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chromatograms. The instrument is operated isothermally at 600C <br /> and the capillary column flow rate is 10 ml/min. These condi- <br /> tions ensure peak retention time stability and prevent contami- <br /> nant buildup within the column. Blanks are run to verify that <br /> the system is free of contamination; as necessary, the instrument <br /> is re-calibrated by injecting standards. These data ensure <br /> system reproducibility. <br /> The chromatograph yields a response in the form of an electrical <br /> signal, measured in volts; this is recorded and integrated across <br /> time by a Shimadzu C-R3A integrator. The peak area is expressed <br /> as volt-seconds (V-sec ) . The instrument is calibrated with a <br /> multicomponent standard consisting of known concentrations of <br /> benzene, toluene, xylene, and ethylbenzene, and with a methane <br /> standard. The integrator calculates and stores the response <br /> ratio, V-sec:ppm. The ratio for each component of the standard <br /> is used to quantify the concentrations of identifiable vapors in <br /> field samples according to their V-sec values. <br /> The concentrations of unidentified compounds are calculated in a <br /> similar manner. Although hydrocarbons produce variable instru- <br /> mental responses, the assumption may be made that all of the <br /> hydrocarbon constituents have response-to-concentration ratios <br /> approximately equivalent to that of benzene and that all quanti- <br /> fications may be based on the ratio for benzene. In the table <br /> describing the results of the assessment, the column entitled <br /> "Peaks Prior to Benzene" represents the sum of the responses in <br /> V-sec for all peaks eluting prior to benzene, proportioned to the <br /> calibrated V-sec response for benzene. Similarly, the column <br /> entitled "Total Volatile Hydrocarbons" represents the sum of all <br /> V-sec responses, proportioned to that for benzene. <br /> The ability of the HNU 421 to ionize a compound depends upon the <br /> structure of the chemicals. The response of its FID to reduced <br /> carbon compounds (e.g. , methane, saturated and unsaturated <br /> aliphatic hydrocarbons, and aromatics ) is excellent. It can <br /> c40/90265 6 <br />