Anal. Chem. 1980, 52,
seen from Table VI, the values for the digested sera obtained by A1F MAS and the ISE show fairly good agreement, while the values for the digested sera are slightly larger than those for the nondigested sera obtained by AlF MAS. There may be a possibility that the differences of analytical results for nondigested and digested sera originate from contamination during the complicated procedures of dry ashing or from the existence of another form of fluorine; e.g., volatile organic fluorine compounds which may not be detected by AlF MAS. However, since A1F MAS has less chance of contamination because of its simplicity and rapidity, the present results may strongly support the existence of fluorine bound to protein, which was suggested by Taves (2, 3 ) .
ACKNOWLEDGMENT T h e authors thank to S. Kamei for providing the blood serum s a m d e s . We also exmess our thanks to S. Fuiiwara and Y. Umwawa for the kind permission to use their electrochemical measurement instrument with an on-line A/D conversion system.
LITERATURE CITED (1) Singer, L.; Armstrong, W. D. Anal. Chem. 1959, 37, 105-108. (2) Taves, D. R. Nature (London) 1968, 217, 1050-1051.
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Taves, D. R. Nature (London) 1968, 220, 582-583. Taves, D. R. Talanta 1966, 75, 969-974. Cox, F. H.; Backer Dirks, 0. Caries Res. 1968, 2 , 69-78. Venkateswarlu, P.; SRa, P. Anal. Chem. 1971, 43, 758-760. Bock, R.; Semmler, H. J. Fresenius' 2. Anal. Chem. 1967, 230, 16 1- 184. Singer, L.; Armstrong, W. D. Biochem. Med. 1973, 8 , 415-422. Venkateswarlu, P. Anal. Chem. 1974, 4 6 , 878-862. Venkateswarlu, P. Clln. Chlm. Acta 1975, 59, 277-282. Venkateswarlu, P. Anal. Biochem. 1975, 66,512-521. Singer, L.; Ophaug, R. H. Anal. Chem. 1977, 49, 38-40. Eksband, J.; Ericsson, Y.; Rosell, S . Arch. OralBid. 1977, 22, 229-232. Belisle, J.; Hagen, D. F. Anal. Biochem. 1978, 87, 545-555. Tsunoda, K.: Fujiwara, K.; Fuwa, K. Anal. Chem. 1977, 49, 2035-2039. Tsuncda, K.; Chlba, K.; Haraguchi, H.; Fuwa, K. Anal. Chem. 1979, 51, 2059-2061. Sawatari, K.; Imanishi, Y.; Umezawa, Y.; Fujlwara, S.Bunsekl Kagaku 1978, 27, 180-183. Tusl, J. Clin. Chem. 1970, 27, 216-218.
RECEIVEDfor review March 19,1980. Accepted June 5, 1980. This research has been supported partly by a Grant-in-Aid for Environmental Science under grant No. 403029, and also partly by a Grant-in-Aid for Special Project Research under grant No. 421704 from the Ministry of Education, Science, and Culture, Japan.
Comparison of Methods for the Determination of Trace Elements in Seawater R. E. Sturgeon,* S. S. Berman, J. A. H. Desaulniers, A.
P. Mykytiuk, J. W.
McLaren, and D. S. Russell
Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada, K I A OR9
Five different analytical methods comprising three instrumental techniques were utilized in a study to determine the trace metal content of coastal seawater. Results obtained using isotope dilution spark source mass spectrometry, graphite furnace atomic absorption spectrometry (GFAAS), and inductively coupled plasma emission spectrometry foilowing trace metal separation-preconcentration (using ion-exchange and cheiation-solvent extraction), or direct analysis (by GFAAS) were collated. Comparison of data between suitably different analytical methods is a practical way of testing the validity of those methods, gives increased confidence in the results obtained, and is especially valuable when standard reference materials are not available.
T h e determination of trace elements in seawater is pursued with great difficulty (I). Quantitation of extremely low concentrations of analyte (0.02-10 pg L-I) accompanied by a matrix consisting of 3.5% dissolved solids imposes great demands on instrumental techniques. Sample preparation schemes designed to both preconcentrate the trace elements and separate them from major interfering components prior to analysis are numerous (e.g., 1-7). All such methods invariably increase sample manipulation and the relatively large amounts of reagents and container surfaces brought into contact with the sample often give rise to unacceptably high and/or random procedural blanks. These problems are exacerbated by a lack of standard reference materials which would permit detection of systematic errors such as contamination or analyte losses introduced during sample manipulation and the presence of matrix or spectral interferences 0003-2700/80/0352-1585$01 .OO/O
perturbing instrument response. A logical approach which serves to minimize such uncertainties is the use of a number of distinctly different analytical methods for the determination of each analyte wherein none of the methods would be expected to suffer identical interferences. In this manner, any correspondence observed between the results of different methods implies that a reliable estimate of the true value for the analyte concentration in the sample has been obtained. T o this end the analysis of coastal seawater for Cd, Zn, Pb, Fe, Mn, Cu, Ni, Co, and Cr was carried out using isotope dilution spark source mass spectrometry (IDSSMS), graphite. furnace atomic absorption spectrometry (GFAAS), and inductively coupled plasma emission spectrometry (ICPES) following trace metal separation-preconcentration (using ion-exchange and chelationsolvent extraction), and direct analysis (by GFAAS). T h e analytical advantages inherent in such an approach are discussed in the present paper.
EXPERIMENTAL Instrumentation. A Varian lechtron atomic absorption spectrometer,model AA-5, fitted w i h a Perjkin-Elmer HGA-2200
graphite furnace and temperature ramp accessory was used for all atomic absorption measurements. Simultaneous background corrections were made using a deuterium lamp. Sample solutions were delivered to the furnace in either 10- or 20-pL volumes using a Perkin-Elmer AS-1 autosampler, and absorbance peaks were simultaneously recorded on a fast response Speed Servo I1 strip-chart recorder (Esterline Corp.) and a digital storage oscilloscope (Gould Advance). Samples were held in acid washed polypropylene cups prior to injection. Pyrolytic graphite coated furnace tubes were used exclusively. A modified Perkin-Elmer graphite tube was used for direct GFAAS determinations of iron and zinc (8). 0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
Spark source mass spectrometric analyses were carried out using an Associated Electrical Industries AEI 702 mass spectrograph using photoplate detection. The line intensities were determined directly from the plates employing an Applied Research Laboratory densitometer fitted with an analog output yielding integrated line intensities. The spectrograph and source assembly have been described in detail in an earlier publication from this laboratory (9). Several elements were analyzed by inductively coupled plasma emission spectrometry following their preconcentration and separation from the seawater matrix. A custom designed instrument consisting of a Plasma-Therm model PT-1ICP source coupled to a Spectrametrics SMI I11 echelle grating spectrometer which was fitted with a 20-channel multielement cassette was used. The instrument has been fully described in an earlier report from this laboratory (10). The trace metal concentrates were introduced into the ICP by means of an ultrasonic nebulizer used in conjunction with a desolvation apparatus, as described in Ref. 10. Data acquisition and manipulation as well as spectrometer operation are under computer control. Reagents. Stock standard solutions (1000 mg L-') of the elements of interest were prepared by dissolution of the pure metals or their salts. Following dilution with high-purity distilled, deionized water their concentrations were verified by standardization against NBS Standard Reference Material 1643 (Trace Elements in Water). Stable isotopes 57Fe,%u, 207Pb,67Zn,'"Cd and 61Ni were used for isotope dilution mass spectrometric analyses. All reagents were purified prior to use. Concentrated nitric, hydrochloric, and acetic acids as well as methyl isobutyl ketone (MIBK) were prepared by sub-boiling distillation in a quartz still from reagent grade feedstocks (11). A saturated solution of ammonium hydroxide (28%) was prepared by isothermal distillation according to the procedure recommended by Zief and Horvath (12). Fresh 5% aqueous solutions of ammonium pyrrolidine-Ncarbodithioate (APDC) were filtered through 0.3-pm membrane filters to remove insoluble material and stripped free of metal impurities by repeated extraction with distilled MIBK. 8-Hydroxyquinoline (oxine) was purified by vacuum sublimation at 120 "C onto a cold finger and 2% solutions of the reagent were prepared in dilute hydrochloric acid. A 1 M ammonium acetate buffer was prepared by combining 77 mL of 28% ammonium hydroxide solution with 57 mL of glacial acetic acid and diluting to 1 L. The buffer pH was adjusted to the desired value as required. Chelex-100 resin (20G400 mesh sodium form, Bio-Rad Laboratories) was purified by batch extraction with 5 M nitric acid prior to use. Coastal seawater was obtained from the Atlantic Regional Laboratory of the National Research Council of Canada, Halifax, Nova Scotia. The samples had a salinity of approximately 32%0 and were taken from relatively unpolluted surface waters off the coast of Nova Scotia. The seawater had been filtered through a nominally 0.45-wm membrane filter, acidified to pH 1.6, and stored in precleaned polypropylene bottles. Procedures. All sample preparations were carried out in a clean laboratory equipped with laminar flow benches and fume cupboards, providing a class 100 working environment. All laboratory ware was thoroughly cleaned in nitric acid (1 + l ) , rinsed with demineralized water, further cleaned with hot aqueous 0.05% APDC solution, and again rinsed with deionized water. Once in use, polypropylene beakers and separatory funnels were washed with only deionized water between runs as frequent thorough cleaning was found to produce highly variable analytical blanks. With the exception of isotope dilution spark source mass spectrometric analyses, the method of standard additions was used for all other analyses in an effort to compensate for the physical and chemical matrix effects present in the seawater ( 2 , 7). All standard additions curves were analyzed by regression procedures to obtain the intercepts. Solvent Extractions. Trace metals were extracted and preconcentrated from 100-mL aliquots of seawater prior to their determination by GFAAS according to the method described by Sturgeon et al. (13). Following their extraction into MIBK, using
a combination of APDC and oxine as chelating agents, the trace metals were further preconcentrated by back-extraction into 1.5 M "OB, yielding a 33-fold preconcentration. Spike recoveries averaged 80%. Two blanks were run concurrently with each solvent extraction preconcentration. Ion Exchange. Preconcentration of trace metals by ion exchange using Chelex-100 resin has been described in detail in a recent publication by Sturgeon et al. (13) and is similar to that reported by Kingston et al. ( 1 4 ) . The volume of seawater taken for each preconcentration was 225 mL for samples analyzed by ICPES and 100 mL for samples analyzed by GFAAS and IDSSMS. At least two blanks were run for ICPES samples and four or more blanks were run for GFAAS and IDSSMS analyses. The trace metal concentrates (and blanks), stripped from the Chelex-100resin and taken up in 10-mL volumes of 2.5 M HN03, provided a virtually matrix-free solution for instrumental analysis (>99.9% rejection of Na, Ca, Mg, K (13)). Spike recoveries averaged 86% for Fe, Cu, Pb, Cd, Zn, Ni, and Co but only 8% for Cr (13). For analysis by IDSSMS, isotopic spikes of Fe, Cu, Cr, Pb, Zn, Cd and Ni were equilibrated with the seawater for 12 h at =40 "C and pH 1.6 prior to preconcentration on Chelex-100 resin. Since Co is monoisotopic, an internal standardization method of calibration was used to quantitatively analyze this element (by comparison to Ni). The trace metal preconcentrates from the resin (=lo mL 2.5 M "OB) were placed in a polymethyl pentene (TPX) beaker, 7 mL of concentrated HCl was added, and the sample was taken to dryness on a hot plate in a clean fume cupboard. Residual ",NO3, introduced into the preconcentrate when the resin was washed with an ammonium acetate buffer prior to trace metal elution with acid, is volatilized in this step. The residue was then taken up in a few milliliters of concentrated HN03or HC1 and again evaporated to near dryness, and the final drops were transferred to the head of the sample electrode with a polyethylene dropper. The electrode was heated in a vertical position on a small electrical heater to =80 OC so that successive drops evaporated quickly. Following completion of sample addition, the electrode temperature was raised to 400 "C to eliminate any organic matter (carried from the resin). The sample electrode was then mounted in the spectrometer source and the surface film of sample was analyzed. Details of optimum instrumental parameters as well as data handling and calibration will be discussed in a subsequent report from this laboratory. Direct GFAAS Analysis. Direct analysis of undiluted seawater for Fe, Mn, and Cd and of 1:l diluted seawater for Zn by GFAAS has been described elsewhere (8, 15). Ascorbic acid (10 mg/mL) was used as a matrix modifier for both Cd and Zn in (€9,respectively. lieu of EDTA (15) and ",NOB Details of the optimized instrumental conditions for analysis of trace metal preconcentrates by GFAAS have been given in Ref. 13 whereas those for analysis of preconcentrates by ICPES have been given in Ref. 10 and will not be repeated here.
RESULTS A N D DISCUSSION Tables I and I1 show the results obtained for two separately collected and stored seawater samples, A and B. The mean concentrations and standard deviations of replicates (after rejection of outliers on the basis of a simple c test-function (16))are given for each method of analysis. Each mean reflects the result of four or more separate determinations by the indicated method. Cu, Ni, Pb, Cr and Co could not be measured by direct GFAAS because of their inherently low concentrations (below GFAAS detection limits) and/or pronounced physicochemical matrix interference effects (8). Manganese could not be determined by IDSSMS because it is monoisotopic. Furthermore, an internal standard method of calibration was not attempted for this element because the yield of Mn by the Chelex-100 ion-exchange preconcentration technique was variable (strongly dependent on the p H of the buffer solution). Although an isotopic spike was available for Cr (Le., 53Cr), analysis of this element by IDSSMS following sample preconcentration by ion-exchange is not reported. Chromium
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
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Table I. Analysis of Seawater Sample A concentration, ng/mL ~~
GFAAS chelation-ex traction
element
direct
Fe Mn Cd Zn cu
1.6i 0.2= 1.6 i 0.1 0.20f 0.04 1.7 f 0.2 ND ND ND ND
Ni
Pb co
ICPES ion exchange
IDSSMS ion exchange
1.5 f 0.6 1.5f 0.1 ND 1.5 f 0.4 0.7 t 0.2 0.4 t 0.1 ND ND
1.4 t 0.1 N D ~ 0.28 * 0.02 1.6 t 0.1 0.7 f 0.1 0.37 f 0.02 0.35 f 0.03 0.020d f 0.003
1.5 * 0.1 1.4 i 0.2 0.24 * 0.04 1.9 f 0.2 0.6 * 0.2 0.33 i 0.08 0.22 lr 0.04 0.018cf 0.008
Precision expressed as standard deviation. Not determined. source mass snectrometrv-internal standard method.
Reconcentrated 100-fold by ion exchange.
a
Spark
Table 11. Analysis of Seawater Sample B concentration, ng/mL element
direct
GFAAS chelation-extraction
Fe Mn Cd Zn
3.7 f 0.3a 2.5 f 0.2 0.05 f 0.01 1.8 * 0.3 ND ND ND ND ND
3.2 i 0.2 1.9 i 0.2 0.06 + 0.01 1.8 i 0.1 0.5 i 0.1 0.46 f 0.03 0.06 f 0.02 0.29 i 0.03 0.015* 0.003
cu
Ni
Pb Cr
co
Precision expressed as standard deviation. spectrometry-internal standard method. a
&I
ion exchange
ICPES ion exchange
IDSSMS ion exchange
3.4 i 0.4 2.2 i 0.3 0.053 f 0.007 2.0 i 0.1 0.51 i 0.03 0.45 i 0.05 0.10f 0.01 0.25 i 0.02 0.018ci 0.008
3.2 t 0.2 2.3 f 0.1 ND 1.6 f 0.2 0.73 i 0.06 0.38 f 0.02 ND ND ND
3.3 f 0.3 N D ~ 0.07 c 0.01 1.9 i 0.1 0.61 f 0.04 0.43 f 0.03 0.11f 0.02 ND 0.028df 0.001
Not determined.
in seawater is poorly retained by the Chelex-100 resin ( ~ 1 0 % recovery of spike (13))whereas Cr retention by the simulated analytical blank is much greater ( ~ 3 0 % (13)),making blank correction for Cr a difficult and semiempirical procedure, especially when the analytical blank is significant relative to the sample ( ~ 2 5 %for these samples). Using such a semiempirical approach to blank correction a value of 0.34 0.03 ppb is obtained for Cr in seawater sample B in fair agreement with the results by GFAAS. This problem can, of course, be overcome by using larger volumes of seawater for preconcentration (=500 mL), thus diminishing the relative importance of the blank correction. Preconcentration of 1-L volumes of seawater (100-fold preconcentration) was required for GFAAS analysis of Cr (and Co) when ion-exchange techniques were used. Cd, Pb, Cr, and Co could not be determined by ICPES using a 25-fold preconcentration of the trace elements as their levels in such concentrates remain below values a t which reliable analyses can be performed (10). Furthermore, no consistent results could be obtained for these elements even when 1-L volumes of seawater were preconcentrated (by a factor of 100) using ion exchange. Chromium, being only weakly retained by the resin, was not sufficiently enhanced in concentration to be determined in such concentrates. Additionally, the larger volumes of seawater tended to magnify any differences in the efficiencies of the exchange columns used for a standard additions analysis, resulting in unacceptable uncertainty in the slopes of the standard additions plots. Overall there is good agreement between the elemental values in relation to the method of analysis. The precision of replicate determination between methods for all elements is comparable. Complete analysis of each sample by all of the methods indicated usually required about two months. The spread in the results may therefore reflect both the real spread inherent in the analytical methods as well as any (obviously
*
Reconcentrated 100-fold.
Spark source mass
~~
Table 111. Analysis of Seawater for Iron and Manganese by Graphite Furnace Atomic Absorption iron
manganese chelationchelationsample direct extraction direct extraction I 1.6 f O.la 1.5 f 0.1 1.22 2 0.09 1.2i 0.1 6.1 f 0.3 6.4 + 0.8 2.5 i 0.8 2.5 i 0.2 I1 _-_ 14.9 i 0.9 14 f 1 __I11 __-__ 1.2 i 0.1 1.3f 0.1 IV V ----1.2 i 0.1 1.3 ? 0.1 a Precision expressed as standard deviation with means calculated from 4 or more replicate analyses.
minor) changes that may have occurred in the sample composition during this time period. Although the data population is too small to carry out a worthwhile rigorous, statistical evaluation of the results, several points merit discussion. The mean concentrations of Fe and Mn in both samples, as measured by direct GFAAS, appear to be slightly positively biased with respect to their mean concentrations determined by other methods. However, our experience with a number of seawater samples has shown that there is no tendency toward higher results for the analysis of Fe and Mn by direct GFAAS and that this deviation may only be an apparent one for the two samples presented here. This is clearly shown in Table I11 where data for the direct GFAAS analysis of a number of different seawater samples are compared with those obtained by analysis using chelation solvent extraction techniques (APDC-MIBK). No systematic biasing of the results is evident. Sample I11 shows obvious iron contamination but the results are included to show that direct analysis can be used over a wide concentration range. No correction for a reagent blank is required for direct GFAAS analysis of Fe and
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
Mn (8) and the instrumental background correction for nonatomic absorption is less that 0.1 absorbance when 20-pL aliquots of seawater are atomized (8). Both of these factors tend to reduce the uncertainty of the analytical result obtained by direct GFAAS. Lead is one of the most difficult elements to determine in seawater because of its extremely low concentration (relative to instrumental detection limits) as well as its relatively high ambient concentration in both reagents and the atmosphere. The poor agreement between the analytical results obtained by chelation-solvent extraction/GFAAS and ion-exchange/ IDSSMS for lead in sample A is indicative of either contamination during analysis, incorrect compensation for blank, or errors in instrument response. T h a t reasonable agreement among the various methods was obtained for the analysis of P b in sample B (at somewhat lower concentration) suggests that the problem encountered with the analysis of sample A was perhaps contamination during sample preparation. The discrepancy between the results obtained by GFAAS and SSMS for Co in sample B may reflect the inaccuracy of calibration by the use of an internal standard for this element by SSMS (i.e., f20%). Agreement between these two methods of analysis for Co in sample A is, perhaps, fortuitous. T h e trace elements determined here are present in a very complex matrix (comprising both inorganic and, perhaps more importantly, organic constituents) and it is not unexpected that both sample preconcentration as well as instrument response may be subject to interferences. To some extent, the use of standard addition methods to compensate for physicochemical matrix effects may alleviate these problems but it cannot guarantee correct results. The use of a number of instrumental techniques coupled with different sample preconcentration procedures provides an additional check against biased results. In no instances would a common response to any physical or chemical interference be expected for all methods of analysis. For example, spectral and chemical interferences in all three GFAAS techniques would not be the same, and those encountered in ICPES are different from those in MS. The good correspondence observed between all methods of analysis (with the exceptions of those noted earlier) leads to the conclusion that a fair estimate of the true value for the analyte concentrations in the stored seawater samples has been obtained. Despite the short equilibration periods (==5min) for ionic spikes added to samples prior to preconcentration (with the exception of IDSSMS, as noted earlier), no apparent problems were encountered with chelation-solvent extraction or ion exchange due to speciation effects (i.e., the degree of extraction or ion exchange of the added ionic spike was equal to that of the native element). This result was perhaps fortuitous in that no soluble strong organic complexes of the trace elements (relative t o APDC, oxine, or iminodiacetate complexes) are indigenous to the waters sampled (although comtal surface waters are usually high in dissolved organic matter, e.g., humates), or, more likely, that the prolonged period of st,orage o analysis of the sample at pH 1.6 (acidified with "OB) prior t irreversibly released all the bound trace metals. Direct analysis by GFAAS is a fast, accurate method for the determination of Fe, Mn, Zn, and Cd in seawater when these elements are present at concentrations above 0.2, 0.2, 0.4, and 0.01 pg L-l, respectively (8, 15) (offshore seawater concentrations of these metals are: Fe -0.5 pg/L, Mn -0.05 pg/L, Zn -0.2 pg/L, and Cd -0.05 pg/L). In this connection it should prove useful as a rapid screening technique for these elements. Below these levels, and for the other elements studied here, chelation-solvent extraction using APDC/ nxine/MIBK in combination with a back-extraction into an
acidic aqueous phase prior to determination by GFAAS is the most useful technique for multielement determinations when small volumes of seawater are available (e,g., 50 X preconcentration on 100-mL aliquot of seawater). If significantly greater preconcentration is required, or if greater volumes of preconcentrate solution are needed as, for example, when analysis is completed by ICPES, the more laborious method of ion exchange (using Chelex-100) may be used. [Jnfortunately, while commercial GFAAS remains relatively inexpensive, it is not a multielement technique and is distinctly lacking when multielement determinations must be made on large numbers of samples. Although ICPES i s a multielement technique, its inferior detection limits (relative to GFAAS for the elements in this study) necessitate the processing of relatively large volumes of seawater. For the present study, 250-mL aliquots were found to be useful for the analysis of Fe, Mn, Zn, Cu. and Ni. Extension of the method to include Cd, Cr, Pb. and Co will require improvements in the preconcentration procediire or alternat,e preconcentration techniques (e.g.,carrier precipitation). Isotope dilution SSMS offers the advantages of multielement capability coupled with high sensit,ivity allowing determinations to be carried out on less than 100 mL of seawater (total). Replicate multielement determinations on many samples can thus be rapidly achieved. TJnfortunately, the method requires a large capital outlay and can process only relatively few samples per day, making routine use unfeasible. The technique serves as an excellent referee method. Comparison of data between such suitably different analytical methods is a practical way of testing their validity in those cases where potential interferences must be overcome and standard reference materials are unavailable. The residts obtained in this study give confidence in the ability of the individual methods to give accurate analytical data and in the effectiveness of the standard addition approach in compensating for physicochemical interferences arising during sample preconcentration and instrumental analysis.
ACKNOWLEDGMENT The authors thank R. Guevremont of the National Research Council of Canada for the filtered, acidified seawater. LITERATURE CITED (1) Riley, J. P.; Skirrow, G. "Chemical Oceanography"; Academic Press: London, 1965: Vol. 3, pp 269-408. (2) Brooks, R. R.; Presley, B. J.; Kaplan. I. R. Talanta 1967, 74. 809 Westerlund. S.Anal. Chim Acta 1978. (3) Danielsson. L.; Magnusson, 6.; 9 8 , 47. (4) Riley, J. P.; Taylor, D. Anal. Chim. Acta 1988, 40, 479. (5) Florence, T. M.; Batley. G. E. Talanta 1976, 2 3 , 179. (6) Batley, G. E.; MatouSek. J. P. Anal. Chem. 1977, 4 9 , 2031. (7) GomiSEek. V . H. S.;Gorenc, B. Anal. Chim. Acta 1978, 9 8 , 39. (8) Sturgeon, R. E.; Berman, S.S.; Desaulniers, A,; Russell, D S. Anal Chem. 1979, 51, 2364. (9) Mykytiuk, A,: Russell, D. S.;Boyko, V. Anal. Chern. 1976, 4 8 , 1462. (IO) Berman, S. S.;Mclaren, J. W.; Willie, S.N. Anal Chem. 1980, 52, 488. (1 1) Dabeka, R. W.; Mykytiuk. A.; Berman, S.S.; Russell. D. S.Anal. Chem. 1976, 4 8 , 1203. (12) Zief. M.; Horvath, J. "Acccuracy in Trace Analysis: Sampling, Sample Handling, Analysis", LaFleur, P. D., Ed.: NRS Special Publication 422, U S . Government Printing Office: Washington. D.C., 1976. (13) Sturgeon, R. E.; Berman, S.S.; Desaulniers. A,: Russell. D. S. Talents 1980, 27, 8 5 . (14) Kingston, H. M.; Barnes, I. L.: Brady, T. J.; Rains, T. C.; Champ, M. A. Anal. Chem. 1978, 5 0 , 2064. (15) Guevremont. R.: Sturmon, R. E.; Berman, S.S . Anal. Chim. Acta 1980, 775,63. (16) Cooper, B. E. "Statlstics for Experimentalists"; Pergamon Press: New York, 1969; p 91.
RECEIVED for review April 1, 1980. Accepted May 27, 11380. This paper was presented a t the 63rd Canadian Chemical Conference, Ottawa, Ontario, Canada, June 8 11, 1980.
