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sampling probes and over multiple sampling events. The vacuum and flow rate should be documented in the field notes. A vacuum of< 10 in Hg should be sufficient for sampling (California EPA, 2003). API Guidance states that a vacuum of< 100 water(<7.4 in Hg) is likely sufficient (API, 2004). Soil vapor sampling is assumed to have failed if a vacuum of> 10 in Hg is obtained or if vapor flow rates are < 10 mL/min (California EPA, 2003). Such failure can occur in some fine-grained soils. A sampling flow rate of < 200 mUrnin is recommended, although in very deep probes or where large diameter tubing has been installed, a rate of up to 1 L/min may be used if the lower now rate makes the time required for sample collection impracticable. The Summa canister system utilizes a flow controller to control the flow rate. The flow controller contains a critical orifice flow restrictor to maintain a constant flow rate over a 0.5 to 8 hour period, even though the vacuum in the canister is decreasing over that time (which would otherwise cause the flow rate to concurrently decrease). A vacuum gauge is built in to the flow controller to monitor sampling progress. The laboratory (e.g., Air Toxics Ltd.) sets up the flow controller for the flow rate specified. Figure 12 shows the range of flow rates for given sampling time intervals. A particulate filter is built into the flow controller device which serves to prevent particulates from fouling the flow controller or entering the Summits canister. Table 2.4.3 Flow Rates for Selected Sampling Intervals (m L/min) Sampling Interval(hrs) 0.5 _ 1 2 -4 8 ._ 12 24 6 L Canister 167 83.3 41.7 20.8 11.5 7.6 3.5 1 L Canister 26.6 13.3 _._ - - .----. —.. Note:Target fill volumes for 6 L and 1 L canisters are 5,000 mL and 800 mL, respectively. Flow Rate(mL/min)= Target Fill Volume(mL) Sampling Interval(min) Figure 12. Range of flow rates for given sampling time intervals using the Air Toxics Ltd. Flow controller(from Air Toxics Ltd.) The recommended sampling time interval for soil vapor samples is approximately 30 minutes,but in any case should not result in a sampling rate>200 mL/min. 3. Sample collection Once the above listed topics have been addressed, after making certain that all connections between the Summa canister, flow controller, and all other portions of the sampling equipment are tight, and arrangements have been made with the analytical laboratory regarding sample shipment and analysis, soil vapor sampling can commence. Sampling of all probes should preferably be completed within a one day time period, with a maximum collection period of one week. Sample collection from a purged soil vapor ETC Soil Vapor Sampling Technical Toolkit 21 Version 1.4,September 7,2005