Carter, J. A.

Donohue, D. L.

Franklin, J. C.

Stelzner, R. W.

UNT Digital Library

3- - ZENVIRONMENTAL AND FUEL MATERIALS ANALYSES BYMULTI-ELEMENT ISOTOPE DILUTION SPARK-SOURCE MASS SPECTROMETRY*J . A. C a r t e r , D. L. Donohue, J . C. Frank l in andR. W. S t e l z n e rOak Ridge Na t iona l Labora to ryOak Ridge, Tennessee 37830ABSTRACT- NOTICE-This report was prepared as an account of wovksponsored by the United States Government. Neitherthe United States nor the United States EnergyResearch and Development Administration, nor any oftheir employees, nor any of tlieii contractors,subcontractors, or their employees, makes anywarranty, express or implied, or assumes any lepalliability or responsibility for the accuracy, completenessor usefulness of any information, apparatus, produci orprocess disclosed, or represents that its use would notinfringe privately owned rights.The isotope dilution method of analysis has been expanded to allow theaddition of twenty or more enriched stable isotopes to a single sample. Themethod combines the survey capability of spark-source mass spectrometry withthe accuracy and precision of isotope dilution techniques. The limit on theprecision of analysis may well be the variability in response of the IlfordQII emulsion. The method has been studied using National Bureau of standardsreference materials in order to determine the accuracy and precision of themethod. The method has been used to measure impurities in a variety ofenvironmental samples at several concentration ranges. A microphotometer hasbeen interfaced with a mini-computer to permit rapid collection of data andsubsequent concentration determinations. The data system and i t s operationwill be discussed briefly.INTRODUCTION p. 11The method of isotope dilution analysis for trace elements has beendescribed by Webster.(1). A known amount of "spike" (tracer) element withnon-normal isotopic abundance is added to the sample. The altered isotopicratio is measured mass spectrometrically, and the concentration of the elementcalculated from the spike concentration and isotope ratios.Usually isotope dilution is used to determine one or two elements with athermal emission mass spectrometer. Leipziger (2) used spark-source massspectrometry isotope dilution (IDSSMS) to determine Cu and Sb in standard samples.Multi-element analyses for up to six elements have been previously reported (3-6).*Work supported by the National Science Foundation - RANN Environmental Aspectsof Trace Contaminants Program under NSF Interagency Agreement AG 389 with theU. S. Energy Research and Development Administration.-2-Isotops dilution adds dimensions of precision and accuracy to spark-sourcemass spectrometry that are not usually obtained by SSMS. The combination ofaccuracy with the broad elemental range and the sensitivity of SSMS provides anew capability in trace metals analysis.Twenty-four stable enriched isotopes have been added to gasoline and watersamples at the 20 ng/ml to 1 ug/ml levels. In addition to being used as spikeisotopes* these enriched isotopes may be used as internal standards for otherelements. Thirty elements have been determined by the combination of methods.-9If the sample element or elements are available in quantities of 10 g,the element can be determined with a precision of 5% and an accuracy of about10 percent.MATERIALS AND METHODSThe analyses were performed with Mattauch-Herzog double-focusing mass spec-trometers, either the JKEI-MS-7 or the MS-702. All data were collected on IlfordQII photographic plates.Sample PreparationWater samples. The water samples were prepared for analysis by adding fromtwo to four micrograms of enriched stable isotopes (acid solution) to 100 ml ofthe water sample. The sample was evaporated to a volume of a few ml; 2-10 ptlof this solution was evaporated on high purity graphite electrodes.Gasoline Samples. The isotope spike solutions were prepared by dilutingthe aqueous stock solutions of the isotopes with acetone. The solutions were90% acetone, 10% water. The concentrations of the isotopes in the acetone spikesolutions were from 0.2 to 0.4 yg/ml of solution.Four methods of equilibration of the spike and sample were used.1. Bromine oxidation was the first method of equilibration. The sampleswere spiked, oxidized with bromine in CCl . The oxidized sample wasfreeze dried and the resulting tar carburized in vacuum at 60°c. Thecarbon was then pressed and sparked with standard spark-source procedures.2. Direct ignitions. The gasoline samples were spiked with the stableisotopes in acetone. The resulting solution was burned in open dishes.The tar remaining after ignition was leached in aqua regia. The acidleachate was dried on graphite, baked for five minutes at 400°C andsparked by the usual spark-source procedure.-3 -3. Burner oxidation. The samples were spiked as above. The equili-brated samples are burned in a lamp using a fiber glass-graphite wick.The chimney of the lamp was connected to an aspirator so that airflow forced the sooty flame up the chimney* The soot and the chimneywalls were leached with nitric acid. This acid solution was dried ongraphite and sparked directly.4. A stable isotope spike solution was made in disti l led HCl to contain1 ug/ml of the following isotopes: Mg, Ca, Cr, Fe, Ni,65rH 67-7 77e 8 7 a 97M~ 1 0 6 ^ 1 1 3 T 1 3 5 O 2 O 3 m 1 A •Cu, Zn, Se, Sr, Mo, Cd, In, Ba, Tl, and iso-topically normal Er. 1.0 ml of this spiked acid was refluxed with10oO ml of gasoline for a period of four hours. During this time,the gasoline partially decomposed as evidenced by the appearance ofbrown-black solid material.The aqueous layer was then drawn off, concentrated, and the entirealiquot deposited on the tips of machined graphite electrodes. Theelectrodes were placed in an oven at 300 °C for 15 minutes to driveoff any organic residue. The electrodes were sparked in the spark-source mass spectrometer to produce a graduated set of exposures from10 to 1 x 10~2 C.The data, regardless of sample preparation, were collected on Ilford QIIphotographic plates. The plates were developed for three minutes in Kodak D-19,short stopped in dilute acetic acid and fixed with Kodak rapid fixer.The density of the lines of the photoplate and the elemental concentrationswere calculated with a computerized plate reader. The microphotometer was con-verted into a single beam instrument, and the output of the photomultiplier wasamplified and digitized. The digitized data were stored in a PDP-8E computeras percent transmittance and element identification data. When data collectionwas complete, the transmittance of the lines was converted to density or in-tensity by the Churchill method, using previously stored emulsion calibrationcurves.Concentration of the individual elements was calculated by use of thealtered isotope ratios according to:/ M W S A M P L E \ (VK\ [(KX/SX) * SK - KK]* I I * I I * •M W v s / K K X / S X ) * SS + KS]-4-whereCS = concentration of sample solution,CK = concentration of spike solution,MW = atomic weight,VK = volume of spike solution.VS = volume of sample solutionKX - relative ion-beam intensity, spike isotope,SX = relative ion-beam intensity, sample isotope,KS. = % abundance (atom % ) , spike isotope in sample solution,SS = % abundance (atom % ) , sample isotope in sample solution,KK = % abundance (atom % ) , spike isotope in spike solution,SK = % abundance (atom % ) , sample isotope in spike solution.The operation of the system and details of construction have been describedby Stelzner, Sites, and KcKown (7).Isotope Spike SolutionsThe isotope spike solutions were prepared using enriched stable isotopesobtained from the Isotopes Division of Oak Eidge Holifield National Laboratory.Whenever possible, the isotopes had an isotopic abundance greater than 90% andchemical purity of 99%. The compounds or elements were dissolved in the appropri-ate acid or solvent, and diluted to stock solutions containing 100 yg/ml of thespike element.The isotope of choice, isotopic abundance and chemical form of the spikeelements, are summarized in Table 1. In addition to the stock solutions of theindividual isotopes, four different solutions of combined isotopes were preparedfor specialized needs. These four solutions, described in Table 2, have provedsatisfactory for water, biological, and fuel samples.Synthetic gasoline samples were prepared in two different forms. Metal-gasoline solutions were prepared by dissolving NBS organo-metallic standards ina mixed solvent of xylene, ethyl alcohol, and hexanes. The SRM solution wasdiluted with ordinary regular gasoline so that the added concentration of eachof the metals was 10 ug/ml in the stock solution. The SRM samples used to pre-pare the standard solutions are summarized in Table 3.-5-Tablg 1. Enriched Stable Isotopes Used in IDSSMSIsotope204Pb194Pti 3 5 L1 1 3 I n1 0 6 Cd1 0 2 P d98_Ru97Mo9 1 Zr8 7 S r7 7 Se73Ge71Ga67Zn6 5Cu6 1 Ni5 7 Fe5 3Cr4 3Ca25MgSpike Abundance94.394.9654.8547.583%93.691.2396.388.475.4589.094.2586.693.2987.2486.199.889.559S.599.5490.4296.481.199.2Normal Abundance1.3729.5232.80.1614.286.567.034.161.220.82.29 .611.236.967.507.77404.1230.911.22.29.540.1310.1Chemical FormPbT12°3PtHgOW 0 3Ba(NO3)2TeInCdOPdRuMoZrO2Sr(NO JSeG e 0 2Ga2°3ZnOCuONiFe2°3Cr2°3CaCO3MgO-6 -Table 2. Enriched Isotope Solutions for IDSSMSSolution 125Mg5 7 FeSolut ion 22 0 4 Ib1 0 6Cd6 5Cu6 7ZnSolution 32 0 4Pb2 0 3 nNormal,,Er1 3 5Ba1 1 3 I n106Cd97Mo8 7 S r7 7Se67Zn6 5Cu6 1Ni5 7 Fe5 3 Cr4 3Ca25MgSolution 42 0 4Pb201Hg1 8 O w150M.Nd1 3 5 Ba1 2 5 Te1 0 6Cd97Mo9 1 Zr8 7 Sr77Se73Ge67Zn61Ni5 7Fe5 3CrTable 3. NBS SRM-1050-1075 Compounds forStandard Gasoline SamplesElement CompoundSi Octaphenylcyclotetrasi loxaneV Bis (1-ph.enyl-l, 3-butanediono) oxovanadium(IV)Fe Tris(l»phenyl-1,3-butanediono) i ron ( I I I )Zn Zinc cyclohexanebutyrateCu B i s (1 -pheny l - l , 3 -bu t aned iono )coppe r ( I I )Cd Cadmium cyclohexanebutyra teSn Dibutyltin bis(2-ethylhexanoate)Cr Tris (1-phenyl-l, 3-butanediono) chromium(III)Ni Nickel cyclohexanebutyrateP Triphenyl phosphater-7-A second se t of syn the t i c sample so lu t ions in gasoline was prepared byd i lu t ion of a metal sulfonate mix, CONOSTAN D-20 (Continental Oil CompanyCONOSTAN, Ponca Cityc Oklahoma). This mixture as purchased contained 20meta l l i c elements a t a concentration of 100 ug/g of the mixture. TheCONOSTAN solut ion was d i lu t ed with regular gasoline t o provide a gasolinestock solut ion with one ug/ml of the twenty elements.RESULTSWater samples have been limited to demineralized water, distilled water,and a few samples of process water. The data in Table 4 summarizes the elementssought in the various water samples. The detection limits reported are; calcu-lated on the basis of the most abundant isotope of the normal element. Thetotal sample volume for all data here was 100 ml.A second type of data was obtained from the demineralized water samples.The samples were high purity, and the added elements were in an isotopicallyenriched state so that several elements were added at masses and concentrationssuch that interferences were highly improbable. Relative sensitivity factors(RSF) are obtained from this data that are useful in other general survey-typeanalyses. The RSF data are compared in Table 5 with previously detenined orestimated RSF values.The impurity content of the gasoline samples depends on the source of thegasoline, type of gasoline, lead content, and unknown factors. Table 6 is asummary of certain elements found in gasoline samples, along with the concentra-tion ranges that have been measured.DISCUSSIONThe first requirement of IDSSMS is that the isotope spikes must s;tay insolution along with the sample elements. Equilibration is not feasible if thesample or the spike is not soluble. The second desirable requirement is thatthe spike isotope be present in the normal element at a low abundance. A thirdrequirement is that the spike isotope be free of interference or be available asa multiply charged ion (odd mass).-8-Table 4. Selected Metallic Impurities in Water SamplesElementAsBeCrCuFeMoNiPbSeSrVZnWater Analysis, ng/mlDistilled Demineralized Process<.l<o'.r32<0.51<1<0.5<0.1<.r0.253<0.51<1<0.5<0 11.0616201521100<0.2 <0.2 250Table 5. Relative Sensitivity Factors Determined by Additionof Stable Isotopes at 20 to 40 Nanogram/ml ConcentrationElementTlInCdMoSeCuNiFeCrCaPrevious RSF20.50.510.70.50.50.50.50.5(a) Assuming that RSF for Er = 1.r-9-IDSSMS provides a second, type of analysis that is not traditional isotopedilution, but a modified internal standard method. It is practical to use anenriched isotope of one element as the internal standard for a second element.123 77We have determined Sb with a Te spike, As with a Se spike and Bi with a204Pb spike. The analysis of these solutions has been duplicated using stand-ard solutions, and the accuracy seems to be about 10%.The analysis of water samples by IDSSMS has detection limits for some-12elements in the range of 1 x 10 g/ml for a 100-ml sample. In addition, theprecision of the analysis is generally from 5 to 10%; photographic plates asion detectors limits the precision to ±5%* The water analysis can be complicatedby the presence of high levels of Ca, Mg and K because of oxide and chloridepolymers. Organic compounds present in the water must be removed by heating at400°C for a few minutes. If not removed, the organic compounds may produce poorsensitivity for metallic elements.The analysis of the gasoline samples is complicated by the fact that themetallic elements except for Pb are present as unknown organic compounds. Forthe.-measurement of the other three metals, the spike isotopes were prepared inaqueous solution, since the cost of synthesis of appropriate organo-metalliccompounds with enriched stable isotopes would be prohibitive. The fact that themetals in gasoline are organo-metallics and are not equilibrated even aftei" SO hrswith isotope spikes is demonstrated by the data in Table 7. The elements wereadded to the gasoline as the-.NBS organo-metallic compounds at abcrt 0.5 vg/ml con-centration range and the sample equilibrated with the isotope spike dissolved in90% acetone and 10% aqueous acid solutions. In some cases, extra acetone wasadded to the gasoline-acetone-spike mix to insure single phase solutions. Thesedata demonstrate the failure to equilibrate between added organo-metallic ele-ments and the appropriate spike. These data were obtained from analysis of theresidue that remained when the gasoline and spike mixture was burned. Examinationof the soot indicated that the normal metallic elements associated with organicligand and was primarily concentrated in the soot which was collected on a coldsurface. The Cd spike remained in the residue, but the normal, cadmium fromthe organo-metallic compound was found in the soot. Sn, Cr and other elementsbehaved in a similar manner.-10-Table 6. Impurity Concentrations in GasolinesElementBiBaSnCdMoSrSeZnCaCoNiFeCtVCuConcentration0.0010.0050.050.001<0.0010.0020.0010.010.050.0050.0050.050.010.00050.01Range, yg/ml- 0.1- 0.5- 0.3- 0.05- 0.05- 0.01- 0.02- 2.0- 0.2- 0.1- 0.1_ 2- 0.5- 0.002- 0.2Table 7. Non-Equilibrated Metals in GasolineElementCdSnNiCrPFound(Pg/ml)0.050.050.10.10.3Added(yg/mi)0.480.470.350.250.61-11-The reflux procedure was then adopted as the method of choice to equili-brate the isotope spikes with the samples. The results obtained from thisreflux procedure showed that the technique essentially assures completeequilibration between the enriched isotope spike and the trace organo-metallicconstituents.CONCLUSIONIso tope d i l u t i o n spark-source mass spectrometry has been shown t o be ava luab le mul t i -e lement method of ana ly s i s fo r water and fue l samples. Thea n a l y s i s r equ i r e s an unequivocal method of e q u i l i b r a t i o n of sample and sp ikeisotopes. The data for the poorly equilibrated gasolines demonstrate thatrecovery of the spike isotope can lead to the conclusion that an accurateanalysis has been made when the sample element has, in fact, been almost com-pletely lost.The analysis of water samples requires a large (100 ml) sample and thepresence of large amounts of Ca, Mg, K and organics in the water may lead toincorrect concentrations for some elements if careful preparation of the samplesis not used.In general, the detection limit for metallic impurities in water is in therange of 5 x 10 g/ml in a 100-ml sample, and the accuracy is in the 5-10%range. The detection limits in gasoline are in the 10 g/ml range for 10 mlsamples with a precision and accuracy comparable to that in the water samples.We have developed a method of refluxing the gasoline with an HC1 solutionspiked with isotopically enriched spike elements. This method of spiking thegasoline samples permits a quantitative analysis with an accuracy of 10% atthe one ug/ml level.ACKNOWLEDGMENTSWork s u p p o r t e d by t h e N a t i o n a l S c i e n c e F o u n d a t i o n - RANN E n v i r o n m e n t a lA s p e c t s o f T r a c e C o n t a m i n a n t s Program u n d e r NSF I n t e r a g e n c y Agreement AG 389with the U. S. Energy Research and Development Administration. Research per-formed at Oak Ridge Holifield National Laboratory, operated by Union CarbideCorporation, under Contract No. W-7405-eng-26, with the Energy Research andDevelopment Administration.-12-LITERATURE CITED1. Webster, R. K. 1959. Isotope Dilut ion Analysis . I n : Advances inMass Speetrpraetry, J. O. Walden, Ed. , Pergammon Press , New York,pp. 103-119.2. Leipniger , F. D. 1965. Analytical Chemistry _37» 1 7 1 -3. Alvarez, R», P. J . Paulsen, and D. E. Kel leher . 1969. Analyt ica lChemistry 41_, 955.4 . Paulsen, P. J . , R. Alvarez, and D. E. Kel leher . 1969. Spectrochim.Acta 24B, 535.5. Paulsen, P. J . , R. Alvarez, and C. W. Nuel le r . 1970. Analy t ica lChemistry 42 ,^ 673.6. Car t e r , J . A., R. L. Walker, and 0 . R. s i t e s . 1973. Trace Impuri t iesin Fuels by Isotope Di lu t ion . In : Nat ional Meeting American ChemicalSocie ty , Vol. 18, No. 4, pp . 78-83.7. S t c l z n e r , R. w., H. S. McKown, and J . R. S i t e s , Computerized Spark-SourceMass Spectrometer Densitrometry, ACS Spring Meeting, Ph i l ade lph ia , Pa.Apri l 6 -11 , 1975.

Environmental and fuel materials analyses by multi-element isotope dilution spark-source mass spectrometry

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Environmental and fuel materials analyses by multi-element isotope dilution spark-source mass spectrometry

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