tomic pectroscopy S

In This Issue:

Application of Solvent Extraction and Acid Hydrolysis of Nb/Ta Separation Methodsfor the Determination of Uranium in Geological Materials, Nb/Ta-type Samples,and Leach Liquors by ICP-OESK. Satyanarayana and M.A. Nayeem........................................................................77

Direct Determination of Trace Elements in Micro Amounts of Biological SamplesUsing Electrothermal Vaporization Coupled to ICP-AESShizhong Chen, Zucheng Jiang, Bin Hu, Zhenhuan Liao, and Tianyou Peng......86

Simple Method for the Selective Determination of AS(III) and AS(V) by ETAAS AfterSeparation With Anion Exchange Mini-columnPatricia Smichowski, Liliana Valiente, and Ariel Ledesma....................................92

Preconcentration and Speciation of Chromium in Natural Waters Using Ion-PairExtraction and Graphite Furnace AASK. Kargosha, M. Noroozifar, and J. Azad................................................................98

Analytical Graphite Furnace Atomic Absorption Spectrometry - A Laboratory GuideG. Schlemmer and B. Radziuk................................................................................103

Spectroscopy

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Vol. 23(3), May/June 2002

77Atomic SpectroscopyVol. 23(3), May/June 2002

*Corresponding author.e-mail: [email protected]

Application of Solvent Extraction and Acid Hydrolysis of Nb/Ta Separation Methods for the Determination of

Uranium in Geological Materials, Nb/Ta-typeSamples, and Leach Liquors by ICP-OES

*K. SatyanarayanaChemical Laboratory, Atomic Minerals Directorate for Exploration and Research

Department of Atomic EnergyP.O. Assam Rifles, Nongmynsong, Shillong – 793 011, Meghalaya, India

andM.A. Nayeem

Chemical Laboratory, Atomic Minerals Directorate for Exploration and ResearchDepartment of Atomic Energy

Begumpet, Hyderabad – 500 016, India

ABSTRACT

Two methods are describedfor the separation and estimationof uranium using (a) solventextraction for geologicalmaterials of silicate matrix,yellow cakes, and leachliquor-type samples and (b) acidhydrolysis for Nb/Ta- bearingsamples by inductively coupledplasma optical emissionspectrometry (ICP-OES). Solventextraction separation of U from1−4 M HNO3 acid medium usingtri-n-butyl phosphate (TBP) ortri-n-octyl phosphine oxide(TOPO) in carbon tetrachlorideselectively separates U from theaccompanying elements indifferent types of geologicalmaterials.

The extracted U content isback-stripped using sodiumpyrophosphate/sodiumtri-polyphosphate or ammoniumcarbonate in the presence ofpentanol-1 for quantitativerecoveries. This is performedbefore aspiration into the plasmafor measurement at the U(II)409.014-nm emission line.However, ammonium carbonateis recommended since it resultsin better signal-to-noise ratios.Acid hydrolysis separationof Nb/Ta-type samplesquantitatively separates U fromthe major matrix elements andthe remaining common elementsdo not influence the U signal inICP-OES.

The silica-rich geologicalmaterials were dissolved byHF-HNO3–H2SO4 treatmentfollowed by dissolution in 4MHNO3 acid before applying thesolvent extraction procedure.In the case of Nb/Ta-bearingsamples, U was separated fromNb and Ta by acid hydrolysis,involving fusion with Na2O2,dissolution in HCl, followedby NH4OH precipitation andhydrolysis in HCl in the presenceof sulfurous acid. The oxychlorideprecipitates of Nb and Ta arefiltered off and the filtratesassayed for U.

The proposed methods wereapplied to some internationalgeological reference standards(GXR-1, SY-2, SY-3, Mica-Fe,NBS-120b, and DH-1a), severalyellow cake samples, leach liquors,and to some Nb/Ta-bearingsamples including two BritishGeological Survey referencestandards, IGS-33 and IGS-34.The results are compared withfluorimetric (in case of Nb/Tasamples) and gravimetric methods(in case of yellow cakes). Bothmethods described are simple,rapid, and accurate showing arelative standard deviation of lessthan 1% at the 2610-µg/g leveland 4.8% at the 33-µg/g level,with the lowest determinationlimit obtained at 25 µg/g.

INTRODUCTION

Uranium (U) is a sparselydispersed element in the earth’scrust with an average crustalabundance of 1.8 µg/g (1). Eventhough the occurrence ofindependent uranium mineralslike Uraninite (UO2), Autunite[Ca(UO2)2 (PO4)2 . 10–12 H2O], Tor-bernite [Cu(UO2)2 (PO4)2 . 12H2O],Carnotite [K2(UO2)2 (VO4)2 .1–3H2O], Uranophane [CaO . 2UO3

. 2SiO2 . 6H2O], and Coffinite[U(SiO4)1-x (OH)4x] is reported (2),their deposition and occurrence isvery rare. Coupled substitutionsand isovalent replacements of ionsof Nb, Ta, Ti, rare earth elements(REEs), Th, etc. by uranium ion areobserved in many minerals likeBetafite [(U,Ca) (Nb,Ta,Ti)3 O9 .H2O], Brannerite [(U, Ca, Fe, Th,Y)3 Ti5O16], and Davidite [(Fe, Ce,La, Y, U, Ca, Zr, Th) (Ti, Fe, V, Cr)3

(O,OH)7]. Accordingly, highuranium values are reported (2) inNb-Ta and other minerals such asBetafite (up to 27.15% UO3),Brannerite (up to 51.76% U3O8),and Davidite (up to 9.8% U3O8),mainly of pegmatatic origin.However, the largest reserves ofuranium are disseminated insediments, usually sandstones andthe majority of smaller deposits arepitchblende veins such as those ofGreat Bear Lake in Canada andKatanga in the Congo.

78

There is an increasing demandworldwide for uranium and itsnuclear products like radium due totheir unique applications in newtechnologies such as pigment andchemical industries, medical andother industrial applications,uranium-fueled nuclear reactors forgeneration of electricity for civilianuse, in addition to their uses formilitary/defense purposes.

The multifarious compositionof geological samples and therefractory nature of many elementsmake multielement analysis ofgeological samples often difficultand time-consuming. The demandfor fast, yet accurate multi-elementanalysis in geological prospectinghas necessitated the developmentof new instrumental methods toreplace some of the classicalmethods as described byWashington (3). Several methodshave been reported (4–16) forthe determination of uraniumin various types of samplesincluding geological materials.Cation-exchange columnchromatography coupled tothermal ionization massspectrometry (10), solventextraction followed byfluorimetry/UV-VISspetrophotometry (7,14),preconcentration on polyurethanefoam coupled to X-ray fluorescencespectrometry (9), ashing of plantsamples followed by instrumentalneutron activation analysis (INNA)(12), inductively coupled plasmamass spectrometry (ICP-MS)(8,11,15,16), or inductivelycoupled plasma optical emissionspectrometry (ICP-OES) (5,6,13)have been applied for theseparation from major matrixelements and estimation ofuranium. The most popular methodroutinely used in our laboratory isthe extraction of uranium from thematrix elements with ethyl acetatein the presence of aluminum nitrateas a salting-out agent, followed byfusion of the dry aliquot using a

mixture of (1:4) NaF/Na2CO3 andestimation by fluorimetry. The useof atomic absorption spectrometry(AAS) is limited to indirect methodswhere trace uranium is concerned(4) due to the refractory nature ofthe uranium oxides, resulting inlow atomization efficiency andpoor sensitivity. Almost all methodsrequire preliminary separation ofuranium from the other elementsthat may interfere with thedetermination of uranium, andalso for better signal-to-noisebackground ratios at trace leveldeterminations. However, analternative method for theestimation of uranium in differenttypes of geological materialsincluding leach liquors and yellowcake samples using ICP-OES isproposed.

In the present study, twodifferent separation proceduresare described for the estimation ofuranium in geological materialsby solvent extraction and inNb/Ta-bearing samples by acidhydrolysis (20,21). Detailed studiesof different solvent extractionseparation systems were conductedincluding tri-n-octyl phosphineoxide (TOPO)/CCl4, anddi-(2-ethylhexyl)phosphoric acid(D2EHPA)/toluene systems fromnitric acid solutions. The solventextraction procedure usingTBP/CCl4 or TOPO/CCl4 is quiteeffective for the quantitative andselective extraction of uraniuminto the organic phase. Differentstripping systems such assodium pyrophosphate, sodiumtri-polyphosphate, or ammoniumcarbonate were attempted tobackstrip the extracted uraniumcontent from the organic phase.Comparing the results of all thesesystems, the ammonium carbonatesystem gave better signal-to-noiseratio for the trace level estimationof uranium. However, quantitativerecoveries could not be obtainedusing ammonium carbonate/sodiumtri-polyphosphate from

D2EHPA/toluene extracts andthe results are presented.

The uranium content inNb/Ta-bearing samples wasquantitatively separated from Nband Ta by acid hydrolysis usingprior fusion of the sample withsodium peroxide, dissolution inHCl, followed by ammoniaprecipitation and dissolution ofthe precipitate in HCl (finalconcentration 10%, v/v) to carryout acid hydrolysis in the presenceof added sulfurous acid. Theprecipitated oxychlorides of Nband Ta are filtered off and thefiltrate is monitored for uraniumvalues by ICP-OES.

The proposed solventextraction-stripping method wasvalidated by applying it to variousinternational reference standardssuch as GXR-1 [Jasperoid, UnitedStates Geological Survey (USGS)],SY-2, SY-3 [Syenites, CanadianCertified Reference Materialsproject (CCRMP)], Mica-Fe Biotite[Centre de RecherchesPetrographiques et Geochimiques,France (CRPG)], NBS-120bPhosphate Rock [National Bureauof Standards, USA (NBS)], andDH-1a Uranium Thorium Ore[Canada Centre for Mineral andEnergy Technology, Canada(CANMET)]. The method was alsoapplied to some “yellow cakes” andleach liquor samples. The acidhydrolysis method was applied toseveral Nb/Ta-bearing samples,including two referencestandards, IGS-33 and IGS-34[Niobates-Tantalates, Institute ofGeological Sciences, U.K (IGS)]and the results are compared withfluorimetric values. The proceduresshow excellent agreement with thecertified values, whereas in case ofIGS-33 and IGS-34, the obtainedvalues are presented as theusable values.

79


EXPERIMENTAL

Instrumentation

Fluorimetric MeasurementsAll fluorimetric measurements

were performed using a Jarrell Ashfluorimeter, Digital Model JarrellAsh, 27-000.

ICP-OES MeasurementsAll ICP-OES measurements were

made using a LABTAM (now GBC,Australia) Model 8410 Plasma scanICP-OES instrument, equipped witha computer-controlled rapidscanning monochromator (focallength 0.75 m), employing a ruledgrating of 1800 grooves/mm in aCzerny-Turner mounting. A moredetailed description of theequipment is reported elsewhere(22,23). The optimum instrumentalparameters and other operatingconditions are given in Table I. Allmeasurements were made undervacuum conditions. Calibration wasdone with a uranium standard (20µg/mL), prepared by serial dilutionof solutions made from 99.99 %specPure oxides (Johnson Matthey,Royston, U.K.). The instrumentaldetection limit of uranium at409.014 nm was found to be0.0686 µg/mL.

Reagents

All reagents and standardsused were prepared fromanalytical grade and specPurechemicals (Johnson and Matthey).Double-distilled water was used forall subsequent dilutions whereverrequired. Uranium standard stocksolution (1000 µg/mL) wasprepared by dissolving 0.2948 gU3O8 in a minimum volume ofnitric acid and made to 250-mL finalvolume, maintaining an overallacidity of 10% (v/v) HNO3.

PROCEDURE

Sample Decomposition

Geological Samples of SilicateMatrix

A 1.0-g sample (150–200 mesh)was digested in a platinum dishwith a mixture of hydrofluoric acid(10 mL, 40 %) and nitric acid (10mL, 8M) to dryness, followed byHNO3 – H2SO4 treatment with afinal dissolution in HNO3. Theresidue, if any, was fused with aminimum of sodium carbonatein a platinum crucible, the cooledmelt was dissolved in the originalfiltrate and made up to 50-mLvolume in a final nitric acidconcentration of 4M.

Leach Liquor SamplesA 2–10 mL aliquot of the liquor

sample was evaporated to drynessin a 100-mL glass beaker, followedby HNO3 treatment (twice, 10 mLof 8M each time) with a finaldissolution in 50 mL of 4M HNO3.

Solvent ExtractionThe sample solution in 4M HNO3

(50 mL) was extracted twice with10 mL each time of 30% (v/v) TBPin CCl4 (pre-equilibrated with 4MHNO3) for a contact time of 5 min,using a separating funnel. Bothorganic extracts (20 mL) containinguranium were back-washed with100 mL of 4M HNO3 and theaqueous layer was discarded. Theorganic extract was back-strippedthree times with 10 mL each of0.5M ammonium carbonate + 3 mLpentanol–1 for a contact time of 5min. The three aqueous layers weremixed, neutralized with HNO3,boiled for 5–10 min, cooled andbrought to 50-mL final volume(or above depending on theconcentration of uranium),maintaining an overall acidity of5% (v/v) HNO3. This solution wasaspirated into the plasma forestimating uranium concentrationvalues at U(II) at 409.014 nm afterprior calibration with the workingstandards.

TABLE I ICP-OES Instrumental Parameters and Operating Conditions

Instrument LABTAM Model-8410 Plasmascan

Rf generator 27.12 MHz(crystal-controlled )

Plasma torch Demountable type, DMT-2000

(GBC, Australia)

Pump Peristaltic, ten-roller, Gilson® Minipuls-2

Nebulizer G.M.K. (V-groove, modified Babington type)

Operating power 1.2 KW

Reflected power <5 W

Viewing height 14-mm above load coil

Argon gas flow rates :

Coolant 14 L/min

Auxiliary 1.0 L/min

Sample 0.8 L/min

PMT voltage 1000 V

Integration time 3 s (n=3)

Solution uptake rate 4.0 mL/min

Sample flush time 10 s

Entrance slit 20 µm and 3 mm height (fixed)

Exit slit 40 µm

Peak search window width 0.12 nm

80

Table II lists the analytical values of uranium inreference materials GXR-1 (USGS), SY-2, SY-3, DH-1a(CCRMP/CANMET), Mica-Fe (CRPG), and NBS-120 b(NBS). The relative standard deviation (RSD) of thesolvent extraction method is 0.55% (at the 2610-µg/glevel) to 4.79% (at the 33.0-µg/g level).

The (%) analytical results for U3O8% in someyellow cake samples are presented in Table IIIand the values compare very well with thewell-established homogeneous gravimetricprocedure. Table IV shows the uranium valuesobtained by the proposed method for some of theleach liquor samples. The values obtained by directestimation in all of the studied samples are on thehigher side due to some spectral and backgroundinterferences.

Acid Hydrolysis (Nb/Ta-bearing Samples)A 0.5-g sample (150-200 mesh) was fused with

10.0 g of Na2O2 in a nickle crucible and the cooledmelt was put in dilute (5% , v/v) HCl and boiled forabout 15 min. Ammonia precipitation was carriedout in the presence of NH4Cl and the hydroxideprecipitate was filtered off through a Whatman 540(15 cm) filter paper. The precipitate was thoroughlywashed with a solution of 5% (v/v) NH4OH in 1%(w/v) NH4Cl, then transferred into the same beakerwith a fine jet of 10% (v/v) HCl, and brought to afinal volume of about 100 mL, maintaining an overallacidity of 10% (v/v) HCl. The solution was boiledgently to complete the hydrolysis of Nb and Ta(about 15 min). A few drops (5–6 drops) of sulfurous

TABLE IIUranium Values Obtained by

Proposed Solvent Extraction Method in Some Standard Reference Materials (SRM)

Using ICP-OES (at 409.014 nm)

Uranium (µg/g)Sl No. SRM Proposed (%) RSD Certifd Valueb

Methoda U (µg/g)

1. GXR-1 33.0 4.79 34.92. Mica-Fe 79.4 3.63 80.03. NBS-120b 125.8 3.00 128.44. DH- 1a 2610.0 0.55 2629.0 *5. SY-2 275.0 2.87 284.0

6. SY-3 647.6 1.77 650.0

a Extraction with 30% TBP/CCl4 (from 4M HNO3solution) and stripping with 0.5M AmmoniumCarbonate + Pentanol-1 system.b Certified values (24, 25*).

TABLE IIIAnalytical Results for (%) U3O8 in Some Yellow Cake Samples at the

409.014-nm Emission Line Using ICP-OES

% U3O8Sample Dissolutiona Gravimetricb (%) RSDc

Procedure Procedure

AMD-15 62.64 62.45 0.66

AMD-16 67.22 67.45 0.88

AMD-17 64.72 64.80 0.34

a Average of five values, obtained by dissolution of the sample (0.2 g)after treatment with HNO3 and residue if any (due to silica) wasfiltered off, ignited in a platinum crucible, treated with HF and finaldissolution in 5% (v/v) HNO3, both filtrate and residue solutionwere mixed and made to 100 ml volume. Suitable aliquot wassubjected to the proposed solvent extraction – stripping method.

b Values obtained by the gravimetric procedure after homogeneousprecipitation of uranium as uranyl ammonium phosphate andsubsequent ignition to uranium pyrophosphate for measurement.

c Relative standard deviation of the above proposed method.

TABLE IVAnalytical Results for Uranium (mg/L) in

Some Leach Liquor Samples Using ICP-OES at the409.014-nm Emission Line

Proposed Method a Direct Estimation b(Average of five values) (Average of five values)

Sample U (mg/L) (%) RSD U (mg/L) (%) RSD

AMD-18 144.4 0.63 147.2 1.06

AMD-19 403.0 0.46 425.4 0.76

AMD-20 247.8 0.38 276.0 0.48

AMD-21 193.0 0.41 216.6 0.68

AMD-22 279.4 0.73 301.2 1.00

AMD-23 165.1 0.53 183.5 0.73

AMD-24 225.1 0.37 242.4 0.58

AMD-25 215.0 0.47 243.4 0.63

AMD-26 375.8 0.59 412.2 0.77

AMD-27 254.4 0.91 274.8 1.06

a Estimated by the proposed method of solvent extractionseparation of uranium using 30% TBP/CCl4 followed by strippingusing ammonium carbonate and final dissolution in 5% (v/v) HClacid solution.b Estimated direct, after evaporation of suitable aliquot followedby dissolution in 5% (v/v) HCl acid solution.

81


acid (5–6% SO2 solution) and aquarter of a Whatman ashless filtertablet were added to the solution,which was then cooled and filteredthrough the same filter paper withthorough washing with 10% (v/v)HCl. The filtrate was made up to100-mL volume, maintaining 10%(v/v) HCl before aspiration into theplasma for measurement of theuranium signal.

The analytical results of thedetermination of uranium in theNb/Ta-bearing samples arepresented in Table V. The resultscompared very well with the valuesobtained by the well-establishedfluorimetric method. The relativestandard deviation of the methodvaries from about 1% (0.8% at the0.210% U3O8 level) to 5.3% (at the0.032% U3O8 level). During the acidhydrolysis stage, sulfurous acid wasadded to act as a coagulant forcolloidal niobic and tantalic acids.

RESULTS AND DISCUSSION

Selection of Emission Line

Considering the expectedelemental concentrations, spectralinterferences, and detection limits,the 14 most sensitive emission linesof uranium, as listed in the Atlas ofSpectral Lines (26), were scannedthoroughly and the ionic emissionline at 409.014 nm was chosen forthe matrix elements. The line givesa detection limit of 0.0686 µg/mL Uwith a net signal-to-backgroundintensity of 21.9 for U, 50 µg/mL.The line is free from almost allmajor elements, except a minorcorrection from Nb, Ta, and Zr.The instrumental detection limitsand background equivalentconcentrations obtained for the14 emission lines studied arepresented in Table VI and theinter-elemental and spectralinterferences studied are presentedin Tables VII and VIII.

(%) U3O8Sample Proposed Fluorimetr. (%)

Methoda Valuesb RSDc

IGS – 33 d 0.032 0.033 5.30

IGS – 34 d 0.433 0.432 0.97

AMD – 1 0.120 0.121 2.17

AMD – 2 0.115 0.116 1.99

AMD – 3 0.130 0.132 1.28

AMD – 4 0.139 0.142 1.64

AMD – 5 0.045 0.046 5.09

AMD – 6 0.126 0.127 1.18

AMD – 7 0.063 0.063 2.36

AMD – 8 0.167 0.168 1.37

AMD – 9 0.072 0.074 2.31

AMD – 10 0.151 0.153 1.11

AMD – 11 0.066 0.066 2.55

AMD – 12 0.210 0.213 0.80

AMD – 13 0.068 0.070 2.45

AMD – 14 0.150 0.151 1.12

a Average of five values, obtainedusing the proposed method byICP-OES at the 409.014-nm line inacid hydrolysis solution.

b Values obtained by pelletfluorimetry (fusion of the samplewith KHSO4 and dissolution incitric acid followed by extractionof uranium using ethyl acetate inpresence of Al(NO3)3 . 9H2O assalting out agent from 10% HNO3acid medium. The residue afterevaporation of the ethyl acetatealiquot was fused with Na2CO3 +NaF (4:1) mixture in a platinumdish and fluorescencemeasurements were made on thecooled pellet.

c Relative standard deviation of the proposed method.

d International Reference Standards (Niobate-Tantalate) from BritishGeological Survey, U.K.

TABLE VAnalytical Results for (%) U3O8 in Some Nb-Ta Samples

TABLE VIDetection Limits (DLs) and Background Equivalent Concentrations

(BECs) of U (50 µg/mL) in 10% (v/v) HCl With Peak Integration Timeof 3 s (n=3). Ip = Peak intensity; Ib = Background Intensity

Wavelength Ip Ib BEC DL Ip-Ib/Ib(nm) (µg /mL)a (µg /mL)b

385.958 3429 100 1.5020 0.0451 33.3

367.007 2092 123 3.1234 0.0937 16.0

263.553 2173 181 4.5432 0.1363 11.0

409.014 2676 117 2.2860 0.0686 21.9

393.203 2086 104 2.6236 0.0787 19.1

424.167 1590 128 4.3775 0.1313 11.4

294.192 348 36 5.7692 0.1731 8.7

385.466 1522 112 3.9716 0.1191 12.6

288.963 702 70 5.5380 0.1661 9.0

256.541 1251 75 3.1888 0.0957 15.7

279.394 1046 91 4.7644 0.1429 10.5

468.907 539 66 6.9767 0.2093 7.2

424.437 939 78 4.5296 0.1359 11.0

286.567 760 61 4.3634 0.1309 11.5a BEC is calculated from: Concentration (µg /mL) x Ib / (Ip – Ib).b DL is calculated based on three times the standard deviation of the blank at 1% RSD

(DL = BEC x 0.03) (ref. No. 23).

82

TABLE VIIInter-elemental and Spectral Interferent Studies

on Different Uranium Emission Lines

Equivalent Uranium Concentration (µg/mL)Interferent/Concn 385.958 367.007 263.553 409.014 393.203 424.167 294.192(100 µg/mL) nm nm nm nm nm nm nm

Nb 1.6737 2.0600 Nil 0.8220 0.4453 0.0815 1.0130Ta 1.8560 0.0620 881.60 0.3189 0.3082 Nil NilFe 0.5613 0.4658 0.2175 Nil Nil Nil NilMn 0.1735 0.0593 4.8000 Nil 0.1346 Nil NilTi 0.2245 0.1355 2.1900 Nil 13.4600 Nil 54.68Sn 0.2857 0.1694 0.5785 Nil Nil Nil 0.1089W Nil 0.1440 1.2600 Nil Nil Nil NilZr 0.0510 0.1186 1.5700 0.3069 0.1767 23.1600 0.1634V 0.3929 0.0762 4.6600 Nil 0.6648 0.1255 0.9259Mo 0.0867 0.0593 2.3700 Nil Nil Nil 0.0545Th 1.9900 14.6900 0.1531 Nil 18.6200 Nil 5.500Ca Nil 0.3049 0.3179 Nil 18.6800 Nil 0.1634Mg 0.3113 0.0932 0.2845 Nil Nil Nil 2.6700Al Nil 0.1355 Nil Nil Nil 0.0456 0.2179Y 0.1750 0.1184 0.5294 Nil Nil Nil 0.2075

Nil = indicates no interference.

TABLE VIIIInter-elemental and Spectral Interferent Studies

on Different Uranium Emission Lines

Equivalent Uranium Concentration (µg/mL)InterferentConcn 385.466 288.963 256.541 279.394 468.907 424.437 286.567(100 µg/mL) nm nm nm nm nm nm nm

Nb 19.5365 0.7699 4.5205 Nil Nil Nil 84.9108Ta 17.7700 Nil 1.2455 20.6480 Nil Nil NilFe Nil Nil 0.1985 1.3600 Nil 0.0783 NilMn Nil 58.1490 16.9500 Nil 0.1762 Nil 0.1700Ti Nil 0.2109 2.4400 Nil Nil Nil 1.8100Sn 0.2500 Nil 0.8225 Nil Nil Nil NilW Nil Nil 0.6098 Nil Nil 8.9116 NilZr 0.2500 Nil 0.9359 Nil Nil Nil 25.5300V Nil 396.9190 4.4400 Nil 0.1409 Nil NilMo 0.4900 Nil 1.1500 0.2798 Nil Nil 9.8300Th 84.92 Nil 298.4800 Nil 79.5300 Nil NilCa 0.1400 Nil 0.8366 Nil Nil Nil NilMg Nil Nil 0.9217 Nil Nil 0.1175 0.8800Al Nil Nil 0.8508 Nil Nil 0.1175 NilY Nil Nil 1.1346 Nil Nil Nil Nil

Nil = indicates no interference.

Effect of Different Acids andTheir Concentrations on theUranium Emission Signal

The effect of different acidssuch as HCl, HNO3, HClO4, H2SO4,and H3PO4 and their concentrationsin the range from 0.5 N to 8.0 N onthe uranium emission signal (20µg/mL) at the six most sensitiveemission lines (385.958, 367.007,263.553, 409.014, 393.203, and424.167 nm) were studied and theresults presented pictorially inFigure 1. The depression in theuranium signal varied up to 37.5%and was severe at the 263.553-nmline in the presence of 8.0 NH2SO4. The depression effect isminimum on the uranium signal atthe 409.014-nm line in thepresenceof HNO3 acid. TheICP-OES, equipped with a G.M.K.nebulizer (modified Babingtontype), provided drift-free analyticalsignals in an aqueous solutionof 0.5–8.0 N HNO3 even after acontinuous run of 3–4 hours.

Solvent Extraction Studies forthe Recovery of Uranium FromMatrix Elements

In order to avoid the spectraland matrix interferences frommajor elements, three differentsolvent extraction systems [30%(v/v) TBP/CCl4, 0.5% (w/v)TOPO/CCl4, and 30% (v/v)D2EHPA/toluene] were studiedand the results are presented inTable IX. Quantitative extractionof uranium was found in allsystems in the presence of somesynthetic major matrix elements,but the recovery was notquantitative in the D2EHPA/toluenesystem using the stripping agents0.2M sodium tripolyphosphate(plus pentanol-1) or 0.5Mammonium carbonate (pluspentanol-1), while quantitativerecoveries were observed using0.2M sodium pyrophosphate (pluspentanol-1) in all the extractionsystems studied. Even thoughquantitative recoveries wereobtained using the ammonium

83


Fig. 1. Effect of different acids on uranium (20 µg/mL).

84

TABLE XAnalytical Results for Uranium in Some Synthetic Nb-Ta Samples

Obtained at the 409.014-nm Emission Line Using ICP-OES After Acid Hydrolysis Separation

Sample/ Uranium (µg)matrix Added Found (%) Recovery

SYN-1 500 490 98SYN-2 500 485 97

SYN-3 500 495 99

SYN-1: Nb2O5 - 350 mg; Ta2O5 - 100 mg; TiO2 - 25 mg; Mn - 25 mg; Fe - 25 mg.SYN-2: Nb2O5 - 100 mg; Ta2O5 - 350 mg; TiO2 – 25 mg; Mn - 25 mg; Fe - 25 mg.SYN-3: Nb2O5 - 250 mg; Ta2O5 - 250 mg; TiO2 - 25 mg; Mn - 25 mg; Fe - 25 mg.

TABLE IXComparison of Different Solvent Extraction/Stripping Systems

for Separation and Estimation of Uranium inSynthetic Mixtures by ICP-OES (at 409.014 nm)

Uranium Concentration (µg)30% TBP/CCl4 0.5%TOPO/CCl4 30% D2EHPA/Toluene

Extraction/Stripping System* Sample No. Added Found Added Found Added Found

1. 0.2M Sodium pyrophosphate (10 mL) + Pentanol-1 (3 mL)200 202 200 203 200 190

2. 0.2M Sodium tri-polyphosphate (10 mL) + Pentanol-1 (3 mL )200 203 200 203 200 30

3. 0.5M Ammonium carbonate (10 mL) + Pentanol-1 (3 mL)

200 198 200 204 200 48

Composition of synthetic mixture: Fe, Al, Ca, Mg - 100 mg each; Na, K, P, Mn, Ti - 50 mg each.* Aqueous solution for extraction: 50 mL (1–4M HNO3);

No. of extractions: 2 (5 min each extraction).Organic solution: 10 mL (extraction); No. of strippings: 3 x 10 mL (5 min each stripping).

carbonate-pentanol-1 strippingsystem in both TBP or TOPO/CCl4extraction systems, TBP/CCl4 isrecommended from an economicalpoint of view and also because TBPin comparison to TOPO is areagent commonly used in mostlaboratories. However, the additionof pentanol-1 during the strippingstage is essential as its presencefacilitates the quantitativerecoveries for uranium values,otherwise the recoveries are notfound beyond 60% of the added orpresent value.

Acid Hydrolysis SeparationStudies for the Recovery of Uranium From Nb/Ta-bearingSamples

Three synthetic niobate-tantalatesamples, prepared by dopingknown amounts of uranium (500µg) were also studied by adoptingthe proposed method; the resultsare presented in Table X.Quantitative recoveries (97–99%)were obtained in the three cases,using the experimental conditionsof the proposed method with theaddition of sulfurous acid, asrecommended in the aboveprocedure. Further, the additionof sulfurous acid helps inproducing a dense and easilyfilterable precipitate, thusfacilitating the quantitativeseparation of Nb and Ta.

CONCLUSION

The solvent extraction-strippingmethod described is rapid, precise,accurate, and highly selective forthe extraction of uranium indifferent types of geologicalmaterials including leach liquorsand yellow cake samples. Themethod offers a detection limit of25 µg/g and above in a variety ofsamples. The proposed acidhydrolysis method for theseparation and determination ofuranium in Nb/Ta-bearing samplesis a simple method and offers thebest results over other methods,

because of the hydrolyzable natureof Nb and Ta. The samplepreparation and matrix separationprocedures are recommended forroutine ICP-OES analysis of samplessuch as Niobate-Tantalate,Pyrochlore, Samarskite, Aeschynite,Betafite, and Brannerite type ofsamples. The acid hydrolysisprocedure also enables thedetermination of the trace rareearth elements Y, Sc, and Th, inaddition to uranium, in the samesample solution, which is notpossible in other separationtechniques.

ACKNOWLEDGMENTS

The authors thank Dr. H.C.Arora, Associate Director(Chemistry), AMD, for his constantencouragement in the R&Dactivities during the course of thesestudies. Further, the authors offertheir sincere thanks and gratitudeto Sri R.K. Gupta, Director, AMD,for his kind permission to publishthese findings.

Received September 4, 2001.Revision received May 28, 2002.

85


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2. Clifford Frondel, Systematic Mineral-ogy of Uranium and Thorium,U.S.G.S. Bulletin No. 1064, U.S.Govt. Printing Office, Washington,D.C. USA, pp 400 (1958).

3. H.S. Washington, The ChemicalAnalysis of Rocks, Wiley, NewYork (1930).

4. J.F. Alder and B.C. Das, Anal. Chim.Acta 94, 193 (1977).

5. C.M. Freeney, J.W. Anderson, andF.M. Tindall, At. Spectrosc. 4, 108(1983).

6. A.M. Marabini, M. Barbaro, and B.Passariello, At. Spectrosc. 6, 74(1985).

7. Y. Kanai, N. Imai, and S. Terashima,Geostandards News. 10, 73 (1986).

8. K. Shiraishi, Y. Takaku, K.Yoshimizu, Y. Igarashi, K. Masuda,J.F. Mcinroy, and G. Tanaka, J.Anal. At. Spectrom. 6, 335 (1991).

9. M.S. Carvalho, M.de L.F. Daningues,J.L. Mantovano, and E.Q.S. Filho,Spectrochim. Acta, Part B, 53B,1945 (1998).

10. P. Goodall and C. Lythgoe, Analyst124, 263 (1999).

11. V.K. Karanda Shiv, A.N.Turanov,H.M. Kuss, I. Kumpmann, L.V. Zad-nepruk, and V.E. Baulin,Mikrochim. Acta 130, 47 (1998).

12. G. Yaprak, N.F. Cam, and G. Yener,J. Radioanal. Nucl. Chem. 238, 167(1998).

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26. R.K. Winge, V.A. Fassel, V.J. Peter-son, and M.A. Floyd, InductivelyCoupled Plasma -Atomic EmissionSpectroscopy - An Atlas of SpectralInformation, Physical SciencesData 20, Elsevier, Amsterdam, TheNetherlands (1985).


Direct Determination of Trace Elements in Micro Amounts of Biological Samples Using

Electrothermal Vaporization Coupled to ICP-AESShizhong Chen, *Zucheng Jiang, Bin Hu, Zhenhuan Liao, and Tianyou PengDepartment of Chemistry, Wuhan University, Wuhan, 430072, P.R. China

INTRODUCTION

Trace elements play animportant biochemical andphysiological role in the regulationof the metabolism and influencehuman growth, development,health, and disease (1–4). Severalinvestigations show that thecontent, speciation, and distributionof trace elements in the living bodyare directly involved with its fluidsand tissues. Since in most cases onlyvery small amounts of the biologicalsamples are available for this type ofanalysis (usually at the mg or mLlevels), the development of a rapid,simple, sensitive method withoutchemical pretreatment is essentialin the clinical, biomedical,nutritional, environmental, andlife sciences.

It is well known that inductivelycoupled plasma atomic emissionspectrometry (ICP-AES) has becomea widely used technique for thesimultaneous multielementdetermination of trace andultra-trace elements in biologicalsamples (5–8). However,conventional ICP-AES generallyrequires chemical pretreatment ofthe sample prior to analysis, whichcan lead to a long analysis time,large sample requirements, highcontamination risk, and loss ofanalyte. Electrothermalvaporization, as an effective sampleintroduction approach for ICP-AES,has excellent features such as hightransport efficiency, small samplerequirements, low absolutedetection limits, and eliminationof the organic or inorganic matrix.Furthermore, various chemical

ABSTRACT

A method is described for thedirect analysis of micro amountsof biological samples byelectrothermal vaporizationinductively coupled plasmaatomic emission spectrometry(ETV-ICP-AES) with slurry sam-pling. The main factors affectingsignal intensities of analytes wereinvestigated. The experimentalresults show that the matrixeffects in fluorination-assistedelectrothermal vaporization(FETV)-ICP-AES were reducedsignificantly. The detection limitsfor Ti, Cu, Cr, Fe, Ca, Y, and Znwere 1.0, 1.2, 2.0, 2.5, 11, 5.8,and 242 ng mL-1, respectively,and the relative standard devia-tions (RSDs) in the range of 2.1%(Ti) - 4.4% (Cr). The recommendedapproach was applied to thedirect determination of trace ele-ments in biological samples (solidor fluid) with satisfactory results.Compared with conventionalpneumatic nebulization (PN) ICP-AES, the proposed method offersthe advantages of no sample pre-treatment, small sample require-ments, and reduction of matrixeffects.

modifiers are required withETV-ICP-AES analysis to improvethe detection power of refractoryand carbide-forming elements(9–11). In our previous works, afluorination-assisted electrothermalvaporization (FETV)-ICP-AES methodusing a polytetrofluoroethylene(PTFE) slurry as the fluorinatingreagent has been described andsuccessfully applied to the directanalysis of trace elements in realsamples (12–14).

The objective of this study wasto develop a method for the direct

analysis of micro-amounts ofbiological samples (solid or fluid)by ETV-ICP-AES with PTFE slurryas the chemical modifier. Theproposed method offers highsensitivity, is rapid and simple, andrequires very small sample amountsand no chemical pretreatment ofthe sample.

EXPERIMENTAL

Instrumentation

A 2 kW power, 27±3 MHz ICPspectrometry source (BeijingSecond Broadcast EquipmentFactory, China) and a conventionalplasma torch were used in thisstudy. A modified graphitefurnace vaporizer was used as thevaporization device. The radiationfrom the plasma was focused as 1:1straight image on the entrance slitof a WDG-500-1A monochromator(Beijing Second Optics, Beijing,China) with a reciprocal lineardispersion of 1.6 nm/mm. Theevolved components were sweptinto the plasma excitation sourcethrough a 0.5-m long Teflon® tube(4 mm i.d.) by a stream of carriergas. The transient signals weredetected with a R456 typephoto-multiplier tube (Hamamatsu,Japan) and a home-built directcurrent amplifier, and the resultsrecorded using a U-135 recorder(Shimadzu, Japan). Theinstrumental and operatingconditions are listed in Table I.

Reagents

The stock standard solutions(1 mg mL-1) for Ti, Cu, Cr, Fe, Zn,Ca, and Y were prepared fromtheir specPure oxides using theconventional method. A 60% (m/v)PTFE emulsion (d<1mm; viscosity,7x10-3 –15x10-3 Pa s) was purchased


87


from the Shanghai Institute ofOrganic Chemistry, China. All otherchemicals used in this work wereof specPure grade or analyticalgrade. Double-distilled water wasused throughout.

Sample Preparation

Human HairHuman hair was washed, dried,

and chopped into smaller pieces(2 mm). Portions of 5.0 mg wereaccurately weighed into amicro-test tube with graduationand immersed with 20 µL ofconcentrated HNO3 for 3 hours.Then, 5.0 µL of 60% (m/v) PTFEemulsion and 2.0 µL of 0.1%Triton® X-100 were added,and diluted to 50 µL withdouble-distilled water foranalytical use.

Human SerumA 20-µL serum sample was

accurately taken into a micro testtube with graduation, 5.0 µL of 60%(m/v) PTFE emulsion and 2.0 µL of0.1% Triton X-100 were added,then diluted to 50 µL withdouble-distilled water formeasurement.

Chinese Medicine Loulu A 5.0-mg powder sample was

accurately weighed into a microtest tube; 5.0 µL of 60% (m/v)PTFE emulsion and 2.0 µL of 0.1%Triton X-100 were then added,finally diluted to 50 µL withdouble-distilled water foranalytical use.

The aqueous standard solutionscontaining 6% (w/v) PTFE wereused for calibration. The slurrysamples above-mentioned weredispersed with an ultrasonicvibrator for 20 min; then themicro test tube was shaken priorto sampling.

Procedure

After the ICP was stabilized, a10-µL sample was pipetted intothe furnace. After being dried andashed, the analyte was vaporizedand carried into the plasma by theargon gas under the selectedoperating conditions. Peak heightmeasurement was used forcalibration.


Vaporization Behavior of theAnalyte

The signal profiles of the analytewith or without PTFE slurry as thefluorinating reagent are shown inFigure 1, where Ti, Cu, and Znwere chosen as the representativesfor refractory, mild volatile, andeasy volatile elements, respectively.As can be seen, the addition ofPTFE greatly changes thevaporization behavior of therefractory element (Ti). Comparedwith no PTFE, an intense analyticalsignal was obtained, and therewas no memory effect in thevaporization process. For themedium volatile element Cu, theanalytical signal without PTFE isslightly weaker than with PTFE.However, for the easy volatileelement Zn, an intense analyticalsignal was observed, no matterwhether PTFE was used or not.Thus it can be concluded that PTFEslurry as an effective fluorinatingreagent can convert the refractorycompound into volatile fluoride.

Ashing Temperature

The selection of an appropriateashing temperature is veryimportant for removing the organicmatrix in biological samples. Figure2 shows the influence of ashingtemperature on signal intensity ofthe analyte with PTFE. Based on theexperimental results, an ashingtemperature of 500oC was chosenfor the simultaneous multielementdetermination. Using the selectedconditions, a complete removal ofthe organic matrix and no analyticalsignal loss of the elements ofinterest was observed.

Vaporization Temperature

Using an ashing temperatureof 500oC, the influence of thevaporization temperature on signalintensity of the elements of interestwas investigated. The results(shown in Figure 3) indicate that

TABLE IETV-ICP-AES Operating Conditions

Incident power 1.1 kW

Carrier gas (Ar) flow rate 0.5 L min-1

Coolant gas (Ar) flow rate 18 L min-1

Observation height 12 mm

Entrance slit width 25 µm

Exit slit width 25 µm

Drying temperature 100oC, ramp 10 s, hold 20 s

Ashing temperature 500oC, ramp 10 s, hold 50 s

Vaporization temperature 2400oC

Clear-out temperature 2700oC

Vaporization time 4 s

Sample volume 10 µL

88

0

10

20

30

40

50

60

c'

c

b'a'

b

a ( A )

Sig

nal i

nten

sity

(ar

bitr

ary

units

)

Time (s)

0 400 800 1200 1600 2000 24000

20

40

60

80

Ti Cu Cr Fe Zn Ca Y

Sig

nal i

nten

sity

( a

.u. )

Ashing temperature ( oC )

1200 1600 2000 2400 28000

20

40

60

80 Ti Cu Cr Fe Zn Ca Y

Sig

nal i

nten

sity

( a

.u. )

Vaporization temperature ( oC )

Fig. 1. Comparison of analyticalsignals for Ti, Cu, and Zn. (A) with PTFE: a = 0.2 µg mL-1 Ti; b =0.6 µg mL-1 Cu; c = 20 µg mL-1 Zn. (B) without PTFE: d = 10 µg mL-1 Ti; e= 3.0 µg mL-1 Cu; f = 20 µg mL-1 Zn; a', b', c', d', e', and f ' are theirresidual signals of the empty firing,respectively.

Fig. 3. Analytical signal versusvaporization temperature with PTFE:Ti, 0.2 µg mL-1; Cu and Fe, 0.6 µgmL-1; Cr, 0.4 µg mL-1; Zn, 10 µg mL-1;Ca, 0.1 µg mL-1; Y, 1.5 µg mL-1.

Fig. 2. Influences of ashingtemperature on signal intensity withPTFE: Ti, 0.2 µg mL-1; Cu and Fe, 0.6µg mL-1; Cr, 0.4 µg mL-1; Zn, 10 µgmL-1; Ca, 0.1 µg mL-1; Y, 1.5 µg mL-1.

Ashing temperature (oC)

Vaporization temperature (oC)

f '

e'

fe

d'd

( B )

89


the analytical signal intensity formost elements (Ti, Cu, Cr, Fe andZn) enhanced with an increase intemperature and reached a plateauat about 2000oC. However, for Yand Ca, a higher vaporizationtemperature (at about 2400oC) isrequired in order to achievemaximum signal intensity. In thiswork, a compromise vaporizationtemperature of 2400oC and avaporization time of 4 s was usedfor the simultaneous multielementdeterminations.

Investigation of Matrix Effect

Under the optimizedexperimental conditions, theinfluence of the main matrixelements (Na, K, Ca, and Mg) onthe signal intensities of theelements of interest in FETV-ICP-AES analysis were examined. Thetolerable amounts of the matrixelements are given in Table II. Itcan be seen, when the matrixconcentrations ranged within 2-5mg mL-1, no significant influenceswere observed. Compared withconventional ICP-AES, the proposedFETV-ICP-AES method remarkablydecreases matrix effects.

In order to explore the reasonsfor reduced matrix effects withFETV-ICP-AES, the vaporizationbehavior of the analyte Y and thematrix elements Na, Ca, and Znwas carried out. The results givenin Figure 4 show that the analyticalsignals successively appeared inthe sequence of Zn, Na, Y, and Ca.In other words, the selectivevolatilization among the elementsexamined took place in thevaporization process. This selectivevolatilization behavior is beneficialto a decrease in the matrix effects.

The experimental resultsalso show that the excitationtemperature in FETV-ICP-AES ishigher than that in PN-ICP-AES. Itis obvious that a higher excitationtemperature is also beneficial forthe excitation / ionization of theanalytes. Thus, it can be concluded

TABLE IIEffect of Matrix Concentration

Element Wavelength Tolerable amount of matrix element (mg mL-1)(nm) K Na Ca Mg

Ti 334.941 5.0 5.0 5.0 5.0Cu 324.754 5.0 5.0 5.0 5.0Cr 267.716 5.0 5.0 5.0 5.0Fe 259.940 5.0 5.0 2.0 2.0Zn 334.502 5.0 5.0 2.0 2.0Ca 317.933 5.0 5.0 - 2.0

Y 371.030 5.0 5.0 5.0 5.0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

20

40

60

80

100

d

c

ba

Sig

nal i

nten

sity

(a.

u.)

Vaporization time (s)

Fig. 4. Analytical signals vs. vaporization time in the presence of PTFE. A = Zn, 5.0 mg mL-1; b = Na, 5.0 mg mL-1; c = Y, 1.0 µg mL-1; d = Ca, 5.0 mg mL-1

using 2400oC.

that the addition of PTFE slurry asthe chemical modifier can greatlyreduce the matrix effects. Thiscan be explained as follows: (a)Selective volatilization occursbetween the analyte and thematrix; (b) compared with thepneumatic nebulization ICPsystem, the ETV-ICP system has ahigher atomization/excitationtemperature; (c) the influence ofthe solvent can be eliminated withthe ETV-ICP system. Further studieson the interference mechanism arecurrently underway.

Detection Limit and Precision

The detection limit is defined asthe analyte concentration yieldingan analytical signal equal to 3 timesthe standard deviation of thebackground noise. The detectionlimits and relative standarddeviations of the proposed methodwere summarized in Table III. Itshould be noted that a less sensitiveZn wavelength was used for thisstudy, while a lower Zn detectionlimited could be achieved with amore sensitive line.

90

Sample Analysis

Under the selected conditions,the concentration of Ti, Cu, Cr, Fe,Zn, Ca, and Y in Chinese medicineLoulu, human hair, and serum wasdetermined directly by using thestandard addition method andworking curve. The analyticalresults are listed in Table IV. Theresults obtained by the proposedmethods were also compared withthe results obtained by pneumaticnebulization (PN)-ICP-AES.

In addition, the standardreference material of human hair(GBW 07601) was analyzed (seeTable V) and the determined valuesare in good agreement with thecertified values.

CONCLUSION

A fluorination-assistedETV-ICP-AES method with slurrysampling has been developed forthe direct determination of thetrace elements Ti, Cu, Cr, Fe, Ca, Y,and Zn in various biologicalsamples. In comparison toconventional pneumaticnebulization ICP-AES, the proposedmethod requires no chemicalsample pretreatment, usesmicro-amounts of sample, andresults in reduced matrix effects.

TABLE III Detection Limit and Precision (n=9)

Element Wavelength Detection limit RSD (%)(nm) (ng mL-1) (pg) (µg g-1 )

Ti 334.941 1.0 5.0 0.012 2.1Cu 324.754 1.2 6.0 0.014 3.5Cr 267.716 2.0 10 0.024 4.4Fe 259.940 2.5 13 0.030 3.6Zn 334.502 242 1210 2.91 4.1Ca 317.933 11 55 0.13 3.3

Y 371.030 5.8 29 0.058 2.9

TABLE IVAnalytical Results of the Elements of Interest in Biological Samples

(n=5)

FETV-ICP-AES PN-ICP-AES

Sample/Element Calibration Standard Calibration Calibration

curvea additionb curveb curveb

Hair (µg g-1 )Ti 2.32 ± 0.31 2.55 ± 0.40 2.97 ± 0.53 2.64 ± 0.33Cu 14.5 ± 2.8 15.2 ± 3.1 16.4 ± 3.6 15.0 ± 2.4Cr 1.12 ± 0.16 1.20 ± 0.20 1.04 ± 0.11 0.98 ± 0.14Fe 35.7 ± 2.7 34.8 ± 3.1 30.9 ± 2.4 31.7 ± 2.0Zn 172 ± 10 168 ± 8.5 159 ± 12 176 ± 9.8Ca 1180 ± 110 1245 ± 108 1064 ± 115 1291 ± 121

Serum (µg mL-1 )Ti 0.14 ± 0.03 0.16 ± 0.02 0.15 ± 0.03 0.13 ± 0.02Cu 1.85 ± 0.31 1.93 ± 0.41 1.78 ± 0.25 1.90 ± 0.33Cr 0.20 ± 0.03 0.18 ± 0.02 0.19 ± 0.04 0.21 ± 0.02Fe 2.31 ± 0.30 2.48 ± 0.44 2.25 ± 0.20 2.51 ± 0.22Zn 3.61 ± 0.42 3.25 ± 0.36 4.03 ± 0.45 4.26 ± 0.50Ca 87.3 ± 9.1 84.9 ± 7.4 91.4 ± 10 95.6 ± 8.7

Loulu (µg g-1 )Ti 31.6 ± 2.4 30.2 ± 1.8 33.5 ± 2.1 35.3 ± 1.9Y 1.31 ± 0.14 1.37 ± 0.18 1.31 ± 0.14 1.35 ± 0.11Cu 18.8 ± 1.6 16.5 ± 3.1 18.8 ± 1.6 20.5 ± 2.0

Cr 6.59 ± 0.63 6.09 ± 0.71 6.59 ± 0.63 5.50 ± 0.98

a Direct analysis with slurry sampling. b Analysis after digestion with HNO3 + HClO4.

TABLE VAnalytical Results of

Standard Reference Material ofHuman Hair (GBW 07601) (n=5)

Element Determined Certifiedvaluea valuea

(µg g-1 ) (µg g-1 )

Ti 2.4 ± 0.3 2.7 ± 0.4

Cu 9.5 ± 1.2 10.6 ± 0.7

Cr 0.41 ± 0.07 0.37 ± 0.05

Fe 61 ± 8 54 ± 6

Zn 205 ± 11 190 ± 5

Ca 2780 ± 224 2900 ± 200

a Calibration curve method withslurry sample.

91


ACKNOWLEDGMENTS

The authors are grateful to theNational Science Foundation ofChina for supporting this project.

Received April 11, 2002

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*Corresponding author.E-mail: [email protected]**This paper was presented at the SeventhRio Symposium on Atomic Spectrometry,Florianopolis, Brazil, April 7-12, 2002.

Simple Method for the Selective Determination of As(III) and As(V) by ETAAS After

Separation With Anion Exchange Mini-column**

*Patricia Smichowskia, Liliana Valienteb, and Ariel Ledesmaa

aComision Nacional de Energia Atomica, Unidad de Actividad Quimica, Centro Atomico ConstituyentesAv. Gral. Paz 1499, 1650-San Martin, Pcia. Buenos Aires, Argentina

bInstituto Nacional de Tecnologia Industrial, CEQUIPE, Laboratorio de Analisis de TrazasCasilla de Correo 157, 1650-San Martin, Pcia. Buenos Aires, Argentina

ABSTRACT

A simple approach isdescribed for the separation anddetermination of inorganicarsenic species using solid phaseextraction and electrothermalatomic absorption spectrometry(ETAAS). A Dowex 1-X8 anionexchange mini-column was usedto separate As(III) and As(V).The chemical (pH, type andconcentration of eluent) andphysical (flow rate of sample andeluent) parameters affecting theseparation were studied. Underoptimized conditions, As(V)showed a strong affinity for themini-column, while As(III) wascollected in the effluent. As(V)was recovered by elution with0.8 mol L-1 hydrochloric acid. Theinfluence of other competingions on the separation of As(III)and As(V) was also evaluated.The detection limit achieved forAs(III) was 4 ng mL-1 and forAs(V) 4 ng L-1.

The relative standarddeviation (%RSD) ranged from0.7 – 1.3% for replicated tap,lake, and well water samplesat the 20 ng mL-1 level. Apreconcentration factor of 100was achieved for As(V) when300 mL of water was processed.Arsenic recoveries (fullprocedure) ranged from92 – 106%.

INTRODUCTION

The toxicological properties ofthe different arsenic species varywidely and inorganic As(III) andAs(V) are the most toxic. Organicspecies, like monomethylarsonicacid (MMA) and dimethylarsinicacid (DMA), have a moderatetoxic effect on humans and biota.Arsenobetaine (AsBet) andarsenocholine (AsChol) arenon-toxic. From the toxicologicaland regulatory point of view, it ismandatory to develop analyticaltechniques aimed at differentiatingbetween these species.

A variety of separation anddetection techniques have beenreported for As speciation analysis(1–4). Inorganic species of Asare most often determined inwaters, soils, and sediments, whilethe organic species are commonconstituents of biological tissues.

Relatively simple methodsbased on the selective reductionof arsenic species by continuoushydride generation have beenproposed (5–7). Although thesemethods offer excellent sensitivity,some analytical limitations suchas molecular rearrangements (8)and incomplete recoveries (9)have been reported. Hyphenatedtechniques based on thecombination of a powerfulseparation technique with asensitive element-specific detectorare among the most promising

approaches to determineselectively As compounds in avariety of matrices. Analyticalmethods reported in the literaturenormally require separationschemes, such as cold trapping(CT), high-performance liquidchromatography (HPLC), gaschromatography (GC), or capillary

electrophoresis (CE) that arerelatively complex for routineanalysis (10-12). It is thereforenecessary to develop otheralternatives for the reliable,sensitive, and low-costdetermination of species at tracelevels in a particular sampleor matrix.

Solid-phase extraction (SPE)using mini- or micro-columns offersadvantages such as simplicity ofoperation, low cost, the possibilityto achieve high preconcentrationfactors, the ability to combine withdifferent detection techniques, andrelative freedom from matrixinterferences. The possibility offield sampling is anotherimportant advantage, whichcombines preconcentration ofwater samples in the field andsubsequent transport of thecolumns for elution and analysisin the laboratory.

Numerous SPE methods forseparation and experimentalapproaches have been proposedin recent years which separate andmeasure inorganic and organicspecies of As. Yalcin and Le (13)employed solid-phase extractioncartridges as low-pressurechromatographic columns for theseparation and subsequentdetermination of inorganic arsenicspecies. Detection limits of 0.2 and0.4 ng mL-1 were achieved forAs(III) and As(V), respectively.Grabinski (14) used a singlecolumn containing both cation andanion-exchange resins to separatefour arsenic species. The overallanalytical detection limit was10 ng mL-1 for each individual

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arsenic species. According toanother study, As(III), As(V), MMA,and DMA were determined bygraphite furnace atomic absorptionspectrometry (GFAAS) afterseparation of the species byion-exchange chromatography (15).In the separation scheme proposed,As(III) was calculated byestablishing the difference.

The aim of this study was todevelop a simple off-line methodbased on the use of a mini-columnfilled with an anion-exchange resinto separate As(III) and As(V) attrace levels. After separation, the Asspecies were quantified by GFAAS.The method’s simplicity and lowcost make it suitable for routineinorganic arsenic speciation analysis.

EXPERIMENTAL

Instrumentation

A PerkinElmer Model 5100 ZLatomic absorption spectrometer(PerkinElmer, Shelton, CT, USA),equipped with a PerkinElmerModel THGA graphite furnace,PerkinElmer Model AS-71autosampler, and longitudinalZeeman-effect backgroundcorrector, was used for theatomic absorption measurements.Electrodeless discharge lamps(EDL, PerkinElmer) were used asthe sources of radiation for Asdetermination. Arsenic wasmeasured at its most sensitive lineat 193.759 nm. Pyrolytically coatedgraphite tubes with pyrolyticgraphite L’vov platforms wereemployed. High-purity Ar (flow rate300 mL min-1) was used to purge airfrom the graphite tubes, exceptduring the atomization step wherestopped flow conditions were used.The analytical measurements werebased on peak area. Autosamplervolumes of 20 µL of samplefollowed by 5 µL of chemicalmodifier were employed for allstudies. Each analysis was repeatedat least three times to obtain theaverage value and its relative

standard deviation (%RSD). Theprogram was optimized usingwater samples spiked with As. Themain ETAAS operating conditionsand matrix modifier used aresummarized in Table I.

Reagents

All chemicals were ofanalytical reagent grade unlessotherwise stated. Deionized water(Barnstead, Dubuque, IA, USA)was used throughout. All solutionswere stored in high-densitypolypropylene bottles. Plasticbottles, autosampler cups, andglassware were cleaned bysoaking in 20% (v/v) HNO3 for24 h. The material was then rinsedthree times with deionized water.Commercially available 1000 mg L-1

As(III) and As(V) standard solutions(Titrisol, Merck, Darmstadt,Germany) were prepared dailyby serial dilutions of the stocksolutions.

A 0.3% (m/v) magnesiumnitrate solution was prepared bydissolving an appropriate amountof Mg(NO3)2 . 6H2O (Merck,Suprapure) in deionized water.The mixed Pd and Mg(NO3)2 matrixmodifier solution was prepared byadding 5 mL of 0.3% Mg(NO3)2

solution and 2.5 mL of 10 g L-1 Pdsolution (Merck) into a volumetricflask of 25 mL and brought to vol-ume with deionized water. Thefinal concentration of the matrixmodifier solution was: 0.06%Mg(NO3)2 and 0.1% Pd.

High-purity Ar was used to purgeair from the graphite tubes.

Column Packing and Conditioning

The resin Dowex 1-X8 (100-200mesh; Cl–; form; analytical grade;Bio-Rad Labs, Richmond, CA, USA)was loosely packed into a glasscolumn (7 cm x 3 mm i.d.). Glasswool plugs were placed at bothends of the column so that the netlength of the resin zone was about5 cm. The method consists of theseparation of As(III) from As(V) onthe acetate form of the Dowex1-X8 ion exchange resin. Beforerunning the sample, the resin wasconverted into the acetate form bypassing 3 mL of 1.0 mol L-1 sodiumhydroxide, followed by 5 mL of4.0 mol L-1 acetic acid at a flowrate of 1.0 mL min-1. Then, themini-column was washed with 10mL of deionized water. Columndegradation was not observed afterseveral weeks of usage.

Analytical Procedure

An aqueous solution containingAs(III) and As(V) was passedthrough the column at a flow rateof 1.0 mL min-1 using a peristalticpump. As(V) was retained in thecolumn while As(III) was collected(in a polyethylene flasks) in theeffluent. A solution of 0.8 mol L-1

was used to elute As(V) from thecolumn. The eluate was collectedin other polyethylene flasks. Aftereach run, the column was washedwith a few mL of 1.0 mol L-1 HCland then with 5 mL of water. Thearsenic concentration wasdetermined in the two fractions byETAAS by injecting 20-µL aliquots

TABLE IGraphite Furnace Temperature Program for As Determination

Parameter Drying Pyrolysis Atomization Conditioning

Temperature (oC) 120 800 2400 2700

Ramp time (s) 1 10 0 1

Hold time (s) 30 20 5 2

Ar flow rate (mL min-1) 300 300 (0) read 300

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into a pyrolytic tube applying theoptimized program given in Table I.All experiments were performedin duplicate.


The difference in dissociationconstants between arsenious acid(pKa1 = 9.2; pKa2 = 12.1; pKa3 =13.4) and arsenic acid (pKa1 = 2.2;pKa2 = 6.9; pKa3 = 11.5) allows toseparate As(III) and As(V) on thebasis of ion exchange. At neutralpH, arsenious acid is present asAs(OH)3 and is not dissociated.For this reason, when using ananion-exchange resin it will not beretained on the column. On theother hand, As(V) will be presentas H2AsO4

– and will be retained onan anion-exchange resin and theseparation of both species ispossible. This separation is pHdependent.

To establish the optimumseparation conditions, chemical andphysical parameters affecting theretention/elution of arsenic specieswere studied. The pH of themedium, flow rate of sample andeluent, maximum sample volume,and minimum elution volume werethe variables considered. Toevaluate the effect of differentvariables affecting the selectiveretention, the separation processwas carried out on each oxidationstate separately. After optimization,mixtures of As(III) and As(V) wereprepared in water to check thecapacity of the resin to retainselectively As(V) in the presenceof As(III).

Effect of pH on As Retention

Five mL of water samplescontaining 100 ng of As(III) werepassed through the column at aflow rate of 0.8 mL min-1. The pHof the samples was variedbetween 3 and 8 (below the pKa1

of arsenious acid and above pKa1

of arsenic acid) because in thisrange the difference in their ionic

property is maximized. Noretention of the uncharged As(III)was observed and it was collectedquantitatively in the effluent. Whena 5-mL water sample, spiked with100 ng of As(V), was passedthrough the column, the negativelycharged As(V) was quantitativelybound to the anion-exchange resin.Pentavalent arsenic was strippedwith hydrochloric acid. In thisscreening experiment, 5 mL of 0.5mol L-1 HCl was used for elution.This study demonstrates that theseparation is possible in a wide pHrange. A working pH of 7 for As(V)retention was selected to performfurther experiments.

Influence of HCl Concentrationon As(V) Elution

Experimental data obtainedusing HCl concentrations rangingfrom 0.05 to 1.0 mol L-1 show thatthe separation of the arsenicspecies improves with increasingacid concentration up to 0.8 mol L-1. No significant changes in As(V)recovery were observed at higherconcentrations. A 0.8 mol L-1 HClsolution was used as the eluent insubsequent studies. Figure 1 showsthe influence of acid concentrationon As(V) recovery.

Influence of Sample and EluentFlow Rates

Different sample flow rates(0.4–1.0 mL min-1) were tested todetermine the efficiency of As(V)retention using in all cases a 20-ngmL-1 standard arsenic solution and aconstant elution flow rate of 0.8 mLmin-1. A sample flow rate of 1.0 mLmin-1 was chosen for further work(Figure 2). Overpressure wasobserved when higher flow rateswere tested. A parallel studyshowed that the recovery of As(V)did not change significantly whenthe elution flow rate (0.8 mol L-1

HCl eluent) was varied from0.4–1.0 mL min-1. In view of theseresults, the samples were passedthrough the column and eluted at aconstant flow rate of 1.0 mL min-1.

Elution Volume

The next step in assessingthe efficiency of As(V)preconcentration was to determinethe minimum volume required tocompletely elute the analyte fromthe mini-column. After running 5mL of 20-ng mL-1 As(V) solutions,increasing volumes (0.1-5.0 mL) of0.8 mol L-1 HCl were successivelyused to elute the analyte retained.

Fig. 1. Influence of HCl concentration on As(V) recovery.

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The eluates were collected andmeasured by ETAAS. Figure 3shows that 3 mL is the minimumvolume of acid required toquantitatively strip the analytefrom the column.

Influence of Sample Volumeon As(V) Retention

A total constant amount ofarsenic (20 ng) in different volumes(5–300 mL) was passed through thecolumn. Figure 4 shows that indeionized water, the amount of

As(V) retained in the columnremains constant up to a volumeof 300 mL. For higher volumes,a significant reduction in theabsorbance signal was observed.

The maximum volume that canbe run through the column withoutany decrease in recovery of As(V)depends on the complexity of thematrix. The recovery achieved waslower when tap, lake, and wellwater spiked with 20 ng of As(V)were passed through the column.

Evaluation of As(V) RetentionCapacity of the Mini-column

The enrichment factor wascalculated as the ratio between themaximum volume of deionizedwater spiked with As(V) that waspossible to pass through thecolumn with respect to theminimum volume of 0.8 M HClrequired to elute the analyte.According to the results obtained,As(V) can be preconcentrated bya factor of 100. The columnproperties remained constantduring about 100 cycles ofretention/elution of As(V). Slightcolumn degradation was observedafter 100 cycles and the columnwas repacked with new resin.

Interference Study

The possible interferencesproduced by different cations,namely, Ca(II), Cd(II), Co(II),Cu(II), Fe(III), Hg(II), Mn(II), Na(I),Ni(II), Pb(II), Sb(III), Se(IV), andZn(II) were studied. To performthis study, 10 mL of samplecontaining 20 ng mL-1 of a mixtureof As(III) and As(V) and theinterfering ion tested was passedthrough the column. As(III) wascollected in the effluent and As(V)was stripped from the column with3 mL of 0.8 mol L-1 HCl. Variationsover (±5% in the analytical signal ofAs were taken as an interference.Under optimized conditions, novariation in As(III) or As(V)recovery was observed in thepresence of up to 1000 ng mL-1

of the ions evaluated except forCu(II), Hg(II) and Pb(II) and Sb(III).A slight depression in As(III) wasobserved in the presence of morethan 750 ng mL-1 of these ions. Thepossible interference from typicalanions (Cl–, NO2, SO4

2–) found inwaters was also investigated.Concentrations higher than 100 ngmL-1 did not allow quantitativeretention of arsenic on the column,which is the major drawback of themethod when seawater samples areanalyzed. The results are set forthin Figure 5.

3 412 16

56

9299 103

0

20

40

60

80

100

120

0.1 0.2 0.3 0.5 1.0 2.5 3.0 5.0

Elution volume (ml)

Rec

ove

ry (

%)

Fig. 2. Influence of sample flow rate on As(V) retention/elution.

Fig. 3. Minimum elution volume of HCl necessary to strip As(V) quantitativelyfrom the column. Figures in the top of the bars indicate the percentage of As(V)recovery.

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Analytical Performance

The detection limits calculatedon the basis of the 3 σ criterion for10 replicated measurements of theblank signal were 4 ng mL-1 forAs(III) and 4 ng L-1 for As(V)(preconcentration factor: 100).The relative standard deviation(RSD) ranged from 0.7 to 1.3% fortap, lake, and well water for 10successive measurements ofsamples containing a finalconcentration of 20 ng mL-1.

Unfortunately, certifiedreference materials (CRM) ofarsenic species are not availableand for this reason a recovery testwas performed (Table II). Althoughit cannot replace accuracy tests,some information is gained aboutthe good performance of theoverall procedure. Differentcombinations of spiked watersamples and elution volumes weretested. The recovery data rangedbetween 92 and 106 %.

0

20

40

60

80

100

120

Mn(II) Sb(III) Co(II) Na(I) Zn(II) Ca(II) Fe(III) Se(IV) Cd(II) Ni(II) Cu(II) Pb(II) Hg(II) NO2- Cl- SO42-

Cations and anions tested

Rec

ove

ry (

%)

Cl - NO2- SO4

2-

Fig. 4. Influence of sample volume on As(V) retention.

Fig. 5. Interference study. As(III) and As(V) concentrations: 20 ng mL-1.Cations concentration: 1000 ng mL-1; anions concentration: 200 ng mL-1.

CI- NO2- SO4

2-

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Application to Natural WaterSamples

To investigate the applicabilityof this method, the content of totalAs, As(III), and As(V) in differentcategories of waters wasdetermined. The calibration graphmethod was employed for thedetermination of total As andinorganic arsenic species. Total Aswas determined by hydridegeneration (HG)-AAS. Groundwatersamples were collected in VenadoTuerto (Santa Fe province,Argentina) from wells where acontamination phenomenon witharsenic had been detected in someareas. Geological studies (16)demonstrated that thiscontamination was from naturalcauses. Water samples werecollected in Teflon® containersthat were previously rinsed withthe sample. Samples were stored atabout -4oC until their analysis in thelaboratory. The analysis was carriedout as soon as possible after

sampling in order to prevent theoxidation of As(III) during storage.The results are set forth in Table III.

CONCLUSION

The use of an anion-exchangemini-column for the separationof As(III) and As(V) and thesubsequent determination of thespecies by ETAAS provides asimple, effective, selective, andlow-cost method for speciationstudies. The methodology is easyto implement in laboratoriesdedicated to routine analysis. Inaddition, the detection limits aremore than adequate in view of themaximum concentration fixed forarsenic in drinking waters bydifferent international regulations.

At no time was there anyevidence that the system allowedany interconversion of species.

TABLE IIIDetermination of Total As, As(III), and As(V) in Real Water Samples

Mean value ± standard deviation (n+3). Concentrations are expressed in ng mL-1.

Sample Total As As(III) As(V)

A 111 ± 4 14.2 ± 0.7 92.8 ± 3.9

B 194 ± 8 16.9 ± 1.0 172 ± 8

C 64.4 ± 2.9 <LOD 59.0 ± 3.3

ACKNOWLEDGMENTS

The authors wish to thank theInternational Atomic Energy Agency(IAEA Research Contract No.11549) for financial support.

Received May 24, 2002.

REFERENCES

1. K.A. Francesconi, P. Micks, and K.J.Irgolic, Chemosfere 14, 143 (1985).

2. N. Violante, F. Petrucci, F. La Torre,and S. Caroli, Spectrosc. 7, 36(1992).

3. W.T. Corns, P.B. Stockwell, L.Ebdom, and S.J. Hill, J. Anal. At.Spectrom. 8, 71 (1993).

4. P. Smichowski, J. Marrero, A.Ledesma, G. Polla, and D. Batistoni,J. Anal. At. Spectrom. 15, 1493(2000).

5. R.K. Anderson, M. Thompson, and E.Culbard, Analyst 111, 1153 (1986).

6. R.S. Braman and C.C. Foreback,Science 182, 1247 (1973)

7. M.O. Andreae, Anal. Chem. 49, 820(1977).

8. Y. Talmi and D.T. Bostik, Anal. Chem.47, 2145 (1975).

9. M.B. Carvalho and D.M. Hercules,Anal. Chem. 50, 2030 (1978).

10. M. Burguera and J.L. Burguera,Talanta 44, 1582 (1997).

11. T. Prohaska, M. Pfeffer, M. Tulipan,G. Stingeder, A. Mentler, and W.W.Wenzel, Freseniusí J. Anal. Chem.364, 467 (1999).

12. M. Van Holderbeke, Y. Zhao, F.Vanhaecke, L. Moens, R. Dams, andP. Sandra, J. Anal. At. Spectrom.14, 229 (1999).

13. S. Yalçin and X.C. Le, Talanta 47,787 (1998).

14. A.A. Grabinski, Anal. Chem. 53, 966(1981).

15. G.E. Pacey and J.A. Ford, Talanta28, 935 (1981).

16. H.B. Nicolli, J.M. Suriano, M.A.Gomez Peral, L.H. Ferpozzi, andO.A. Baleani, Environ. Geol. WaterSci. 14, 3 (1989).

TABLE IIRecovery of Mixtures With Different Concentration Ratios

of As(III) and As(V)Mean value ± standard deviation (n=3).

Concentrations are expressed in ng mL-1.

Arsenic added Arsenic found Recovery (%)

As(III) As(V) As(III) As(V) As(III) As(V)

Tap 5 5 49 ± 0.2 4.7 ± 0.2 98 94

Well 1 10 50 9.2 ± 0.4 46 ± 1.9 92 92

Lake 1 0 0.5 – 0.53 ± 0.03 – 106

Lake 2 50 100 47 ± 1.7 105 ± 5 94 105´

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Preconcentration and Speciation of Chromium inNatural Waters Using Ion-Pair Extraction and

Graphite Furnace AAS*K. Kargoshaa, M. Noroozifarb, and J. Azadc

a Chemistry and Chemical Engineering Research Center of IranP.O. Box 14335-186, Tehran, Iran

b Department of Chemistry, Shahid Beheshti University, Evin, Tehran, Iranc Department of Chemistry, Azzahra University, Vanak, Tehran, Iran

INTRODUCTION

The two primary oxidationstates of chromium in naturalwaters, Cr(VI) and Cr(III), differsignificantly in their biological,geochemical, and toxicologicalproperties (1,2). The totalchromium concentration inunpolluted rivers is in the 0-50 µg/Lrange, while seawater containsaround 0.04 µg/L (3). Chromium(VI) probably exists in naturalwaters at the mg/L or lowerlevels and must therefore bepreconcentrated prior to analysiswith methods that are usually notsensitive enough to directly detecttrace Cr(VI). The most commontechniques for the determinationof chromium are atomic absorptionspectrometry (AAS) employingflame (FAAS) or graphite furnaceatomization (GFAAS), inductivelycoupled plasma optical emissionspectrometry (ICP-OES), andinductively coupled plasma massspectrometry (ICP-MS), althoughby themselves these techniquesonly yield information on totalconcentrations. This is the reasonwhy sample pretreatment methodssuch as ion exchange (3) solventextraction (4), microwave field (5),fast protein anion-exchangeliquid chromatography (6), andsolid-phase extraction (7,8), whichincludes analyte element separationand preconcentration, are requiredin order to determine the lowlevels of individual Cr species evenwith the most sensitive techniquessuch as GFAAS.

ABSTRACT

The speciation analysis ofchromium was studied usingsequential ion-pair extraction ofCr(VI) and Cr(III) [previouslyoxidized to C1–(VI)] withTetrabutylammoniumbromide(TBAB) as the counter ion], incombination with graphitefurnace atomic absorptionspectrometry (GFAAS). TheCr(VI) in 100-mL acidicsamples is extracted intomethyl isobutylketone (MIBK)as TBAB-Cr(VI) ion-pair;then back-extracted andpreconcentrated into 5 mL ofacetate buffer (pH=5), and finallydetermined by GFAAS.

The calibration curve waslinear up to 5 µg/L of Cr(VI) witha sample injection volume of 10µL. The detection limit (threetimes the standard deviation ofthe blank) was 3 ng/L. The RSDwas 2.32% (n=4) for Cr(VI) and2.39% (n=4) for Cr(III). Theinfluence of probableconcomitant species in naturalwater on the determination ofchromium was studied. Therecovery was 97.5–101.5% forCr(VI) and 99.5–104.5% forCr(III). For verification of theaccuracy, some natural waterreference samples were utilized.

The present paper describes thedevelopment of a speciative GFAASprocedure for the determinationof Cr(VI) and Cr(III) in seawater,using tetrabutylammoniumbromide(TBAB) as the counter ion for theselective ion-pair extraction ofCr(VI) from acidic solutions intoMIBK and preconcentration of thision-pair by back-extraction intoacetate buffer. By oxidation of

Cr(III) to Cr(VI) with sodiummetaperiodate, the Cr(III)concentration can then bemeasured by GFAAS.

EXPERIMENTAL

Instrumentation

For this study, a PerkinElmerModel 1100 B atomic absorptionspectrometer was used,equipped with an HGA®-700graphite furnace (PerkinElmerInstruments, Shelton, CT USA). Theinstrumental parameters (Table I)were optimized for maximumabsorbance. The samples weredirectly injected onto the tube wallof the pyrolytically coated graphitetubes using a PerkinElmer ModelAS-70 Furnace autosampler. Arotator Model AR.14 (E.L.M.) wasused for shaking the solution.

Reagents

All reagents were of analyticalreagent grade. Doubly distilled anddemineralized water was used toprepare all solutions.

Stock solutions of 100 mg/L ofCr(III) and Cr(VI) were preparedby dilution of a Titrisol® stocksolution (Merck) and from K2CrO4

(Merck), respectively. TheHAC/AC- buffer, pH 5, wasprepared by mixing 5.8 mL ofCH3COOH (Merck, 99%) and158 mL of 0.4 mol/L alkalinesolution and diluting to 500 mL.The alkaline solution was preparedby dissolving sodium hydroxide(Merck, 98.5%) in water and storedin a polyethylene bottle. The stocksolution of TBAB 0.02 mol/L wasprepared from this reagent inmethyl isobutylketone (MIBK)

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(Mak & Baker). Metaperiodate 0.1mol/L solution was prepared fromits sodium salt and was stored in adark bottle.

Procedure

For the Cr(VI) determination,100 mL of the samples or standardsolutions [containing both Cr(III)and Cr(VI)] was pipetted into a250-mL separatory funnel, and 4 mLHNO3 65% and 10 mL of TBABsolution were added. The ion-paircompound was extracted byshaking the mixture for 5 min.The two phases were allowed toseparate and then the organicphase was transferred into a newseparatory funnel, 5 mL of acetatebuffer solution added, thoroughly

mixed and the ion-pair compoundback-extracted from MIBK into theacetate solution. Ten µL of theacetate-separated phase were usedfor GFAAS measurement of Cr(VI).For the determination of totalchromium in these binarysolutions, apply the sameprocedure but before addingHNO3 to the separatory funnel,add periodate and alkaline solution(5 mL of each) and shake for 5 min.Sodium metaperiodate in alkalinesolution oxidizes the Cr(III)into Cr(VI).


Ion-Pair Extraction Conditions

Tetrabutylammoniumbromide[R4N+Br–] as a counter ion reactswith oxyanions of chromium inacidic aqueous solution and resultsin Cr(VI)–TBAB ion-pair (9). Thision-pair can be separated fromother species (cations, anions)present in sample solution byselective extraction into anorganic solvent.

In the present work, the Cr(VI)content of a 100-mL water samplewas reacted with TBAB and theformed Cr(VI)–TBAB ion-pair wasfirst extracted into MIBK andthen back-extracted into 5 mLHAC/AC– buffer solution. Finally,the Cr(VI) content of the buffersolution was determined by GFAAS.

The Cr(VI)-TBAB ion-pairextraction conditions wereoptimized. For this purpose,solutions containing 1 µg/L ofCr(VI) and Cr(III) were preparedand tested using the proposedmethod.

Several water-immisible organicsolvents such as chloroform, MIBK,and carbontetrachloride weretested as the extracting solvent.MIBK is recommended as aconvenient solvent for this work.Chloroform did not extract theion-pair at all and the extractionof this ion-pair withcarbontetrachloride was notquantitative.

The Cr(VI)–TBAB ion-pair isstable only in acidic solutions. Theeffect of acidity on this stability, theefficiency of the ion-pair formation,and hence the extraction of thision-pair into the organic solventwere investigated by adding variousvolumes of nitric acid (65%) to100-mL volume of 1-µg/L Cr(VI)samples and measuring theabsorbances of the extractedCr(VI). The results are shown inFigure 1. By increasing the volume

TABLE IOperating Parameters of GFAAS

Step Atomization Program for the Estimation of CrTemp. (oC) Hold Time

Dry 90–120 15.0 secAsh 1100–1200 10.0 secAtomization 2400 5.0 sec

Background correction D2 lampMeasurement Peak heightArgon flow 10 L/minSample volume 10 µLWavelength 357.9 nmSlit width 0.7 nm

Lamp current 7.0 mA

V (Nitric acid, 65%) ml

0.15

Ab

sorb

ance 0.1

0.05

0

0 5 10 15

Fig. 1. Acidity dependence of the Cr(VI)–TBAB ion-pair stability: [TBAB]=0.02 M,shaking time=5 min and [Cr(VI)] = 1 µg/L.

100

of acid added, the efficiency ofextraction (absorbance) increases.Although the absorbance reached afixed value by adding 4 mL of theacid, we chose 5 mL as theoptimum acid volume to ensurethat sufficient acid is available fortotal chromium determination. Inaddition, we studied the effect ofpH on the back extraction of theion-pair into the HAC/AC– buffer.Working at a pH<4 or pH>6, thesignal decreased. It was found thatthe HAC/AC– buffer at pH=4.5provides the best conditions forback extraction and stability ofthe ion-pair.

The influence of shaking timeon the efficiency of the ion-pairformation and extraction wasstudied using a rotator. TheCr(VI)–TBAB ion-pair is formedand extracted into MIBK bymixing 100 mL of chromiumsolution with MIBK containingTBAB. The mixture was thoroughlyshaken using the rotator. Theion-pair of this separated MIBKwas then back-extracted into theHAC/AC– buffer solution. Theinfluence of shaking time on theefficiency of these two extractionprocesses was tested by evaluatingshaking times of 1 to 7 min. It wasfound that a 5-min shaking time isrequired for each of the aboveextractions. When shorter shakingtimes were used, the Cr(III) wasnot quantitatively extracted.

By adding different volumesof MIBK to 100-mL chromiumsolutions and measuring theamount of the extracted Cr(VI), itwas found that the 10-mL volumesof MIBK gave the best results. Byback-extracting the ion-pair fromMIBK into different volumes ofHAC/AC– buffer solution, 5 mLof this buffer was found to beoptimum.

Finally, the effect of TBABconcentration on the extractionefficiency was studied by using

10-mL volumes of MIBK withdifferent concentrations ofTBAB (see Fiqure 2). The resultsin Fiqure 2 show that TBAB 0.02mol/L provides the best results.

The applicability of thisprocedure for speciation andquantitative analysis of chromiumwas tested by analyzing threedifferent standard solutions ofchromium: Cr(III) standard solution(1 µg/L), Cr(VI) standard solution(1 µg/L), and binary standardsolution of Cr(III) and Cr(VI)(1 µg/L each). Table II lists theabsorbances of these solutionsfor extracted chromium underoptimized conditions of extraction.In analyzing the single-componentCr(III) solution and also the totalchromium determination of binarysolution, Cr(III) was oxidized toCr(VI) by sodium metaperiodateprior to ion-pair formation andextraction.

As can be seen in Table II, theextraction and determination ofCr(VI) using the proposed methodis quantitative and the presence ofCr(III) does not cause interferences(samples S1, S2, and B1). Theseresults also show that totalchromium [and Cr(III)]determination is possible byapplying the method described(samples S4 and B2).

Interferences

The influence of probableconcomitant species in naturalwater on the extraction anddetermination of Cr(VI) and Cr(III)(total chromium) was studied. Thetolerance limit was set as theconcentration of foreign ions thatproduced an error of <5% in thedetermination of Cr(VI). Closeresults were obtained working witheither Cr(III) or Cr(VI) solutions.The results in Table III show thatthese cations and anions cause nosevere interference effects.

Calibration Curve, DetectionLimit, and Reproducibility

The calibration curve was linearup to 5 µg/L of Cr(VI) with acorrelation coefficient of r=0.9993.The intercept and slope of thecalibration curves obtained fromsingle and binary solutions ofCr(III) and Cr(VI) were statisticallycomparable.

The detection limit (evaluatedas the concentration correspondingto three times the standarddeviation of the blank signal) was0.003 µg/L for Cr(VI).

The RSDs, evaluated byrepeated analysis of the standardsolutions of Cr(VI) and Cr(III) witha concentration of 1 µg/L (n=4),were 2.32% and 2.39%,respectively.

[TBAB] mol/L

Ab

sorb

ance

0.15

0.1

0.05

0

0 0.02 0.04 0.06 0.08

Fig. 2. TBAB concentration effect on the ion-pair extraction: Volume of nitric acid (65%) =5 mL, shaking time = 5 min and [Cr(VI)] = 1 µg/L.

101


TABLE IIAnalysis of Single and Binary Standard Solutions

of Cr(III) and Cr(VI) Extracted Under Various Conditions

Samplesa,b Absc RSD (%)

S1 0.117 2.32

S2 0.001 2.34

S3 <0.001 2.40

S4 0.115 2.39

B1 0.118 2.33

B2 0.231 2.31

a Solutions and conditions of extractions: S1 = Cr(VI) solution without oxidation; S2 = Cr(III) solution without oxidation; S3 = acetate buffer solution with oxidation; S4 = Cr(III) solution with oxidation; B1 = solution of Cr(III) and Cr(VI) without oxidation; and B2 = solution of Cr (III) and Cr(VI) with oxidation.

b [Cr(III)] = [ Cr(VI)] = 1 µg/L. c Based on four measurements.

TABLE IIIEffect of Foreign Ions on the Determination of Cr

(VI), 1.5 µg/L

Interferent Tolerancea

F–, Cl–, SO42–, PO4

3–, CH3COO–

S2O42–, S2O5

2–, S2O82– >700b

Ca2+, Mg2+, Co2+, Al3+, Cr3+ >550b

Fe3+, Ni2+, Pb2+, Cd2+ >300b

Cu2+, Mn2+, Sn2+ >200b

VO3–, MnO4

– 100

a Maximum weight ratio of interfering species to Cr(VI)giving an error of <5%.

b Maximum amount tested.

TABLE IVNatural Water Sample Analysis and Recovery Studies

Cr added (µg/L) Cr founda (µg/L) Recovery (%)Sample Cr(III) Cr(VI) Cr(III)b Cr(VI) Cr(III) Cr(VI)

Underground water 0.00 0.00 0.80±0.018 2.23±0.052 – –

2.00 2.00 2.84±0.068 4.32±0.098 101.5 104.5

Oman Seawater 0.00 0.00 1.60±0.038 2.30±0.055 – –

2.00 2.00 3.56±0.082 4.38±0.105 98.0 104.0

Untreated water from Zahedan City 0.00 0.00 0.00 3.83±0.078 – –

2.00 2.00 1.95±0.048 5.89±0.190 97.5 103.0

Hamoon Lake water 0.00 0.00 0.00 1.81±0.048 – –

2.00 2.00 2.00±0.066 3.80±0.081 100.00 99.5

a Based on four measurements.b Obtained from the difference between total chromium and Cr(VI).

TABLE VChromium Concentration in Reference Water Samples

Using Proposed Method and Comparison to Certified Values

Sample Chromium (µg/L)Proposed methoda Certified valueb

LGC 6010 (Hard drinking water) 48.5±1.19 49.0

LGC 6011 (Soft drinking water) 48.2±1.16 48.0

CASS-3 (Near shore seawater) 0.089±0.002 0.092

SLRS-4 (River water) 0.32±0.008 0.33

a Average of three determinations.b Provided by LGC (UK) and MRC (Canada).

102

Recovery Study and ReferenceSample Analysis

A recovery study was carried outby measuring the Cr(VI) and Cr(III)concentration in four differentnatural water samples spiked withCr(III) and Cr(VI). The originalchromium concentration of thesesamples was also determinedbefore spiking. The recovery,established by the aboveexperiment, was 97.5–101.5%(n=4) for Cr(VI) and 99.5–104.5%(n=4) for Cr(III) (see Table IV).

Finally four reference watersamples [two LGC drinking water(UK) and two NRC natural watersamples (Canada)] were subjectedto the proposed method and theirtotal chromium concentration wasdetermined. The results in Table Vshow good agreement between thecertified values and those obtainedby the method proposed.

CONCLUSION

The results of this study showthat ion-pair extraction results ingood separation of Cr(III) andCr(VI) and simultaneouspreconcentration. Back-extractionwas applied to achieve furtherpreconcentration by a factor of 20.Chromium concentration in realsamples down to the 6-ng/L level ismeasurable by using this extractionmethod in combination withGFAAS analysis. The method issimple and sensitive and using theoptimized conditions described hasbeen successfully applied to theanalysis of two kinds of naturalwater reference samples.

Received March 12, 2002.

ACKNOWLEDGMENT

M. Noroozifar thanks Sistan andBaluochestan University for provid-ing the financial support for thisstudy.

REFERENCES

1. J. O. Nriagu,”Chromuim in “The Nat-ural and Human Environments,”,John Wiley & Sons, New York(1998).

2. K. Vercoutere and R. Cornelis, “Qual-ity Assurance for EnvironmentMetal Analysis,” Elseiver, Amster-dam, The Netherlands (1995).

3. J.F. Pakow and G.E, Janauer, Anal.Chim. Acta 69, 97 (1974).

4. W.J. Wang, Anal. Chim. Acta 119,157 (1980).

5. I. Kubrakova, T. Formanovsky, N.Kuzmin, and G. Tsysin, Analyst119, 2477 (1994).

6. R. Milacic and J. Stupar, Analyst 119,627 (1994).

7. E. Vassileva, K. Hadjivano, and T.Stoyehev, Analyst 125, 693 (2000).

8. T. P. Rao, S. Karthikoyan, B. Vijay-alekshmy, and C. Lyer, Anal. Chim.Acta 369, 69 (1998).

9. W. J. Maeck, M. E. Kussy, and J. E.Rein, Anal. Chem. 34, 1602 (1962).

103


Analytical Graphite Furnace Atomic Absorption Spectrometry

-- A Laboratory Guide -- Authors: G. Schlemmer and B. Radziuk

This book provides insight into the theoretical and practical aspects of graphite furnace AA, making it the perfect reference resource for all laboratories wanting to

use their graphite furnace more effectively.

Using an easy-to-follow style, the reader is guided from method development to calibration and validation of the instrument to the use of accessories and software

in modern graphite furnace AA.

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Part No. B051-1731, Analytical Graphite Furnace AAS – A Laboratory Guide, is available online from PerkinElmer Instruments at www.orderessentials.com

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