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«� probe should begin within 10 minutes of purging. Leak testing should be performed concurrently with sampling as described above. To begin sampling, open the valve on the Summa canister. As the canister fills, observe the pressure gauge on the flow controller to ensure that the vacuum in the canister is decreasing over time. If the flow controller is working correctly, the planned sampling completion time will be reached when the pressure has decreased to 5 in Hg. Note that low permeability soils characterized by low soil vapor flow rates may require sampling to cease before the canister pressure has decreased to 5 in Hg. Quality control (QC) of soil vapor samples must be addressed through the collection of field blanks and field duplicates, and the transport trip blanks. A field blank should be collected at the site during sampling activities from a certified air source (supplied by the laboratory). At least one trip blank should be obtained from the analytical laboratory for each sampling day (event). The trip blank contains laboratory grade ultra pure air and is intended to provide evidence of contaminants entering the sample containers during handling and shipping. At least one duplicate sample should be obtained each day of sampling, or from at least 10 % of the samples obtained. A duplicate sample should be collected by using a splitter located upgradient of the flow controller, with separate sampling tubes connecting the splitter to two Summa®canisters. The flow controller must be set such that the flow rate from the sampling probe is < 200 mL/min; this will double the required sampling time since two canisters are being filled simultaneously. After collection, canisters must not be chilled since contaminants may condense in the canister at low temperatures. Make certain that all samples are correctly and clearly labeled. Follow standard chain-of-custody procedures, including noting the final canister vacuums and serial numbers of the canisters and flow controllers. See Section III A for canister maximum holding time information. Document all procedures, sampling times, conditions, problems, etc. Although the California EPA (2003) guidance recommends against using air freight for shipping,air shipment of canisters is acceptable. IV. Analytical techniques The analytical methods used are specific to the components analyzed. Also, the level of quantification required can vary between components. For example, fixed gases such as 02, CO2, N2, and CH4 may have less stringent quantification requirements than those for VOCs (e.g., benzene). Before sampling and analysis begins, refer to applicable state and/or regional regulations to ensure that all requirements are complied with in sampling and analysis, including any requirement for state certification of the analytical laboratory. Ensure consistency of analytical methods between sampling events, as this can help reduce uncertainties in data results and interpretation. The use of field monitoring devices such as a PID is not recommended for analyzing soil vapor sample concentrations. ETC Soil Vapor Sampling Technical Toolkit 22 Version 1.4, September 7,2005