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Anal. Chem. 1985, 57, 2428-2429
Extraction of Molybdenum with a Multielement Liquid Organic Extraction System J. Robert Clark*' Geology Department, Colorado School of Mines, Golden, Colorado 80401
John G. Wets US. Geological Survey, Denver, Colorado 80225 Clark and Viets developed a liquid organic extraction system for concentrating and separating a large number of trace elements from analytically interfering matrices ( I ) . The Methyl isobutyl ketone-Amine synerGistic Iodide Complex (MAGIC) extraction system has been shown t o extract Cu, Ag, Au, Zn, Cd, Hg, Ga, In, TI, Sn, Pb, As Sb, Bi, Se, Te, Pt, and Pd (I). Aliquat-336, Alamine-336, methyl isobutyl ketone, and heptane, combined in an organic phase, are used to extract these metals from an aqueous phase that contains potassium iodide, ascorbic acid, potassium bromide, potassium chloride, and hydrochloric acid. Seeley and Crouse (2)demonstrated that both Alamine-336 and Aliquat-336 have an affinity for Moe+ when it is present as a chloride complex. Rao (3) used 3% Aliquat-336 in methyl isobutyl ketone (MIBK) t o extract the phosphomolybdate complex. Because of the reducing conditions produced by the ascorbic acid and K I in the aqueous phase of the MAGIC system, any Mo in that phase is probably in the +4 valence state and likely occurs as iodide complexes. Molybdenum has been shown to extract over a narrow acid normality range under these conditions into a mixture of Aliquat-336 and MIBK (4). T h e extraction described here provides a more predictable separation over a significantly wider acidity range.
EXPERIMENTAL SECTION Apparatus. Molybdenum determinations were made with a Perkin-Elmer Model 603 atomic absorption spectrophotometer, using a nitrous oxide-acetylene flame and a nitrous oxide burner head. A Teflon nebulizer was used, because MAGIC organic extracts dissolve both stainless steel and platinum-rhodium nebulizers. The standard Perkin-Elmer spoiler was used in the burner chamber, because it offers substantially improved sensitivity with these organic extracts over any other burner system (including impact beads) that the authors have tested. A 1pg/mL Mo organic standard produced an absorbance of 0.035. Flame conditions were those indicated in the manual supplied by the instrument manufacturer, except that the nebulizer was leaned to 75% of maximum absorbance (using an organic Cu standard, Cu lamp, and air-acetylene flame) in order to achieve maximum absorbance per volume of sample aspirated. Reagents. All chemicals used were reagent grade. Aqueous solutions were prepared with distilled, deionized water. Aliquat-336 (tricaprylmethylammonium chloride) and Alamine-336 (tricapryl tertiary amine) were obtained from Henkel Chemical Corp., Minneapolis, MN. A stock aqueous salt solution was prepared by dissolving 400 g of KI, 400 g of ascorbic acid, 100 g of KC1, and 100 g of KBr in 1 L of H 2 0 (final volume was about 1.5 L). The dissolution process is endothermic and requires about 2 h of mixing (the authors placed the reagent bottle in a roller mill until dissolution was complete). A stock organic extracting solution was prepared by mixing 100 mL of Aliquat-336,50 mL of Alamine-336,100mL of heptane, and 300 mL of MIBK in a 1-L volumetric flask and diluting to volume with MIBK. Both the stock salt solution and the stock organic extracting solution were stored in clean, brown glass, reagent bottles. 'Present address: U.S. Geological Survey, M.S. 973, Denver, CO
80225.
A 1000 pg/mL molybdenum aqueous standard was prepared by dissolving 1.8402 g of ammonium heptamolybdate tetrahydrate [(NH4)6Mq0N.4H20],in 100 mL of HzO in a 1-L volumetric flask. Fifty milliliters of concentrated H3P04and 30 mL of H20z(30%) were added, and the solution was diluted to volume. A blank organic extract was prepared for use in mixing organic standards and for zeroing the instrument. Five hundred milliliters of organic extracting solution was shaken in a separatory funnel with an aqueous phase containing 200 mL of H20,200 mL of stock salt solution, and 200 mL of concentrated HCl (final acid normality was 4 N). The aqueous phase was drained off and discarded. This blank organic extract preparation was similar to the procedure used for the test extracts, described below, ensuring approximately the same viscosity and burning characteristics. A Mo standard was also prepared in an organic matrix. Approximately 50 mL of the blank organic phase was used tQ dissolve 0.285 g of molybdenum pentachloride. The solution was diluted to 200 mL with more of the blank organic phase, yielding a 500 kg/mL stock organic standard. A 5 pg/mL Mo organic standard was prepared by diluting 1mL of the 500 pg/mL standard to 100 mL with the blank organic extract. The remaining 200 mL of blank organic extract was set aside for use as a blank during atomic absorption tests. Positive displacement pipets were used to measure the small volumes of organic fluids for standard dilutions. Automatic pipets, which have a column of air between the piston and the liquid, overmeasure most organic liquids. Procedure. Extraction tests were conducted in 18 X 150 mm Corning brand disposable screw-cap culture tubes with Teflonlined caps. A 12-mL aqeuous phase was mixed in each tube. First, a total of 8 mL of concentrated HC1 and deionized water were mixed in proportions to give the desired final acid concentration (based on 12 mL volume) in each of the tubes. Tubes were prepared for final HC1 normalities of 0, 1,2,2.6,2.8, 3.0, 3.2, 3.4, 3.6, 3.8, 4.0, 5, and 6. Four milliliters of the stock salt solution and 25 p L of the 1000 pg/mL aqueous Mo standard were added to each tube, and the contents were mixed. The tubes were allowed to stand for 30 min, after which 5 mL of organic extracting phase was added. The tubes were capped, shaken vigorously for 1min, and centrifuged. The aliphatic solvent in the organic phase reduces the loss of that phase, due to solubility in the aqueous phase, to less than 5%. Therefore, a complete extraction would have given essentially 5 pg/mL Mo in the organic phase. Molybdenum determinations were made from the organic phases by flame atomic absorption. The absorbances from the test extracts were compared to that of the 5 pg/mL organic standard described above to determine the efficiency of extraction.
RESULTS Under the conditions of these tests, the extraction of Mo appeared to be a function of HCl normality, with virtually complete separation above 3.6 N (Figure 1). A test was conducted to determine if the extraction was a function of just HC1 concentration or of overall acidity. When the aqueous phases of the test sequence were all prepared a t 1N H2S04 in addition to the HCl in each, the resulting extraction curve was essentially identical with that shown in Figure 1,when plotted against total acid normality instead of HCl normality. Another test was performed to determine if Mo was being extracted as an iodide complex. Three aqueous solutions, which were 4 N in HC1 and 26% (w/v) ascorbic acid, were prepared in culture tubes. One contained 20% (w/v) KC1,
0003-2700/85/0357-2428$01.50/00 1985 American Chemlcal Society
(f
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Anal. Chem. 1905, 57, 2429-2430
ACKNOWLEDGMENT The authors are grateful for critical reviews provided by Richard M. O'Leary and Richard F. Sanzalone.
l@Jr 90
Registry No. Mo, 7439-98-7.
LITERATURE CITED
5 a
(1) Clark, J. R.; Wets, J. G. Anal. Chem. 1081, 53, 61-65. (2) Seely, F. G.;Crouse, D. J. J . Chem. Eng. Data 1066, 7 7 , 424-429. (3) Rao, P. D. At. Absorpt. News/. 1071, 10. 118-119: (4) Wets, J. G.; Clark, J. R.; Campbell, W. L. J . Geochem. Expor. 1084, 20,355-366.
40 30 20
10
"-
0
1
2
3
4
5
6
HCi NORMALITY
Flgure 1. Extraction curve for molybdenum.
another was 20% (w/v) KBr, and the third was 20% (w/v) KI. When 25 pg of Mo was extracted from these solutions into 5 mL of organic phase, the KC1 gave only a 16% extraction, the KBr gave only a 22% extraction, and the KI provided an essentially total extraction.
RECEIVED for review December 13, 1984. Resubmitted May 21, 1985. Accepted May 21, 1985. This research was an outerowth of a Ph.D. thesis moiect bv J. R. Clark on trace eleGent distributions a t Orchai Mine, Matagami, Quebec, Harold Bloom, advisor. Mention of manufacturers' names in this report is for descriptive purposes only and does not imply endorsement of their reagents or equipment by the US. Geological Survey.
Microreactor for Postcolumn Reaction Gas Chromatography/Mass Spectrometry with Fused Silica Capillary Columns Alan L. Chaffee* and Imants Liepa
CSIRO, Division of Energy Chemistry, Lucas Heights Research Laboratories, Private Mail Bag 7, Sutherland, New South Wales, 2232 Australia Reaction gas chromatography/mass spectrometry (GC/ MS), although not widely used, can be a valuable adjunct to the standard GC/MS analysis of complex mixtures. It is especially valuable in cases where the mass spectra of individual components are not sufficiently informative or distinctive to enable confident structural assignments or to differentiate between various isomers (1). In reaction GC/MS, a vapor phase microreactor is filled with catalyst (or some other reagent) and positioned either before the column (position A, in which case the injector port is often suitable) or between the column and the mass spectrometer (position B). The latter position is particularly interesting since it enables components to be separated on the basis of their original molecular structure and then converted individually to specific derivatives whose mass spectra provide additional or more useful structural information. Reaction GC/MS can be regarded as an extension of reaction GC which has been used widely for the determination of carbon skeletons and the positions of functional group substitution (2,3). Reaction GC/MS has been applied to the structural investigation of, inter alia, olefins (4), alcohols (5), sulfides (6),and cyclopropanes (4)but has apparently only been used in combination with packed GC columns (7) to date. The approach has therefore failed to take advantage of the vast improvements in chromatographic resolving power and ease of handling provided by commercially available fused silica capillary columns. The present paper describes a simple microreactor for use with fused silica capillary columns at position B and illustrates its application in the determination of the skeletal structure of olefins derived from the Kolbel-Engelhardt synthesis.
EXPERIMENTAL SECTION The microreactor consists of a single piece of glass-lined tubing (60 mm long X 0.5 mm i.d. X 1/16 in. o.d., S.G.E. Scientific) as 0003-2700/85/0357-2429$01.50/0
illustrated in Figure 1. For the determination of olefin skeletal structures, the microreactor was vacuum packed with finely powdered Adams catalyst (platinum oxide, < 75 wm Johnson Matthey Chemicals, Ltd.). It was then connected to the outlet of the fused silica capillary column (50 m BP-5, 0.22 mm id., S.G.E. Scientific)at one end and to a piece of uncoated fused silica capillary tubing (0.2 m x 0.22 mm i.d.) at the other end. Connections were made so that the capillary tubing butted tightly against the catalyst packing at both ends using conventional capillary to stainless steel tubing unions (S.G.E. Scientific). The capillary tubing was in turn connected to the mass spectrometer inlet. The microreactor was housed in a separately constructed oven situated on top of the gas chromatograph of the JEOL DX-300 GC-MS system to facilitate independent temperature control for the microreactor via a 0-400 "C temperature controller (RKC Instrument, Inc.). The small internal diameter of the capillary tubing (0.22 mm) generally ensured that the catalyst did not escape (under vacuum) from the microreactor into the mass spectrometer. As an added precaution, small lengths (approximately 5 mm) of very fine nickel wire (36 swg) were packed tightly between the catalyst and capillary tubing at the exit end. For the analysis of olefins, neat synthesis liquids were injected into the gas chromatograph (50/1 split, 300 "C) and the oven was programmed from 10 "C at 4 "C min-' using hydrogen as carrier gas (linear flow 30 cm s-l). The microreactor and GC/MS interface ovens were maintained at 300 "C. The mass spectrometer was operated in the electron impact mode (ionizing potential 70 eV, ionizing current 300 PA) using a source temperature of 220 "C and a data acquisition rate of 1 scan (mlz 35-600) per second (JEOL DA-5000 data system).
RESULTS AND DISCUSSION Carrier gas flow rates for fused silica capillary columns are very low (ca. 0.5 mL min-l) by comparison with those of packed columns. Since individual peaks may elute from the column in volumes of the order of 20-50 bL, it is imperative that the effective dead volume in the postcolumn reaction 0 1985 American Chemical Society
