Anal. Chem. 1081, 53, 61-65
81
Multielement Extraction System for the Determination of 18 Trace Elements in Geochemical Samples J. Robert Clark' Department of GsoW,Cobrado School of Mlnes, Golden, colored0 80401
John G. Wets U.S. Oeobgkal Survey, Golden, Cobrado 80401
A Methyl tsokrtyl ketone-Amhe synerGietlc I o d d e Complex (MAGIC) extractlon system has been devekped for use In gwchdcal expkratkn which separates a maxbnun lymber OfbaceelementsfranMerferhgmatrfces. Extrectknanes for 18 of these bace elements are presented: Pd, Pt, Cu, Ag, AU, m, M,~ a1%, n,~ nm, , AS, 81, h,and le. he acld nonnaltty of the aqueous phase controls the extraclkn Into the organk phase, and each of these 18 elements has a koad range of HCI normaWty over whlch ll te quantltattvdy extracted, maklng H possJMe to detennlne all 18 trace de merits from a slngle sample dlgestlon or leach solutlon. The extract can be analyzed dlrecUy by flame atomic absorption or Inductlvdy coupled plasma emission spectroscopy. Most of these 18 elements can be detemJned byfiamekm atomic absorplkm after speclal treatment of the wgank extract.
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Organic extractions have played a very important role in the development of geochemical analysis. Although many of the colorimetric extraction methods which were widely used in exploration geochemistry (1)have been replaced by atomic spectrometry, organic separations are still neceasary to isolate and concentrate specific elements from the interfering matrices of geological materials (2-5). Many of the rock forming elements when present at crustal levels have been observed to produce both positive and negative errom in trace determinations made by flame atomic absorption (Figure 1). The use of deuterium background correction may result in serious negative errors (Figure 1). Flame conditions (Figure 1) and burner alignment can influence the magnitude of these errors. Although the very high temperature of inductively coupled plasma has virtually eliminated the chemical matrix interferences of earlier emission sources, sample matrix still may produce both direct spectral and stray light interferences. Flameless atomic a b sorption is much more susceptible to background absorption from small amounts of matrix salts than is flame atomic a b sorption. Sample matrix can also produce errors due to alloying and to isomorphous substitution of a trace element in a matrix component. Matrix-related errors in these three techniques of sample analysis can often be eliminated by using the method of standard additions. However, this is an undesirably slow procedure for use in an exploration laboratory. Organic extraction methods provide a viable alternative, because the desired trace elements in the sample digestion solution are extracted into a phase that is identical with the standard extracts and which is free from the interferences of the geological sample. Exploration geochemistry is experiencing rapid growth in the utilization of multielement analyses followed by sophisticated statistical treatment of the data. The development of automated spectrometry is helping to solve the logistical
problem facing the analyst. However, there is an obvious need for more multielement extraction procedures that would simplify methods of sample preparation and minimize matrix interferences. Recent work by Viets (61, Hannaker and Hughes (7), Nakagawa (B), and others has been directed a t solving this problem. A rapid organic extraction technique, which has been devised specifically to extract a maximum number of trace elements useful in exploration geochemistry, is proposed in this study. Halide, mostly iodide, complexes of Pd, Pt, Cu, Ag, Au, Zn, Cd,Hg, Ga, In, T1, Sn, Pb, As,Sb, Bi, Se, and T e are extracted into an organic phase that contains Alamine 336, Aliquat 336, and methyl isobutyl ketone (MIBK). This extraction procedure is compatible with several methods of sample leaching and digestion and could easily be adapted for use in other areas of trace analysis. The applications of this extraction to various sample preparation methods will be described in later papers. Alamine 336 (tricaprylyl tertiary amine) and Aliquat 336 (tricaprylyl methyl ammonium chloride, a quaternary amine salt) have been UBed in a variety of metal extraction schemes. Seeley and C r o w (9) performed a study of the extraction of 63 elements using Alamine 336 and Aliquat 336 from solution of various LiCl-HCl concentrations. Analytical extraction procedures using Aliquat 336 or Alamine 336 have been reported for gold (IO), tungsten and molybdenum (11, 12), cadmium and zinc (13,14) and uranium and thorium (15,161. The extraction scheme described in this paper is a modification of two previous multielement extraction systems developed by Vieta (6) and by Viets and Clark (17). Extraction c w e s for this second system (I 7) will be found in an appendix in ref 18 by Clark. EXPERIMENTAL SECTION Apparatus. Trace-element determinations were made with a Perkin-Elmer 360 and a Perkin-Elmer 603 atomic absorption spectrophotometer. An air-acetylene flame was used for all elements except Sn, for which a nitrous oxide-acetylene flame was used. A eingle-slot 10-cm burner was used for the airacetylene flame and a universal burner head was used for the nitrous oxide-acetylene flame. Hollow cathode lamps were used for Pd, Pt, Cu, Ag, Au, Zn, Ga, In, and Pb determinations. Perkin-Elmer electrodeless discharge lamp were used for Cd,Hg, T1, Sn, As, Sb, Bi, Se, and Te determinations. The Sb and Bi lamps were found to be identical, emitting spectral linea for both elements. Although labeled as single-element lamps, they were used interchangeably for determinations of both Sb and Bi. For certain high salt aqueous solutions and for As and Se determinations, deuterium background correction was used. It was found that MAGIC extracts will slowly dissolve etainlw steel nebulizers. Therefore, a Teflon nebulizer should be used whenever these extracts are being analyzed by flame atomic absorption. Platinum-rhodium nebulizers were not tested for resistance to dissolution by MAGIC extracts. Reagents. All chemicals used were reagent grade except for hexane. Practical grade hexane was found to be sufficiently free of contamination for use in these tests. All chemicals should be
Ms ertlcle not subkt to U.S. Copyright. PuMished 1980 by Me Amer)can Chemical Sockty
62
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
Not Corrected 7
3h.
0
5
10 Fe IN PERCENT
15
20
Flgure 1. Apparent Cu,
pb, and Ag concentrations determined by flame atomic absorption in the presence of Fe.
checked for purity because one brand of reagent grade potassium iodide was found to be contaminated with heavy metals. Aqueous reagent solutions were prepared by using distilled, deionized water. Alamine 336 and Aliquat 336 were obtained from General MiUs Chemical Division, Minneapolis, MN. All reagent solutions, standards, and digestion procedures used with this extraction must be kept free of nitrates and acetates, both of which will produce drastic degradation of the extraction coefficients for most of the extracted elements. Aqueous standards were made up in hydrochloric acid or hydrobromic acid solutions. Combined lo00 pg/mL. aqueous standards were prepared for As, Sb, Bi, Se, Te, and Au, for Pb and Ag, for Ga and In, for Pd and Pt, and for Cd, Hg, and T1. Individual lo00 gg/mL aqueous standards were made for Cu, Sn, and Zn. Organic loo0 pg/mL standards were prepared for Sn, Pb, Ga, In, and Hg by dissolving their chloride salts in the MAGIC organic solution. Organic As, Sb, Se, and Te standards were made by dissolving their oxides in a minimum amount of ethanol or concentrated hydrochloric acid and then diluting to volume with MAGIC organic extracting solution. MAGIC salt solution was prepared by adding 400 g of L-ascorbic acid, 100 g of potassium bromide, 100 g of potassium chloride, and 400 g of potassium iodide to a dark glass reagent bottle. One liter of distilled, deionized water was added, and the bottle was shaken periodically until all the contents of the bottle were dissolved. The resulting solution had a final volume that was just slightly less than 1.5L. This solution will keep for several weeks in a cool dark place, but it should not be refrigerated. MAGIC organic extracting solution was prepared by adding 50 mL of Alamine 336, 100 mL of Aliquat 336, and 100 mL of hexane to a 1-L volumetric flask. Next, 500 mL of MIBK was added, the contents were swirled, and the solution was diluted to volume with MIBK and thoroughly mixed. The solution was stored in a water-free brown glass reagent bottle. Procedure. Extractions were prepared at selected intervals of aqueous phase HC1 normality. A specific volume of aqueous multielement standard was added to 16 X 150 mm Corning disposable culture tubes such that the fiial concentrations of the desired trace elements were 20, 8, or 2 ppm in the extract, depending upon the relative sensitivities of the elements. A total
of 8 mL of concentrated 12 N HCl and water were then added to each test tube, the HC1 and water being in specific proportions tr, provide a range of 0-6 acid normality in the final solution. Then 4 mL of MAGIC salt solution was added, giving a total of 12 mL of aqueous phase. The acid normality interval between consecutive extractions varied from 0.1 to 1.0, depending upon how critical that portion of the extraction curve was judged to be. Slight adjustments in the acid normality were calculated for the volume and acid strength of the standard solutions added to each tube. A 5-mL portion of MAGIC organic extracting solution was added to each culture tube, and the tubes were stoppered with No. 5215-5 Kimble stoppers, which had been presized to fit the tubes. The contents were shaken mechanically for 1min, after which each tube was centrifuged and allowed to stand overnight. All extraction curves were determined by atomic absorption spectrometry. Extraction percentages were determined by comparison of the extract to an organic standard, by analysis of the aqueous phase, and by the relative absorbance of the extracts. Because the viscosity of the extracts varied slightly with the acid normality of the aqueous phase, organic standards were prepared accordingly, MAGIC organic solution was first shaken as a blank over a matched aqueous solution, and then a portion of that extract was separated and mixed with an appropriate amount of concentrated organic standard. The aqueous phases tended to clog the nebulizer with precipitated salt, especially at high acid normalities. Therefore, extracted aqueous phases and standards mixed to match the salt content of those aqueous phases both were diluted prior to analysis. RESULTS AND DISCUSSION Extensive tests were performed during the course of this study to determine the optimum system for achieving the largest number of trace element extractions over the broadest range of conditions. Tests were also performed to determine which halide ion forms the predominant extracted complex for each element. The final compositions of both the organic phase and the aqueous phase reflect the results of these tests. Alamine 336 and Aliquat 336 were chosen for this study due to their ability to extract a large number of trace elements over a wide range of acidities. Critical pH control, common to many organic extractants, is thus avoided. All 18 elements covered in this study are extracted significantly by both of these reagents in the presence of at least one of the halide ions included in the aqueous phase. MIBK was chosen as the carrier for the organic phase because of its proven affinity for forming oxonium ion association pairs with a large number of halide complexes. When extractions were run with MIBK only, all the trace elements, except Zn,showed at least partial extractions from a HCl-KI media. Zinc extracts partially when HBr is substituted for HC1. Therefore, when Alamine 336, Aliquat 336, and MIBK are combined in the organic phase, all the elements studied, except zinc, are extracted by at least two discrete mechanisms. Both amine complexing agents and the MIBK are competing for the same halide complexes. This produces a synergistic action in which the combined effect of the three extractants is much greater than their effects taken singly. Extraction curves for the period 4 elements studied, Cu, Zn, Ga, As, and Se, are shown in Figure 2. Each data point below 100% extraction is shown by a dot. Tests conducted in this study and previous studies (6,17,18) indicate that Cu, Zn, As, and Se are extracted mainly as iodide complexes. Gallium is extracted as a chloride complex, extracting completely only when the C1 content of the solution reaches the necessary level. Large quantities of iodide in the system actually suppress the extraction of Ga slightly. Selenium and As both form an elemental precipitate at low acid normalities. This precipitate reaches a maximum at about 1N HCl. Zinc is not completely extracted beyond about 1.7 N HC1, but because of the constant low slope of that portion of the extraction curve the percentage of Zn extracted was found to be very predictable. An error of 1N in HCl concentration
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981 100,
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HYDROCHLORIC ACID NORMALITY
Figure 2. The extraction of Cu, Zn, Ga,As, and Se as a function of HCi concentration.
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Figure 4. The extraction of Pt, Au, Ag, TI, Pb, and Bi as a functbn of HCI concentration.
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1
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F w e 5. The effect of hexane in the organic phase on the extraction
Figure 3. The extraction of Pd, Ag, Cd, In, Sn, Sb, and Te as a function of HCI concentration.
of Sb, Bi, and Zn.
will produce an error of less than 10% in the Zn concentration of the extract. This upper end of the extraction curve is presently being used regularly for Zn analyses with no discernible loss in precision. Figure 3 is the set of extraction curves for the period 5 elements studied, Pd, Ag, Cd, In,Sn, Sb, and Te. Pd, In, and Sb are extracted completely at all HCl normalities tested. The extraction of Te is complete except when relatively large amounts of As or Se are present. The deviation away from 100% extraction is around 1 N HC1, the maximum As-Se precipitate point, and is due to occlusion of Te in the precipitate. Tin extracts well at acidities above 1N. The depression of the Ag curve where KCl precipitates is probably due to the substitution of a small portion of the Ag present for K in the precipitate. Pd, Ag, Cd, In, Sn, Sb, and Te were found to be extracted mainly as iodide complexes. Te and P d will also be extracted as chloride or bromide complexes (3,9),and Sn is extracted a little more readily when HBr is used instead of HCl in the system (18). However, Sb is not extracted well in the presence of an excessive amount of bromide ion (18). Extraction curves for the period 6 elements studied, Pt,Au, Hg, T1, Pb, and Bi, are shown in Figure 4. Pt, Hg, and Bi are extracted completely throughout the normality range of the test. Gold mirrors the extraction curve of Te (Figure 3) in that there is a slight drop in the Au content of the extract over the range of the heaviest As and Se precipitation. This is also due to occlusion of Au in the precipitate. Pt, Au, Hg, T1, Pb, and Bi were found to be extracted mainly as iodide complexes, although Pt, Au, Hg, Bi, and T1 will also extract as bromide and chloride complexes (3,9,19). Pb and T1 both fall away from a 100% extraction beyond the acid normality
at which KCl precipitates, as does Ag, with Pb having the most drastic drop in extraction coefficient. This also is apparently due to the substitution of Pb and Tl for K in the precipitate. Hexane was added to the organic phase, because it was found to improve the physical properties of the extract at higher acid normalities and thus improve the extraction coefficients of a number of metals. Figure 5 shows the improvement for Sb, Bi, and Zn. The organic and aqueous phases become slightly more miscible as HC1 normality increases. This effect is much more pronounced with HBr. As the miscibility of the two phases increases, the partitioning of elements between the phases begins to decrease. It was also noted that K and Ca (17,18)in the aqueous phase begin to migrate into the organic phase as HC1 normality increases and the two phases become more miscible. This problem also becomes drastically worse when HBr is substituted for HCl. The presence of 10% hexane in the organic phase satisfactorily eliminates this problem (17, 18). Tests were run to determine the effect of time on As and Se extraction curves. It was observed that the precipitates, which were heavy initially, were reduced in volume after 24 h. The organic phase had absorbed this portion of the precipitates during the 24 h, significantlyimproving the extraction curves, as is shown in Figure 6 for Se. During trials, lightcolored precipitate would sometimes form when lo00 pg each of As, Sb, Bi, Se, and Te were combined in one of the extraction tests. This precipitate would also be absorbed by the organic phase after standing overnight. Alamine 336, Aliquat 336, and MIBK will not extract interfering matrix elements such as Na, K, Mg, Ca, Al, or Si. The extracts were tested for K, Ca, and Mg after gross amounts of these salts were added to the aqueous phase. No
64
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
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HYDROCHLORIC ACID NORMALITY
Figure 6. The extraction of Se into the organic phase as a function of time and HCI concentration. 100-
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HYDROCHLORIC ACID NORMALITY
Flgure 7. The extraction of Fe and Mn as a function of HCI concentration. more than 0.3 wg/mL of any of these elements was found in the extract at acid normalities below 5 N HCl. Although Fe and Mn will be extracted by this organic system as halides under oxidizing conditions, they will not extract significantly in the presence of both K1 and ascorbic acid as shown in Figure 7. Individually, each of these reagents will reduce the iron to the ferrous state and greatly suppress its extraction. But, acting together, they form a powerful reduction couple and essentially eliminate the extraction of iron. It was determined that the amount of ascorbic acid should be at least 10 times larger than the amount of Fe in the aqueous phase. As is indicated in Figure 7, if there are any residual oxidants in the aqueous phase at the time of extraction, such as a trace of NO,, Fe will extract significantly. Because Fe may reduce slowly in some samples, sample solutions should stand for 30 min after addition of the salt solution and prior to extraction to allow time for complete reduction. Figure 8 illustrates the optimum HC1 normality extraction ranges for all 18 elements. Except for Zn, they can all be extracted quantitatively between 2.25 and 4.2 N HC1. As has already been stated, Zn can be predictably extracted in this range. It is evidient that Sn, As, and Ga can be separakl from the rest of these elements by sequential extraction. This would be an advantage when analyzing massive ores of Cu, Zn, Pb, or Ag for As, Ga, or Sn, where the ore metals could themselves be a matrix problem. A periodic trend for the extraction of these elements was observed during the course of this study. The elements to the upper right on the atomic chart extract leas efficiently than those to the lower left. An earlier system developed during the course of this study, which did not extract over as broad
a range as the present system, and which did not precipitate KCl below 5 N HC1, exhibited this periodic trend more clearly (17). In stripping the metals out of the extracts for flameless atomic absorption analysis (discussed in the following paper in this issue) this same trend is apparent. Preliminary inveatigation has shown that there are solubility limits for each metal-iodide complex in the MAGIC extracts. For example, the solubility limit for Cu in the organic phase lies somewhere between 0.3 and 1.2%. Excess Cu forms CUI precipitate, a white material which clings to the boundary of the organic phase. Copper, silver, or lead iodide precipitates can usually be driven into the organic phase by adding an extra gram or two of KI or extra MAGIC organic phase to the sample tube and shaking the sample again. A more viable remedy is to redigest the sample, reducing the amount of sample material used. The solubility limit for Se in the organic phase, over 3 N HC1, is about 80 rcg/mL. As and Te are soluble at least up to 3000 pg/mL. Ag, Au, and Bi are soluble in the organic phase above the 2000 pg/mL concentration level. One sample extract was determined to contain 4% Sb. Solubility limits for the remainder of the 18 elements have not been investigated. Poisoning effects of a large amount of one extracted element on the extraction of other elements have not yet been observed. Additional work is needed in these areas. Pt and Pd are the only two extracted elements that can be analyzed directly out of the organic phase by flameless atomic absorption. There is a varying tendency for the remaining 16 elements to be volatilized from this organic matrix during the charring stage of the analytical cycle of the graphite furnace. Some are partly volatilized while others are lost completely. Therefore, it is necessary to strip the metals out of the extracts prior to flameless analysis. Procedures for stripping MAGIC extracts are presented in the following paper in this issue. The MAGIC extraction system is compatible with a variety of sample digestion techniques used in geochemical analysis. However, it cannot be used with sample preparation procedures that use nitric acid, nitrate salts, acetic acid, or acetate salts unless the offending ions are destroyed or removed f i t . Fluoride ions left over from H F digestions will interfere with the extraction of some elements. As much fluoride as possible
Anal. Chem. 1081, 53, 65-70
should be fumed off during the digestion procedure. Residual fluoride ions can then be complexed with aluminum ions (6).
ACKNOWLEDGMENT The authors are grateful for the critical reviews provided by Harold Bloom and Samuel S. Goldich of the Geology Department of the Colorado School of Mines and by T. T. Chao, John R. Watterson, and Harry M. Nakagawa of the US. Geological Survey. LITERATURE CITED Ward, F. N.; Lakin, H. W.; Canney, F. C. et al. Geol. Surv. Bull. U . S . 1063, No. 7752. Motooka, J. M.; Mosier, E. L.; Sutley, S. J.; Viets, J. 0.Appl. Spectrosc. 1070, 33, 456. Watterson, J. R.; Neuerbvg, G. J. J . Res. U.S. Geol. Surv. 1075, 3 , 191. Rubeska, I.; Koreckova, J.; Welss, D. At. Absorpt. News/. 1077, 76, 1. Chao, T. T.; Sanzobne, R. F.; Hubert, A. E. Anal. CMm. Acta 1078, 96, 251. Viets, J. G. Anal. Chem. 1078, 50, 1097. Hennaker, P.; Hughes. T. C. Anal. Chem. 1077, 46, 1485. Nakagawa, H. M. Geol. Surv. Bull. (U.S.) 1075, No. 7408, 85. Seeb, F. G.; Crowe, D. J. J . Chem. Data 1066, 7 7 , 424. Groenwald, T. Anal. Chem. 1068, 40, 853.
85
Rao, P. D. At. Absorpt. Newsl. 1070, 9 , 131. Rao, P. D. At. Absorpt. Newsl. 1071, 70, 118. McDonald, C. W.; Moore, F. L. Anal. Chem. 1073, 45, 963, McDoneld, C. W.; Rhodes, T. Anal. Ctwm. 1074, 46, 300. Rigall, L. Anal. CMm. Acta 1078, 96, 199. Berbano, P. 0.; Cospito, M.; Rigali, L. Anal. Cblm. Acta 1078, 96, 199. Viets, J. G.:Clark, J. R. "Abstracts of Papers", 176th Natlonal Meeting, of the Arrtarkan Chemical Society, Mlami Beach, FL, Sept 1978; American Chemical Society: Washington Dc, 1978; HIST 39. (18) Clark, J. R. h . D . Thesls, cdorado School of Mines, Golden. CO,in preparation. (19) Morrison. G. H.; Freiser, H. "Solvent Extraction in Anaiytlcal Chemlstry"; Wlley: New York, 1966; Chapter 11. (20) Clark, J. R.; Viets, J. G. Anal. Chem., folkwing paper in thls b. (11) (12) (13) (14) (15) (16) (17)
RECEIVED for review July 1,1980. Accepted September 29, 1980. This extraction system was developed as an outgrowth of a Ph.D. thesis study by J. Robert Clark and as an extension of previous research performed by John G. Viets. Analytical facilities of the exploration geochemistry laboratories of the Geology Department at the Colorado School of Mines and the analytical facilities of the US.Geological Survey, Denver, CO, were used for the development of this system. Mention of manufacturer's names does not imply endorsement of their reagents or equipment by the US.Geological Survey.
Back-Extraction of Trace Elements from Organometallic-Halide Extracts for Determination by Flameless Atomic Absorption Spectrometry J. Robert Clark' Department of &ology, Colorado School of Mines, &Men, Colorado 8040 1
John G. Viets U.S. Geological Survey, &Men, Colorado 8040 1
The Methyl Isobutyl ketone-Amine synerGlstk Iodkle C m piex (MAGIC) extraction system offers the advantage that a large number of trace elements can be rapldly determined wlth a single sample preparation procedure. However, many of the elements extracted by the MAGIC system form volatlle organometallic haYde salts when h a organk extract Is heated In the graphlte furnace. Hlgh concentratlons of some -elements such as Cu and Zn extracted by the system from anomalous geological samples procluce serbus Interferences when certaln other elements are determined hy flameless atomlc absorpllon. Strlpplng systems have been developed uslng solutions of HN03, H2S0,, and CHSCOOHIndividually or combined wHh H202In order to clrcumvent these problems. WHh these systems most of the elements in the organic extracl can be sequentlaily stripped Into an aqueous phase. Organometanlc volatlnzatbn and the most serlous Interelement Interferences, therefore, can be elhnlnated by strlpplng with varlous combinations of reagents in a serles of steps.
Many trace elements found in geological samples cannot be accurately determined by flameless atomic absorption in the presence of typical geological matrices. Rock components such as iron and calcium can produce background absorption of a magnitude that typical continuum source correction
systems either grossly over or under compensate. Sulfur and phosphorus are minor constituents of most rock samples, and Se and As readily substitute isomorphously in many S and P containing minerals, respectively. During the drying and charring of acid rock digestion solutions in the graphite furnace, Se will be lost with the fumesof H$Ol and As with the fumes of H3P04 Therefore, Se and As cannot be determined in the presence of the element with which each is frequently associated in geological samples. Gold and silver are subject to alloying effects with metals such as Fe, Cu, and Zn, and the accuracy of flameless determinations of Au and Ag is enhanced if they are separated from as many other matrix components as possible. Most of the serious matrix problems encountered in analyzing geological samples by flameless atomic absorption can be eliminated by employing various extraction methods to separate the desired metal from the digestion solution. The MAGIC extraction system described previously ( I ) offers the additional advantage that a large number of trace elements can be determined from a single sample preparation procedure that does not require tedious pH adjustments. However, the MAGIC extraction does not eliminate some alloying effects or background interferences, because in many anomalous samples some of the extracted elements will be present in sufficiently high concentrations to produce analytical problems. Furthermore, many of the elements extracted by the
This article not subject to US. Copyright. Published 1980 by the American Chemical Society
