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s J , <br /> free product or product containing soils is typically rich in the <br /> "more volatile low boilers" which are the compounds in the upper <br /> left hand portion of Figure 1 --2, which have low boiling points. <br /> The vapor plume arising from the dissolvd constituents is <br /> relatively enriched in the compounds that are more soluble and <br /> more mobile in ground water which are the compounds in the lower <br /> right hand corner of Figure 1 -3 (typically the aromatics such as <br /> benzene and toluene ) . <br /> On the basis of these properties it can be seen that associated <br /> with a hydrocarbon release into soil, or into shallow ground/ <br /> water is a vapor phase that can be detected by SVA techniques. <br /> Through the collection and analysis of soil vapor the SVA tech- <br /> nique permits a rapid, cost-effective delineation of the approxi- <br /> mate extent of elevated levels of subsurface hydrocarbons. <br /> Soil vapor samples are collected from driven vapor points. Prior <br /> to sample collection, a vacuum pump is used to purge the vapor <br /> from the probes, to ensure that any soil vapor samples collected <br /> are representative of vapor in the soil surrounding the probe. <br /> The vacuum pressure reading on the purging apparatus is recorded; <br /> this vacuum pressure is related to the soil ' s gas permability and <br /> is of assistance in data interpretation. The samples are <br /> collected through a septum located immediately ad3acent to the <br /> probe on the sampling apparatus. The sample is collected with a <br /> microsyringe and in3ected into a gas chromatograph for analysis. <br /> The Photovac 10550 is a portable, programmable, integrating gas <br /> chromatograph with a photoionization detector ( PID ) . The PID is <br /> a nondestructive flow-through detector that uses high-energy <br /> ultraviolet radiation as its ionization source The high energy <br /> radiation ionizes compounds , generating an energy increase in the <br /> detector that is interpreted as a signal The signal is ampli- <br /> fied, integrated, and recorded as a chromatographic peak iapor <br /> samples are in3ected into the gas chromatograph, separated on an <br /> analytical column, sensed by the detector , integrated, and <br /> 4 <br />