micromolar amounts of Ca2+,Co2+, Fez+, Mn2+, and Zn2+. Application of the procedure to 0.5-mL portions of the spent medium showed no detectable NTA remaining. Recovery of 10 pg additions of NTA (20 ppm) was quantitative within normal experimental reproducibility and no interfering effects from the medium or the unremoved bacteria were observed.
ACKNOWLEDGMENT The gift of the bacterial mutant from P. T. S. Wong of the Canada Center for Inland Waters, Burlington, Ontario, Canada, is gratefully acknowledged.
LITERATURE CITED (1) D. Murray and D. Povoledo, J. Fish. Res. Board. Can., 28, 1043 (1971). (2)Lars Rudiing, WaterRes., 6, 871 (1972). (3)W. Aue, C. Hastings. K. Gerhardt, J. Pierce, M. Hill, and R. Moseman, J. Chromatogr, 72, 259 (1972). (4)C. B. Warren and E.J. Malec, J. Chromatogr., 64,219 (1972). (5) Y. K. Chau and M. E. Fox, J. Chromatogr. Sci., 9,271 (1971). (6)Lars Rudling, Water Res., 5, 831 (1971).
(7)A. E. Pierce, “Silylation of Organic Compounds”, Pierce Chemical Co., Rockford, Iil., 1968. (8)W. C. Butts and W. T. Rainey, Anal. Chem., 43,538 (1971). (9)W. C. Butts, Anal. Lett., 3, 29 (1970). (IO)T. Hashizume and Y. Sasaki, Anal. Biochem., 21, 316 (1967). (11) C. W. Gehrke, H. Nakamoto, and R. W. Zumwalt, J. Chromatogr., 45,24 (1969). (12)R. J. Stolzberg and D. N. Hume, Anal. Left., 6, 829 (1973). (13)W. R. Supina, ”The Packed Column in Gas Chromatography”, Supeico Inc., Beliefonte, Pa., 1974,pp 91-94. (14)P. T. S.Wong, D. Liu, and B. J. Dutka, Water Res., 6, 1577 (1972). (15) K. Freudenberg, H. Molter, and H. Walch, Naturwissenschaften, 30, 87 (1942). (16)G. F. Longman, M. J. Stiff, and 13. K. Gardiner, Water Res., 5, 1171 (1971). (17)Anders Ringbom, “Complexation in Analytical Chemistry”, interscience Publishers, New York, N.Y., 1963. (18)L. G.SilOn arid A. E. Martell, “Stability Constants of Metal Ion Complexes”, The Chemical Society, London, Special Publication No. 17 (1964),No. 25 (1971). (19)R. J. Stolzberg and D. N. Hume, Environ. Sci. Techno/.,9,654 (1975).
RECEIVEDfor review September 16,1976. Accepted December 9, 1976. This work was supported, in part, by the U.S. National Science Foundation under Grant GP33950.
Chemical and Physical Considerations in the Use of Atomic Absorption Detectors Coupled with a Gas Chromatograph for Determination of Trace Organometallic Gases G. E. Parris,’’ W. R. Blair, and F. E. Brinckman Inorganic Chemistry Section, National Bureau of Standards, Washington,D.C. 20234
A -crommerclal atomic absorption spectrophotometer with a heated graphite-tube furnace atomizer (HGA) was adapted as a detector for a gas-llquld chromatograph. The combined system was applled to the determinationof elements (i.e., As, Se, Sn) known to be methylated by mlcroorganlsms. The system was optimized by assessing the effects of varying the atomlzatlon temperature, the Inner surface of the furnace (Le., fused slllca, alumina, bare graphlte and pyrolytic carbon surfaces) and the carrler gas (Le., pure argon and argon wlth hydrogen). Using conservative, statlstically-based numerical techniques, the system detection limits for arsenlc, selenium, and tin (introduced as trimethylarsine,dimethylselenlum and tetramethyltln gas solutions with nitrogen dlluent) were found to be 5 ng As, 7 ng Se, and 12 ng Sn. To obtaln these Ilmlts, the bare graphite furnace was run contlnuously at about 1800 “C while the compounds were eluted from the chromatograph wlth argon to which 10% hydrogen was added. Optlmlzatlon of the furnace conditions requires an understanding of the thermodynamics and kinetics associated with thermal and chemical decomposition of the analyte compounds.
Ubiquitous biogenesis of labile organometallic compounds containing a variety of toxic heavy metals covalently bound to methyl groups is now apparent ( I , 2 ) . The need for analytical techniques combining chemical separation and highsensitivity, element-specific detection in environmental studies relevant to metal transport was outlined in previous publications (3-5). In the case of microbial transformations of metals, particularly in those situations in which organoM
Present address, EPA Office o f Toxic Substances, W H - 5 5 7 , 4 0 1 Street, S.W., Washington, D.C. 20460.
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
metallic metabolites (e.g., (CH3)2Hg, (CH&As, etc.) volatilize across water-air or lipid-air interfaces, a consideration of instrumental capabilities and limitations led to the conclusion that combined gas chromatography-atomic absorption (GC-AA) techniques best fulfilled the requirements for speciation at nanogram levels. Several GC-AA systems have been described in the literature. Some of these systems can be indirectly employed for speciation of trace organometals following a separate preconcentration step (6-9) while others permit direct determination of biogenic metal products during the growth of microorganisms. One system exemplifying the latter approach was applied to examination of bacterial respirant atmospheres ( 5 ) .This system employed reductive combustion of organomercurials in a flame ionization detector (FID) of a conventional GC followed by cold vapor atomic absorption detection of resultant elemental mercury gas. This technique (IO) offers a simple GC-AA system but since it depends on two unique features of elemental mercury (i.e., high volatility and monomeric vapor), the system cannot be considered as a general application of GC-AA technology. Several other GC-AA systems offer more promise as a general analytical tool. Segar ( 1 1 ) described the use of an AA detector in which the GC effluent was directed via a tungsten transfer tube into a high-temperature flameless atomic absorption furnace. There, decomposition of the effluent chemical compounds, and volatilization and atomization of the transported analyte element M, are essentially simultaneous and can probably be regarded as a single process: C,H,M,
-2700 “ C
--+x C + y H + z M
Another interesting development is the application by Chau and co-workers (6-8) of an electrically-heated, hydrogen-air diffusion flame, silica-tube furnace. The actual mechanism
.
--
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IIGA
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I I
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Figure 1. Schematic diagram of the GC-AA system
of atomization of the analyte element, introduced into the furnace as a volatile compound along with other elements, is unclear in this system inasmuch as the chemical reactions associated with different thermal regions present in this reactor presently represent an area of high-temperature flame chemistry of metals only recently receiving intensive study (12).Thus, while Robbins has recently summarized (13)many of the chemical considerations involved in efficient atomization of samples in flameless AA methods, characterization of detectors employing hydroxyl-mediated transport processes in Hz-air flames mainly relies on trial and error. In addition, the effect of varying the nature of the atomizer surface has not received the attention it deserves in either flameless or diffusion flame detectors (14). EXPERIMENTAL GC-AA Apparatus. The overall system with associated peripherals is outlined in Figure 1.A dual column Hewlett-Packard Model 5700 gas chromatograph (GC) with flame ionization detectors (FID) was fitted with glass columns lk-inch 0.d. by 6 feet long, supplied by Supelco Inc., Supelco Park, Bellefonte, Pa. 16823. Columns were packed with 5% SP-2100 (methyl silicone) and 3% SP-2401 (fluoropropyl silicone) on 80/lOO mesh Supelcon AWDMCS support. The GC oven was programmed for isothermal operation at 40 “C. Argon, flowing at 20 ml per minute, was used as carrier gas. Where required, H Zwas added to the carrier gas with mixing accomplished by the hydrogen jet of the cold FID detector. The FID collector assembly was modified to connect, via Swagelok fittings, to a small bore (0.027-inch i.d.), heated, programmable, stainless steel transfer tube, manufactured by Chemical Data Systems, Inc., Oxford, Pa. 19363. The GC injection ports, FID detector, and transfer tube were maintained a t 100 “C for all experiments conducted. The total column effluent from the GC was conducted through the transfer tube to the detector, a Perkin-Elmer model 360 atomic absorption spectrophotometer fitted with a deuterium background corrector and a HGA-2100 graphite furnace atomizer employing a temperature dial control power supply. The end of the transfer tube was connected to the internal gas purge passageways of the graphite furnace by means of a stainless steel T fitting. (See Perkin-Elmer HGA-2100 Graphite Furnace manual, pp 14,Perkin-Elmer Corp, Norwalk, Conn., 1974.) Column effluent was symmetrically introduced into the graphite furnace from both ends, passing to the central orifice where the effluent exited the furnace. The signal produced by the detector was processed by an Infotronics automatic digital integrator (Infotronics Corp. 8500 Cameron, Austin, Texas 78753) and printed out on a teletype. The signal was simultaneously attenuated as necessary and recorded on a strip chart recorder. Preparation and Calibration of Standard Gas Mixtures. Dilute solutions of trimethylamine, tetramethyltin, and dimethylselenium in high purity nitrogen (
