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Laboratory Procedures <br /> All of the samples collected during the field phase of the <br /> survey were analyzed according to EPA Method 602 on a gas chromato- <br /> graph equipped with a flame ionization detector (GC/FID) , but using <br /> direct injection instead of purge and trap. Analytes selected for <br /> standardization were: <br /> methyl tertiary butyl ether (MTBE) s ` <br /> benzene <br /> toluene <br /> ethylbenzene <br /> meta- and para- xylene <br /> _ ortho-xylene <br /> These compounds were chosen because of their utility in evaluating <br /> the presence of petroleum products such as fuels, lubricating oils, <br /> and non-halogenated solvents. <br /> FID Total Volatiles values were generated by summing the areas <br /> of all iritegratel chromatogram peaks and calculating using the. in- <br /> strument response factor for toluene. Injection peaks, which also <br /> E contain the light hydrocarbon methane, were excluded to avoid the <br /> skewing of Total Volatiles values due to injection disturbances <br /> and biogenic methane. <br /> 13 The analytical equipment was calibrated using an instrument- <br /> response curve and injection of known concentrations of the above <br /> standards. Retention times of the standards were used to identify \ <br /> ! _ the peaks in the chromatograms of the field samples, and their <br /> response factors were used to calculate the analyte concentrations. <br /> rs <br /> The tabulated results of the laboratory analysis of the soil gas -- <br /> samples are reported in micrograms per liter (ug/1) in Table 1. <br /> Map .sample points with no data shown indicate that the analyte <br /> concentrations in the samples were below the detection limit. <br />