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APPENDIX C: USE OF LOW-MOLECULAR-WEIGHT STANDARDS TO IDENTIFY <br /> EARLY-ELUTING COMPOUNDS <br /> As noted in the text, an unidentified peak was found in the chro- <br /> matograms for soil gas samples from Chevron property 9-0265 . The <br /> characteristics of the peak, a strong response, very short reten- <br /> tion time, and longer tailing, suggests that the peak is the <br /> response to a low-molecular-weight, non-polar hydrocarbon. To <br /> identify the unknown constituents, a number of standards were <br /> made up (Table C-1 and following chromatograms ) and run in the <br /> HNU 421 gas chromatograph. On the basis of the retention times <br /> and peak shapes of these standards, it is believed that the <br /> unidentified peaks ( see site chromatograms, Appendix B) are <br /> responses to a small alcohol, a small alkane, or a combination of <br /> alcohols and alkanes coeluting. Small alkanes and alcohols were <br /> suspected because they can be formed from natural soil organics <br /> or petroleum hydrocarbons by microbial biodegradation. Studies <br /> of microbial decomposition show that alcohols, organic acids, and <br /> other small hydrocarbons can be formed from natural soil mate- <br /> rials by aerobic or anaerobic processes (Odum 1971 ) . The decom- <br /> position of petroleum-related aromatics ( such as benzene, <br /> toluene, xylenes, and ethylbenzene ) can also form small alkanes <br /> (API 1985 ) . At the subject site, it is believed that natural <br /> soil organics are the precursors, because no detectable levels of <br /> the petroleum aromatics were found. <br /> c40/90265 C-1 <br />