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�s :---- '. s a" screening is below the top of the water table) , bccauae nail r}.aa "ill net be properly tran3ported. The 0011 vapor and monitoring well heddapace were collected through a septum with a microbyringo and injected into ac Photovac 10550 gags chrotts;togr4ph for analysis. rile Photovac 10S50 in a portable, program-mable, integrating gaa0 chrortakograph with a photoionization detector WID) , The PID is a nondestructive flew-through detarcto: that useu high energy ultraviolet radiation as its ionization source. Vapor samples arca infected into the gas chromatograph, separated on an analytical column, and sensed by the detector. The high energy radiatioli ionizes cargounda, generating an energy increase in the detector which appears as an electrical signal, measured in VOI M this is integrated across time by the instrument to give a value for the peak in Volt-secondsV-sec) , The instrument is calibrated with standards corasistin; of 10ene and 10 toluene in air. For the reading Of each standard,1zthe inatrunPpm ant stores the known concentration and the V-50C response to it. The ratio for the standard, V-aec.ppm, is then used to quantity the concentrations of identifiable, vapors in field ZarV103 according to their V-sec values. The concentrations of unidentified compounds are calculated in a similar manner. Although petroleum hydrocarbons Produce variableinstrumental responses, the ass=Ution may be made that all of the hydrocarbon constituents have response-to-Concentration ratios approximatelY equivalent to that of benzene and that all quantifications may be based on the ratio for benzene. In the table of results, the Column entitled "Peaks Prior to In represents the_s -of-t3.e-ros - - rtaasa s -V-aec for all ----- - eluting Prior to benzene, Peaks Proportioned to the calibrated V-sec response for benzene. Similarly, the Columna entitled "Total E Detected Patroleua Hydrocarbons- or "Total Volatile Hydrocarbons" CJIV16.@1A