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a� The flow rate for purging should be the same as the flow rate used for subsequent sampling (< 200 mL/min). Guidance documents from different agencies recommend different purge volumes, ranging from 1 to 5 purge volumes (CSDDEH, 2002; API, 2004). Chevron recommends that 3 volumes be purged unless otherwise required by applicable guidance. The purge test data (calculated purge volume, purging rate, and duration of purging) should be recorded for each soil vapor sampling point. It is important to ensure that the same purge volumes and rates are used at a given probe for each sampling event. E. Leak testing Leakage of atmospheric air into the sampling equipment during sampling can compromise sample integrity and dilute measured soil vapor hydrocarbon concentrations, possibly to the point that the concentration is below the method detection level (i.e., a "false negative"). Contaminants in ambient air can also enter the sampling system and be detected in the sample from a non-contaminated sampling probe (i.e., a "false positive"). Air leakage can occur at the land surface into the probe and, more likely, through loose fittings in the above-ground sampling equipment. Leakage of air into the below-ground sampling system is unlikely if the probe has been properly constructed and a proper bentonite or concrete surface seal (described earlier) has been emplaced. Shallow gas probes (5 ft below grade) and temporary (direct-push) probes are most susceptible to leakage(i.e., short-circuiting)due to the minimal thickness of overlying soil and the potential for leakage around rods. Sampling equipment must be thoroughly inspected to ensure tight fittings between all components. Be aware that leakage locations may not be obvious. Elevated OZ concentrations in samples from deeper depths in multi-level probes may be indicative of leakage, but in some cases this alone would not provide definitive evidence for leakage. If Oz concentrations remain high with increasing depth and hydrocarbon concentrations are also high, this may provide evidence that leakage is occurring. Thus 02 is a semi-qualitative method of assessing leakage in soil vapor samples. To minimize the potential for leakage, the soil vapor sampling rate should be kept at <200 mL/min (see Section F 2 below). Repair or replacement of the sampling probe may be necessary if it is determined that leakage through the probe is occurring. Refer to state or regional guidance to determine if a prescribed course of action applies for probe replacement. Tracers are used to test for air leakage into the sampling system. Numerous compounds have been used for this purpose; however, isopropanol (store-bought rubbing alcohol) is used most commonly and is recommended. In California, the analytical laboratory must have a detection limit of 5 10 gg/L for isopropanol (5 4000 ppb,) (California EPA, 2003). Outside of California, Chevron suggests a detection limit of 100 pg/L unless applicable regulations dictate otherwise. Other compounds such as butane and propane may be present as soil contaminants and should not be used as leak detection compounds. A common method of leak testing involves soaking paper towels with isopropanol, placing the paper towels in plastic bags, and wrapping the bags around the above-ground ETC Soil Vapor Sampling Technical Toolkit 18 Version 1.4, September 7,2005