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Significant carbon isotope fractionation was observed during FeS-mediated reductive dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE). Bulk enrichment factors (E(bulk)) for PCE were -30.2 +/- 4.3 per thousand (pH 7), -29.54 +/- 0.83 per thousand (pH 8), and -24.6 +/- 1.1 per thousand (pH 9). For TCE, E(bulk) values were -33.4 +/- 1.5 per thousand (pH 8) and -27.9 +/- 1.3 per thousand (pH 9). A smaller magnitude of carbon isotope fractionation resulted from microbial reductive dechlorination by two isolated pure cultures (Desulfuromonas michiganensis strain BB1 (BB1) and Sulfurospirillum multivorans (Sm)) and a bacterial consortium (BioDechlor INOCULUM (BDI)). The E(bulk) values for biological PCE microbial dechlorination were -1.39 +/- 0.21 per thousand (BB1), -1.33 +/- 0.13 per thousand (Sm), and -7.12 +/- 0.72 per thousand (BDI), while those for TCE were -4.07 +/- 0.48 per thousand (BB1), -12.8 +/- 1.6 per thousand (Sm), and -15.27 +/- 0.79 per thousand (BDI). Reactions were investigated by calculation of the apparent kinetic isotope effect for carbon (AKIEc), and the results suggest that differences in isotope fractionation for abiotic and microbial dechlorination resulted from the differences in rate-limiting steps during the dechlorination reaction. Measurement of more negative E(bulk) values at sites contaminated with PCE and TCE may suggest the occurrence of abiotic reductive dechlorination by FeS.

The study of natural products in plants, animals, bacteria, fungi, and other living systems has long been of interest to chemists, biochemists, and analytical chemists. Generally, however, these scientists are concerned only with the isolation and identification of compounds present in living systems. A relatively small but rapidly growing scientific discipline has emerged over the past two decades involving scientists primarily interested in the fate of organic material and associated natural products after their demise and deposition in the sedimentary record and conversion into fossil fuels or chemical fossils. Organic compounds present in the original source material whose carbon skeleton is preserved throughout the geological record are referred to as biomarkers. In this Report the authors will introduce some of the basic concepts of petroleum geochemistry and the key role that analytical chemistry has played in the development of this discipline. They will also discuss areas in which geochemical research is most likely to develop in the next few years, and they will examine the major analytical challenges the organic geochemist will meet in conducting geochemical research during the next decade.