried by a nitrogen purge into a heated quartz cell. Whenthe stibine gas is atomized in the cell, a peak absorbance signal is produced withthe height being proportional to the amount of analyte in the sample.Water samples received by the National Water Quality Laboratory(NWQL) for analysis of metals are preserved to a pH of less than two using nitricacid to prevent loss of metals from solution. Digestion of the sample withpotassium persulfate with heat under pressure decomposes any organically boundantimony. A subsequent reduction step, the addition of potassium iodide to thesample, guarantees that all of the antimony is present in the 3 valence state.Following the valence-state adjustment, sodium borohydride (NaBH4) andhydrochloric acid (HC1) are added to produce stibine. The stibine is swept, usingan inert gas purge, through a gas-liquid separator into a heated (800 C) quartzcell. There, the stibine molecules are decomposed to atomic vapor, and anabsorbance signal is produced proportional to the antimony concentration in thesample.This report describes a method for determining antimony in samples ofnatural water containing at least 1 }0,g/L using automated-hydride A AS. Themethod supplements other methods of the U.S. Geological Survey (USGS) fordetermination of inorganic substances in water that are described by Fishman andFriedman (1989). This method was implemented in the NWQL in 1979.
ANALYTICAL METHODParameters and Codes:Antimony, dissolved, 1-2055 ( ig/L as Sb): 01095Antimony, suspended recoverable, 1-7055 ( ig/L as Sb): 01096Antimony, whole water recoverable 1-4055 ( ig/L as Sb): 010971.Application1.1 This method may be used to analyze water and water-suspendedsediment containing at least 1 ag/L of antimony. Samples containing antimonyconcentrations greater than 20 jag/L need to be diluted.1.2 Suspended recoverable antimony is calculated by subtractingdissolved antimony from whole water recoverable antimony.1.3 Whole water recoverable antimony in samples of water-suspendedsediment may be determined after each sample has been thoroughly mixed byvigorous shaking, and a suitable portion of sample has been rapidly withdrawnfrom the mixture.2.Summary of methodOrganic antimony-containing compounds are decomposed by a potassiumpersulfate and hydrochloric acid digestion. The resultant decompositionproducts, along with inorganic antimony originally present, react with potassiumiodide, hydrochloric acid, and finally with sodium borohydride to form stibine.The stibine is stripped from solution with the aid of nitrogen and then reduced toantimony atoms in a tube furnace placed in the optical path of an atomicabsorption spectrophotometer at 217.6 nm.3.Interferences3.1 A number of ions, especially transition metal cations, interfere withborohydride reductions. In most natural water, concentrations of metal cationsare several orders of magnitude below the levels causing interference (Andreaeand others, 1981).3.2 Absence of interferences from selenium and arsenic (which alsoform gaseous hydrides) was verified at concentrations of 100 ag/L. Higherconcentrations were not tested.
4.Apparatus4.1Atomic absorption spectrophotometer and recorder.4.2 Refer to manufacturer's manual to optimize instrument for thefollowing:Grating.UltravioletWavelength.217.6 nmSource (electrodeless discharge lamp).Antimony4.3 Autotransformer, variable', Superior1 powerstat type 3 PN1010 orequivalent.4.4 Pyrometer, portable, 0 to 1,200 C; Thermolyne Model PM-20700 orequivalent.4.5 Gas-liquid separator, Pyrex, packed with 3- to 5-mm Pyrex beads(fig. 1). Cooling of the condensing column to 4 C is required. The nitrogen gasflow rate is adjusted for maximum sensitivity by analyzing a series of identicalstandards. An optimum flow rate usually ranges from 100 to 150 mL/min.4.6 Tube furnace, quartz, 10-mm inside diameter x 100-mm length witha quartz eyelet at each end of the tube to anchor nickel-chrome wire and tubefused to the center with a 2-mm inside diameter quartz tube. Wrap the tubefurnace with approximately 5.5 m (18 ft) of 26-gage nickel-chrome wire andcover with an insulating ceramic fiber cloth. Mount lengthwise in the opticalpath of the atomic absorption spectrophotometer.4.7 Technicon AutoAnalyzer II, consisting of sampler with a manifoldand a proportioning pump (fig. 2).5.Reagents5.1 Water: All references to water shall be understood to mean ASTMType I reagent water (American Society for Testing and Materials, 1991).lrThe use of trade, brand, and firm names in this report is for identificationpurposes only and does not constitute endorsement by the U.S. GeologicalSurvey.
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0\Water -Sodium borohydrideHydrochloric acidPotassium iodideAirAi\4 10-turn coilsTo sampler wash receptacle25-turncoilFigure 2. Antimony manifold.Peristalticpump3.9 mL/min3.9 mL/min3.9 mL/min1.2 mL/min2.0 mL/min3.9 mL/min(milliliters per minute)Flow RateTo gas-liquidseparator25-turncoil
5.2 Antimony standard solution I, 1.00 mL 100 g Sb. Dissolve 0.100g Sb metal (99.999 percent) in a minimum amount of aqua regia made fromultrapure acids. Add water to increase rate of dissolution, and dilute to 1 ,000 mLwith water. As an alternative, a commercially prepared standard solution may beused and diluted accordingly.5.3 Antimony standard solution II, 1.00 mL 10.0 ig Sb. Dilute 50.0mL antimony standard solution I to 500.0 mL with water.5.4 Antimony standard solution III, 1.00 mL 1.0 g Sb. Dilute 50.0mL antimony standard solution II to 500.0 mL with water.5.5 Antimony working standards. Prepare a blank and 1,000 mL eachof a series of antimony working standards by appropriate dilution of antimonystandard solution HI with water. Each standard should also contain 0.4 percentconcentrated HNO3 by volume.Antimony standardsolution chloric acid, 12M, concentrated (specific gravity 1.19).NOTE: All chemicals are reagent grade purity unless otherwise specified.5.7 Hydrochloric acid, 6M. Add 500 mL concentrated HC1 (specificgravity 1.19) to water, and dilute to 1 L.5.8 Potassium persulfate solution, 18 g/L. Dissolve 18 g K2S2Og inwater, and dilute to 1 L.5.9 Potassium iodide solution, 100 g/L. Dissolve 100 g KI in water, anddilute to 1 L.5.10 Sodium borohydride solution, 5 g/L. Dissolve 5 g NaBH4 and 40 gNaOH in water, and dilute to 1 L.
6.Procedure6.1 Pipet a volume of well-mixed sample containing less than 0.400 jigSb (20 mL maximum) into a 20- x 150-mm borosilicate test tube.6.2 Pipet 20-mL blanks, standard reference materials, and a complete setof standard solutions containing from 1 to 20 ig/L antimony into 20- x 150-mmtest tubes.6.3 Add 3.0 mL K2S2O8 solution and 1.0 mL of concentrated HC1 toeach test tube. Cover tubes with plastic caps, and autoclave at 11 lb/in2 at 115 Cfor 20 minutes.6.4Set up analytical manifold as shown in figure 2.6.5 Using a variable autotransformer, apply voltage as needed to the tubefurnace to maintain a constant temperature of 800 C. Calibrate the tube furnacetemperature using a portable pyrometer with the thermocouple placed in themiddle of the tube.6.6Feed all reagents through the system, using water in the sample line.6.7 Condition the tube furnace by running two aliquots of a 40- ig/Lundigested standard, followed by six aliquots of a 20-jig/L undigested standard.6.8 With a 10-mV recorder, 20 jig/L of antimony will give a peak approximately 60 percent of full scale. If the sensitivity drops by 30 percent ormore, replace the tube furnace or treat it by swabbing the furnace with hydrofluoric acid, followed by water rinses. (CAUTION: Follow appropriatelaboratory safety procedures when using hydrofluoric acid.)6.9 Set up sample tray to be analyzed. Place digested standards in trayafter beginning with a blank sample. Place individual digested standards, blanks,or Standard Reference Water Samples of varying concentrations about everyeight positions; then fill the remainder of the tray with unknown digestedsamples.6.10 Remove the sample line from the wash solution when the baselinestabilizes, and begin the analysis.
7.Calculations7.1 Prepare an analytical curve by plotting the height of each standardpeak against its respective antimony concentration. This curve may be generatedusing an appropriate computer program. A second-order polynomial function(y ax2 bx c) usually provides improved concentration estimates than does themore conventional linear model (y mx b).7.2 Determine the concentration of dissolved or whole water recoverableantimony in each sample by comparing its peak height to the analytical curve.Any baseline drift is taken into account when computing the height of a sample orstandard peak.7.3 To determine the concentration of suspended recoverable antimony,subtract concentration of dissolved antimony from the concentration of wholewater recoverable antimony.8.Reporting of resultsReport antimony, dissolved (01095), whole water recoverable (01097), andsuspended recoverable (01096), concentrations as follows: concentrations lessthan 10 ig/L are reported to the nearest microgram per liter; results greater thanor equal to 10 jag/L are reported to two significant figures.9.Precision9.1 Precision expressed in terms of percent relative standard deviationfor dissolved antimony for 20 replicate analyses, by one operator, is as follows:Mean( ig/L)1.63.44.610.3Standard deviation( qg/L)0.33.35.581.05Relative standard deviation(percent)20.710.312.510.2
9.2 Precision expressed in terms of percent relative standard deviationfor whole water recoverable antimony for eight replicate analyses, by oneoperator, is as follows:Mean( ig/L)1.41.52.012.0Standard deviation(jiig/L)Relative standard SSION OF RESULTSPrecision and BiasU.S. Geological Survey Standard Reference Water Samples (SRWS) wereused to evaluate the precision of the method for determining antimony byautomated-hydride atomic absorption spectrophotometry. Precision data forseven different SRWS's are listed in table 1. Replicate analyses were performedon each sample over a period of several days. An indication of the bias of themethod, as well as antimony values determined by interlaboratory studies, arealso listed in table 1.10
Table \.--Precision and bias of antimony method[NWQL, National Water Quality Laboratory; SRWS, Standard Reference Water Sample of theBranch of Quality Assurance; AAS, atomic absorption spectrophotometry;M-g/L, micrograms per liter; n, number of replicates; --, no 69.45.412.0NWQLautomated-hvdride 3.710.210.213.76.8Interlaboratory igma values USGS Branch of Quality Assurance changed statistical reportingmethods.The automated-hydride AAS means generally agree with the meansobtained by interlaboratory analysis, and the results all fall within one standarddeviation of the interlaboratory mean value. The percent relative standarddeviations range from 5 to 24 percent. These data also are shown in figure 3. Inaddition, the precision of the automated technique is substantially better than themanual method (13-50 percent relative standard deviation) as reported byFishman and Friedman (1989, p. 69).The Student's t-test indicated a statistically significant difference at the 95percent level between the interlaboratory means and the hydride AAS means forseveral of the SRWS. Those differences may be attributed to the one-laboratory,one-operator, one-method operation for the automated-hydride system comparedto the multiple laboratory, multiple operator, multiple methods (flame AAS, ICP,and manual hydride) used in determining the interlaboratory most probable value(MPV). The differences between the automated-hydride values andinterlaboratory MPVs become much less significant when considering theprecision of the method and the reporting limit (nearest microgram per liter forvalues less than 10 jig/L, and two significant figures for values at or greater than1011
16DCNational Water Quality Laboratory automated-hydride atomicabsorption spectrophotometry standard deviationPHInterlaboratory program standard deviation(none for pseudosigma value)OODC12u 10QOgtDCOwm oLINE OF EQUAL RELATION6ILJJ9DC4QQLJJ'r f"468101214INTERLABORATORY MEAN, IN MICROGRAMS PER LITERFigure 3. Relation between antimony mean concentrations determined byautomated-hydride atomic absorption spectrophotometry and interlaboratory means for several Standard Reference Water Samples.1216
Spike RecoveryTo further determine the accuracy of this method, 17 natural-watersamples and blanks were spiked with known concentrations of antimony, thenprepared and analyzed according to the automated-hydride AAS method. Theresults from the spike study are listed in table 2. Recoveries ranged from 90 to114 percent.The spike recovery studies on the 17 blanks and samples were repeatedfour times to give another measure of the precision of the method. Replicate dataare listed in table 2, indicating percent relative standard deviations in the 3- to10-percent range, depending on concentration.Replicate samples also were prepared and analyzed on four separate days togive an additional measure of method repeatability on actual sample matrices.These data are listed in table 3 and shown in figure 4. Again, for actual sampleswith varying antimony concentrations (dissolved and whole water recoverable),the percent relative standard deviations range from 3 to 10 percent. For samplesnear the detection limit of 1 (J-g/L, the precision decreases, with percent relativestandard deviations in the 20- to 50-percent range.13
Table 2. Recovery of antimony in surface-water samples, micrograms per liter]Mean results, in .66.45.410.08.16.45.15.0Table 3.--Sample replication listing mean, standard deviation, and relativestandard deviation for four separate analyses, micrograms per iation(ue/L)Relative standarddeviation .81.71.57.49.79.68.69.812.31.412.014
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Detection LimitThe method detection limit (MDL), using the procedure from the U.S.Environmental Protection Agency (1992, p. 565-567), is defined as the minimumconcentration of a substance that can be measured and reported with 99 percentconfidence that the analyte concentration is greater than zero. The theoreticaldetection limit was determined to be 0.71 ig/L, on the basis of three times thestandard deviation of multiple blank determinations. An antimony standard wasprepared with a concentration of 3 ig/L (approximately three times thetheoretical detection limit). This standard then was analyzed ten timesnonconsecutively. A mean and standard deviation were calculated from the datalisted in table 4 to determine the MDL. According to the U.S. EnvironmentalProtection Agency (1992, p. 565-567), the MDL was calculated to be 0.42A reporting limit of 1 (ig/L was chosen as appropriate for this analysis.Table 4.--Determination of the antimony method detection limit[fig/L, micrograms per liter]ReplicateConcentrationnumber ( ig/L)12.822.832.942.952.762.773.082.892.910 3,2Mean( igTL)Standard deviation (fig/L)Number of pointsDegrees of freedomt value (99 percent confidence)Method detection limit16 2.87 0.149 10 9 2.821 0.42
CONCLUSIONSThe automated-hydride atomic absorption spectrophotometric method forantimony gives reproducible and accurate results for natural-water samples. Theprecision and bias are equal to or better than a manually operated antimonyhydride system. The automated system also provides the operator with ease ofanalysis and minimizes errors from incorrect or variable reagent additions thatmight occur during manual-hydride A AS analysis.REFERENCESAmerican Society for Testing and Materials, 1991, Annual book of ASTMstandards, Section 11, Water: Philadelphia, v. 11.01, p. 36, 45, and 434.Andreae, M.O., Asmode, J.F., Foster, P., and Van't dack, L., 1981,Determination of antimony(in), antimony(V), and methylantimony speciesin natural waters by atomic absorption spectrometry with hydridegeneration: Analytical Chemistry, v. 53, no. 12, p. 1766-1771.Fishman, M.J., and Friedman, L.C., eds., 1989, Methods for determination ofinorganic substances in water and fluvial sediments: U.S. GeologicalSurvey Techniques of Water-Resources Investigations, book 5, chap. Al,545 p.U.S. Environmental Protection Agency, 1992, Primary drinking-waterregulations, maximum contaminant levels (Appendix B to part 136,National primary drinking-water regulations): U.S. Code of FederalRegulations, Title 40, parts 100-149, revised as of July 1, 1992, p. 565567.17
4.6 Tube furnace, quartz, 10-mm inside diameter x 100-mm length with a quartz eyelet at each end of the tube to anchor nickel-chrome wire and tube fused to the center with a 2-mm inside diameter quartz tube. Wrap the tube furnace with approximately 5.5 m (18 ft) of 26-gage nickel-chro
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