This method is for peak, ceiling and Time Weighted Average (TWA) determinations of Volatile Petroleum Hydrocarbons (VPH), specifically benzene, toluene, ethylbenzene, p&m-xylene, and o-xylene.  There is the possibility that interactions between analytes may reduce the breakthrough volumes and change desorption efficiencies. Samples are analyzed using a Varian 3800 Gas Chromatograph equipped with a 2200 ion trap mass selective detector (MSD) and an 8400 auto injector. Helium is used as the carrier gas and a compatible high resolution capillary column.  Results are compared to a calibration curve.  A separate computer program is used to convert concentrations to ppbv.  This procedure assumes working knowledge of the software analysis packages currently in use. For detailed SOP click here.