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
88
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
MAGIC system cannot be determined readily in the extract by flameless atomic absorption due to organic interferences. Copper and zinc have the highest natural abundances of all the elements extracted by the MAGIC system, and they are also the most frequently found in anomalous concentrations. Therefore, they produce the most serious interference problems. Above a threshold of about 500 ppm, using a 1O-pL sample, Cu will produce background absorption during the determination of many elements. Copper will also produce an alloying effect, which changes atomization rates, for most of the elements extracted. Anomalous Zn concentrations produce a serious background absorption problem for only one of the elements extracted, Te. High Zn concentrations produce alloying and mass flushing effects which produce the most serious interferences for In, Sn, and Sb. Many of the elements that are extracted by the MAGIC system are either volatilized partially or totally during the charring cycle of the graphite furnace if the organic extract is not pretreated or stripped. Many attempts at eliminating volatilization by matrix modification in the graphite furnace were unsuccessful. It is probable that the element-halide complexes attached to the Aliquat-336 are lost as methylmetal iodides, which typically have low boiling points (2). The elements which are not lost during the charring cycle are those that are the most refractory such as platinum and palladium. These two metals can be determined directly from MAGIC extracts in the graphite furnace without any pretreatment or stripping. A series of experiments were conducted to determine if pretreatment of the extracts would eliminate some interferences. It was thought that stripping (3)the extracted elements into an aqueous phase would eliminate the organic interferences. Some elements which are easily extracted by methyl isobutyl ketone (MIBK) alone possibly might be retained in the organic phase even though their bonds to Aliquat-336 are disrupted by the stripping procedure. These elements could then be determined by flameless atomic absorption from the organic phase utilizing suitable matrix modifications. It was found that both of these approaches produced desirable results and that some interelement interferences could be eliminated by stepwise pretreatment of the extracts. Dilute nitric acid solutions, which are an acceptable sample medium for graphite furnace determinations, had been used previously for stripping zinc (4,cadmium (5), and uranium (6) from Aliquat-336. It had been noted earlier that the nitrate ion had a poisoning effect on the extraction of some of the elements in the MAGIC system (7). Hydrogen peroxide was included in the stripping solutions in the initial investigation (8),because Hz02 will oxidize iodide to iodate in an acid medium. It was found that with mixtures of dilute HN03 and H2Oz,sequential strippings of elements from the extra& were made possible by increasing the HN03 and HzOz concentration in steps (8). Tests were conducted by using mixtures of acetic acid and hydrogen peroxide, because it had been observed that CH,COOH would produce adverse effects on the extraction of some elements. Stripping with sulfuric acid and hydrogen peroxide was also studied. Dilute H 8 0 4 has no effect on the extraction mechanisms in the MAGIC system. Therefore, the effect observed in this set of tests was the progressive decrease in the activity of iodide as the peroxide concentration and acidity increased. Mixtures of dilute HC1 and H202were also studied, but the results were of little value with regard to stripping of extracts. Although the HC1-HZOz tests did provide some corroborative data concerning extraction mechanisms, the results will not be discussed. EXPERIMENTAL SECTION Apparatus. AU stripping determinations were made by atomic absorption spectrometry using either Perkin-Elmer Model 360
Table I. Stock Stripping Solutions
acid
HNO, HNO, HNO, HNO, HNO, CH,COOH CH,COOH CH,COOH CH,COOH CH,COOH Haso, Haso, Haso,
vol % overconcd all vol % concd [acid], (30%) vol % acid M H,O, H,O, 20 3.1 0 0 20 3.1 6.7 2 13.3 4 20 3.1 20 3.1 33.3 10 10 1.55 66.7 20 20 3.5 6.7 2 20 3.5 13.3 4 20 3.5 33.3 10 20 3.5 66.7 20 10 1.75 66.7 20 20 3.6 0 0 20 3.6 6.7 2 20 3.6 13.3 4
concd acid: overall H,O, 1O:l 5:l 2:l 1:2 1O:l 5:l 2:l 1:l 1:2
1O:l 5:l
or 603 instruments. An air-acetylene flame was used for all determinations except for Sn, for which a nitrous oxide-acetylene flame was used. Operating conditions were those recommended by the instrument manufacturer. A single-slot 10-cm burner was used for the &acetylene flame, and a universal burner was used for the nitrous oxide-acetylene flame. Hollow cathode lamps were used for Cu, Ag, Au, Zn, Ga, In, and Pb determinations. Electrodeless discharge lamps were used for Cd, Hg, T1, Sn, As,Sb, Bi, Se, and Te determinations. Deuterium background correction was used only for the As and Se determinations. Reagents. All chemicals were reagent grade except for hydrogen peroxide, which was electronic (transistor)grade. It cannot be stressed too strongly that the HzOzused for stripping MAGIC extracts must be unstabilized. Stabilized peroxide retards the stripping reactions and alters some distribution curves. Also the stabilizing agent in some brands of H202is a source of Sn contamination. These brands of HzOzshould be avoided entirely because their unstabilized product often contains traces of Sn. Aqueous reagent solutions were prepared by using distilled, deionized water. Alamine-336 and Aliquat-336 were obtained from General Mills Chemical Division, Minneapolis, MN. AU standards, the MAGIC salt solution, and the MAGIC organic extracting solution were prepared as previously described (I). An organic extract stock solution was prepared by mixing 150 mL of concentrated (12N) hydrochloric acid, approximately 40 mL of various standard solutions, 210 mL of water, and 200 mL of MAGIC salt solution (I)in a separatory funnel. Then 250 mL of MAGIC organic extraction solution was added, and the contents were vigorously shaken. The resulting stock organic extract contained 20 pg/mL Ag, Au, Cd, Hg, Ga, In, T1, Pb, Sb, Bi, Se, and Te, 0.8 pg/mL Cu and Zn, and 40 pg/mL Sn. After the mixture was allowed to stand overnight, the aqueous phase was discarded and the organic phase was stored in a glass bottle. Stock stripping solutions were prepared at specific ratios of concentrated acid to total HzOzcontent; Le., the volume of H202 reagent used to make the stock solution was adjusted for the concentration of the product supplied, in this case 30% HzOp The ratios of acid to H202for the various stock solutions were arbitrarily chosen to provide a wide range of stripping conditions. Table I outlines the mixtures in the stock stripping solutions. Stock solutions containing HNO,, CH3COOH, and H2O2and Hfi04, CH,COOH, and HzOzwere prepared in a similar manner. Procedure for Determining Stripping Curves. Dilutions of the stock stripping solutions were made in 16 X 150 m m Coming disposable culture tubes. The total aqueous volume in each of 5 mL, and the acid strengths varied from 0 to 10% concentrated acid by volume in incrementa of 1%. Five milliliters of stock organic extract were added to each tube, which was stoppered with a Kimble no. 5212-5stopper. The racks of tubea were allowed to react at room temperature for 48 h with periodic shaking. Aqueous to organic ratios were always 1:1,since different ratios produce different stripping curves. Some of the stripping reactions occur almost instantly, while most proceed very slowly, especially those involving the oxidation of iodide. The ratio of phase contact area to total liquid volume affects the rate of reaction directly, and, testa performed with only 1 mL of each phase in the same
ANALYTICAL CHEMISTRY, VOL. 53,NO. 1, JANUARY 1981
;-::
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s 6-7
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l
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VOLUME PERCENT CONCENTRATED
HNO3 IN AQUEOUS PHASE
PPt Aqueous Phase
--___ - --_- 100% 90 ~
in Organic Phase lo 100% in Organic Phase 100% in Aqueous Phase - - - - - - - - - 90 to 100% in Aqueous Phase
L
--I
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i Pbdd I--
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....................
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D VOLUME PERCENT CONCENTRATED
Flgure 1. HN0,:H,02
- $i
--.
--i
Ga
0
.I
1
1
2
"03
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3
1
4
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5
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E IN AQUEOUS PHASE
Element distributions when stripping with mixtures of "0,: (A) HNO, only; (e) concentrated HN03:H20p = 1O:l; (C) concentrated = 5:l; (D) concentrated HN03:H,02 = 2:l; (E) concentrated HNO,:H2O2 = 1:2 (see Table I).
diameter tubes stripped somewhat faster than did the testa described above. In a few instances stripping was accelerated by warming a rack of tubes in a 35 "C water bath for 1-2 h and then allowing them to equilibrate overnight at room temperature. All stripping curves were determined by flame atomic absorption spectrometry. In most instances, stripping determinations were made from the organic phase with the stock organic extract as a standard. In those few cases where there was a significant change in the viscosity of the organic phase, stripping curve determinations were made from the aqueous phase, using aqueous standards in a matched matrix. All determinations were performed in as short a time as possible, because the organic to aqueous ratio decreased with each determination, producing a slow shift in some of the stripping curves. RESULTS AND DISCUSSION Initially Pt and P d were included in this study. When stripping tests were performed with the other elements present in the extract, unpredictable stripping curves were produced not only for Pt and Pd but also for Te, Au, Ag, and Se. Metals such as Pb and S n were affected to a lesser extent. These
results were probably due to the formation of complex intermetallic halides and precipitates. Since the geological samples that will contain enough Pt and Pd to produce these effects will be encountered in only extremely rare instances, and since Pt and P d can be determined directly from MAGIC extracts by flameless atomic absorption, both elements were dropped from further stripping study. The stripping systems described render each of the studied elements, except Hg, in a form that is amenable to graphite furnace determinations. Some elements can be determined from the organic phase and some from the aqueous phase. A few can be determined from the organic phase at one point in the system and the aqueous phase a t another point. Many of these elements require matrix modification in the graphite furnace. A discussion of the analytical programs and of matrix modifications is a lengthy topic which will be published separately at a later date. Detailed stripping curves were determined for Cu, Ag, Au, Zn, Cd, Hg, Ga, In, Tl, Sn, Pb, As, Sb, Bi, Se, and T e in a
88
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
:
ASe "I -'
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A VOLUME PERCENT
6
7
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C
CONCENTRATED CH3COOH IN AQUEOUS PHASE
-- - - - - - - - 19 00 0 %to Se
- -- - - - ----
--- - - - - - -
In Organic Phase 100% in Organic Phase 100% in Aqueous Phase 90 to 1 0 0 % in Aqueous Phase
VOLUME PERCENT CONCENTRATED CH3COOH IN AQUEOUS PHASE
Figure 2. Element distributions when stripping with mixtures of CH3COOH: (A) concentrated CH3COOHH202 = 1O:l; (B)concentrated CH3COOH:H202 = 5:l; (C) concentrated CH,COOH:H202 = 2:l; (D) concentrated CH3COOH:H202= 1:l; (E) concentrated CH3COOH:H202 1:2 (see Table I).
HN03-H202 system, a CH3COOH-H202system, and a H2S04-H20z system. It is not possible to present all the curves for each element in each stripping system. Instead, summary graphs are provided. The detailed curves will be available in an appendix in a thesis by Clark (9). In the summary graphs, the distributions of the 16 elements are indicated as being in either the aqueous phase or the organic phase. In all three stripping systems As, Ga, and Zn can be quantitatively removed from the organic phase simply by backwashing (3) the extract with water or very dilute acids. These three elements are the most weakly extracted of all the elements studied in the MAGIC system. Therefore, by backwash of the extract, Zn can be separated from Te and In, two elements for which it interferes. Nitric acid has a bleaching effect on the extracts by oxidizing the iodide to iodate. When H202is mixed with "OB,
the bleaching reaction is stronger and occurs at lower acid strengths. The nitrate ion also tends to replace iodide complexes attached to the amine ion exchange agents, an effect that becomes more pronounced as the activity of iodide decreases. A set of tests were performed with dilute HN03 only (Figure lA), a set with HN03:Hz02= 1 0 1 (Figure lB), a set with HN03:H202 = 5:l (Figure IC),a set with HN03:H202 = 2 1 (Figure lD), and a set with HN03:H202 = 1:2 (Figure 1E). Most of the elements pass into solution in the aqueous phase as they are stripped out of the organic phase. Silver and gold, however, precipitate. Most of the stripping curves are compressed toward the left side of the diagrams as the H202content increases. Tin,antimony, bismuth,and selenium present rather odd stripping curves. Tin will precipitate a t more than one point depending on its valence and the acid strength. The stripping of Se, Te, Bi, and Hg is much more
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
I I
se TI
Ag
4 I
Cd
I
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69
Se
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,
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VOLUME PERCENT
CONCENTRATED H2S04
I
1
1
1
2
3
1
4
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1
5
u
1
6
1
7
,
1
8
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9
/
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100% in Organic Phase -_-__-- 90 to 100% in Organic Phase 100% in Aqueous Phase - -
..... -.
90 to 100% in Aqueous Phase
Flgure 3. Element distributions when stripping with mixtures of H2SO4: (A) H2S04only; (B) concentrated H2S04:H202 = 1O:l: (C) concentrated H2S04:H,02= 5:l (see Table I).
complete if CH3COOH is included in equal volume to HN03 in the stripping solution. Of the various stripping solutions tested, the HN03-HzOz combination has the strongest stripping effect. In the HN03system and in the following systems, Sn4+and Sb3+are more soluble in the organic phase than are Sn2+and Sb6+. Acetic acid mixtures (Figure 2) provide some useful alternatives to HN03 solutions. Since Te is found in most Cu ores, it is important to separate it from Cu to get an interference-free determination. In the presence of acetic acid and HzOz,Te is readily stripped into the aqueous phase, while Cu remains in the organic phase if the proportion of HzOzis small. Tellurium will not be stripped unless a small amount of H2Oz is present. A portion of the Sb in the extract precipitata when any amount of HzO2 is present. At low ratios of CH3COOH:HZOzIn, Pb, Cu, Cd, Bi, and Se are either partially or totally stripped, while Hg, Au, Ag, and T1 are not affected. Tin and gallium are reextracted back into the organic phase as the H2O2content of the aqueous phase increases. Arsenic is partially reextracted at low CH3COOH concentrations. Data are summarized for dilute HzS04in Figure 3A, for HzSO4:HZO2= 1 0 1 in Figure 3B, and for H2SO4:HzOZ= 5:l in Figure 3C. By backwashing the extract with 6% H#04, Zn can be separated from Sb, In, and Te, for which it produces an interference. Stannous iodide is 85 % stripped by dilute HzS04,but the addition of a small amount of H202results in the reextraction of tin as stannic iodide, separating it from interfering Zn. However, Sb partially precipitates when H202 is included in the HzSO4 solution. At 6% HzS04 and with H ~ O 4 : H Z O=z 5:1, Sn, Ag, T1, Au, and Hg remain in the organic extract while the other elements are either partially or totally stripped. This is a particularly useful separation since Zn and Cu interfere with Sn, Ag, T1,and Au. At lower acid strengths and high proportions of H202,Ga is partially reextracted. If CH,COOH is included in these stripping solutions, the changes in the various stripping curves are minor. These stripping systems can be combined in sequential steps any number of ways to provide the analyst with the desired separations. For example, the following two-stage
Dilute Portion of
Flameless A P
Extract
PI Pd
1 Flame A A Cu
Zn
Pb A q
Cd
Flameless A A As Ga . Aqueous Phase A q Sf1 Tl.Orqiiriic Phase
/
J HNO3 Strip
Flameless A A Au Organic Phase
c HN03.ti202 S l r ~ p
4
Fldrneless A A Akr O r g m c Phase Cd
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61
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Example flow chart fa processing MAGIC extracts for trace element determinations. stripping procedure (Figure 4) can be used to eliminate most of the common interelement interferences produced by geologic materials on various flameless atomic absorption determinations. First, backwash l mL of extract with l mL of 6 vol % H 3 0 4 in a 16 X 100 mm culture tube. Set aside 0.5 mL of the organic phase and discard 0.5 mL of the aqueous phase. Add 20 pL of unstabilized Hz02(30%), stopper, shake, and let stand overnight. Silver, tin, and thallium are apparently no longer bound to Aliquat-336 and can be determined from the organic phase by using chromic acid or ascorbic acid as matrix modifiers while As and Ga can be determined from the aqueous phase. After all the other desired determinations have been performed from this strip, 75 p L of HN03 can be added. When the extract has turned pale Figure 4.
70
Anal. Chem. 1981, 53, 70-74
yellow, Au can be determined from the organic phase. Add 0.5 mL of 5 vol % concentrated “03-5 vol % concentrated CH3COOH in concentrated Hz02to the 0.5 mL of organic phase set aside above. Stopper the test tube and let stand overnight. Gold can be determined from the organic phase, and Cd, In, Sb, Bi, Se, and Te can be determined from the aqueous phase. Gold is a little more sensitive in this strip and has been separated from all potential interferences, but the determination must be made within 48 h before the higher Au concentrations begin to precipitate. High Cu samples may require further treatment. Copper, lead, and zinc and some cadmium and silver determinations are more easily made by diluting a portion of the original extract with methyl isobutyl ketone to give an overall dilution factor of 101. These determinations can then be made by flame atomic absorption. Just as a periodic trend was observed while developing the MAGIC extraction system, a similar trend was observed in the various stripping tests. Generally, elements to the upper right of the periodic chart are stripped much more easily than those to the lower left. Selenium is the most significant exception to this observation in that it is much more difficult to strip from MAGIC extracts than is tellurium. On the basis of the observations made during the course of this study, it appears that the periodic trend is centered on germanium, which was not included in this study. Assuming that Sn and Sb are kept in their lower valence states, the farther an element is below Ge or to the right or left of Ge on the periodic chart, the more strongly it is held by the MAGIC extract, within the limits studied. The distribution diagrams presented here can be used to devise a sequential stripping scheme that will produce the trace element separations desired by the analyst. Because of the high degree of separation of trace elements with these methods, deuterium background correction is rarely needed in making trace element determinations from geological
samples by flameless atomic absorption spectroscopy. Detailed instrumental programs and matrix modification procedures for the graphite furnace will be published later. Applications of the organic extraction and stripping systems are currently being investigated for use with induction coupled plasma emission spectroscopy for the separation of these trace elements from the variable interferences encountered in geologic materials.
ACKNOWLEDGMENT The authors are grateful for the critical reviews provided by Harold Bloom, L. Graham Closs, and Samuel S.Goldich of the Geology Department of the Colorado School of Mines and by T. T. Chao and Harry M. Nakagawa of the US.Geological Survey. LITERATURE CITED (1) Clark, J. R.; Viets, J. G. Anal. Chem. preceding paper In this Issue. (2) ”Handbook of Chemistry and physics”, 59th ed.;the Chemical Rubber Co: Cleveland, OH, 1978. (3) Morrlson, G. H.; Frelser, H. “Solvent Extractlon In Analytlcal Chemistry”; Wiley: New York, 1968; Chapter 9. (4) McDonald, C. W.; Rhodes, T. Anal. Chem. 1974, 46, 300. (5) McDonaH, C. W.; W e , F. L. Anal. Chem. 1973, 45, 983. (8) Barbano, P. Q.; Rigall. L. Anal. Chlm. Acta 1978, 96, 199. (7) Viets. J. G. Anal. Chem. 1978, 50, 1097. (8) Viets, J. G.; clerk,J. R. “Abstracts of Papers” 176th Natbnal Meetlng of the Amerlcan Chemical Society, Miami Beach, FL, Sept 1978; American Chemical Society: Washlngton, DC, 1978; HIST 39. (9) Clark. J. R. ph.D. Thesls, Colorado School of Mlnes, Odden, CO, in preparatlon.
RECEIVED for review July 1, 1980. Accepted September 29,1980. This study was conducted in the exploration geochemistry laboratories of the Geology Department of the Colorado School of Mines and in the laboratories of the US. Geological Survey. Use of brand or manufacturer’s names in this paper is for descriptive purposes only and does not constitute endorsement by the U S . Geological Survey.
Two-Factor Minimum Alpha Plots for the Liquid Chromatographic Separation of 2,6=Disubstituted Anilines Bohdan Sachok, John J. Stranahan, and Stanley
N. Demlng’
Department of Chemistry, University of Houston, Houston, Texas 77004
The combined effects of two mobile phase factors, percent methanol and concentration of octanesulfonate, on the ret e n t h thws of five 2,tkliwbstltuted anllbres were determined by fitting a mechanlstlc model to experlmental data obtalned at nlne factor comblnatlons. A multifactor optimlzatlon strategy based on minlmum alpha plots (MAPS) was used to predict the factor combination giving optimal separation5 5 % methanol and 1.5 mM octanesulfonate. The anlllnes could be determined at a level of 2 pmol by use of electrochemical detection.
Reversed-phase high-performance liquid chromatography enjoys widespread success primarily because of the large number of variable factors that can be adjusted in the polar mobile phase to give improved chromatographic performance
(1). Today, the individual effects of many of the factors can be predicted with a relatively high degree of accuracy. Less well understood, however, are the combined effects of the mobile phase factors. Chemical and physical interactions cause the individual effect of one factor to be different at different levels of a second factor (2);thus, observations of chromatographic behavior at one factor combination often give false predictions at other factor combinations. For realization of the full power of the many factors associated with the reversed-phase eluent, multifactor experimental designs, multifactor mathematical models, and multifactor optimization strategies need to be employed. Optimization of chromatographic separation is especially difficult. Because the order of peak elution can be different with different chromatographic conditions (3-5),several sete of locally optimal experimental conditions usually exist. Locating the global (or overall) optimal set of chromatographic
0003-2700/81/0353-0070$01.00/00 1980 Amerlcan Chemical Society
