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,• Tracer Research Corporation contamination. ' Groundwater samples were collected in 40 nil VOA vials that were filled to exclude air and capped with Teflon-lined septa caps. Approximately half of the liquid Iin the bottle was decanted, the vials were shaken vigorously, and a sample of the headspace from the container was injected into the gas chromatograph (GC). IHeadspace analysis is the preferred technique when a large number of water samples are to be performed daily. The method is more time efficient for the measurement of volatile organics than direct injection of the water sample into the GC because there is less chance of semi-volatile and non-volatile organics contaminating the system Depending upon the partitioning coefficient of a given compound, the GC may be more sensitive to headspace analysis than to direct injection analysis. The precision and accuracy of both methods are similar. 5.0 ANALYTICAL PARAMETERS The GC was calibrated for headspace analysis by decanting 20 ml of the known standard, leaving approximately the same amount of headspace as in the water I headspace samples. The standard bottle was resealed and shaken vigorously for 30 seconds. An analysis of the headspace in the bottle determined the Response Factor I (RF) which was then used to accurately estimate the standard concentrations. The GC was also calibrated for soil gas analysis by comparing the detector's response to a known aqueous standard to the detector's response to the soil gas vapor. The instrument calibrations were checked periodically throughout each day to monitor the response factor and retention time. I 5.1 Analyte Class ' The soil gas and groundwater samples were analyzed for the following analyte class and compounds: I Analyte Class: Hydrocarbon benzene, toluene, ethylbenzene, and xylenes (BTEX) total volatile hydrocarbons (TVHC) I - May�..2� 1992 Page 3 1-92-358