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Section No 6 <br /> 0 Revision No 0 <br /> Date September 5, 1991 <br /> Page 14 of 55 <br /> r Activate pump, adjusting flow rate to that required by test method <br /> using flow control need valve, -- <br /> s Purge a volume of gas equivalent to three times the estimated <br /> internal volume based upon flow rate and duration <br /> Immediately after the sampling apparatus has been purged of stagnant <br /> gases, attach the sample container and commence the sample collection for the <br /> prescribed period. Set timer and assure that the sample container is <br /> properly labeled Record data as directed by Gas Purging and Sampling Log <br /> Before terminating sample collection, monitor the final flow rate and <br /> record Remove the sample container from the sampler using the quick- <br /> disconnect fitting <br /> 6 3 5 2 Grab Air Sampling System - Grab air sampling will be utilized <br /> to collect subsurface soil-gas samples or characterize source emissions <br /> For grab air sampling, a positive pressure gradient is induced between the <br /> specified sample source and sample container by either of two means <br /> pumping or cryogenic/evacuation techniques <br /> Grab air samples will be collected by a pump with unrestricted air flow <br /> to purge the sampling train and then to collect the samples, alternatively <br /> following purging by pumping The sample will be "grabbed" by the pressure <br /> differential established by canister evacuation and maintained by <br /> condensation using a liquid nitrogen bath The latter method allows <br /> collection of a sample with minimal exposure to sources of potential <br /> cross-contamination <br /> Collection of subsurface soil-gas samples by the grab method will employ <br /> special techniques as described below <br /> 6-14 <br />