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5.3.1 Collecting Soil Gas Samples with Sampling Manifolds Air Toxics provides soil gas sampling manifolds in order to facilitate meeting the numerous quality control guidelines for collecting soil gas data. Two of the most critical aspects of soil gas sampling are purging the lines and preventing leaks.The manifold setup allows for automatic leak-checking of the canister sample train without the application of a leak check compound to the canister fittings.When the purge can is opened and closed,it creates a vacuum within the canister lines and fittings. If this vacuum is maintained,the train is considered leak-free. Because there is only one connection— the probe tubing to sample train—the chance for leaks is greatly reduced. The manifold's in line gauge system used with a purge canister enables the sampler to determine the appropriate purge volume.Typically,purge volumes of 3 to 5 times the volume of the train tubing are used,and once the purge volume.is calculated the lines can be purged by opening the purge canister valve and monitoring the decrease in vacuum.This decrease in vacuum is proportional to the volume purged through the lines. There is a suggested flow rate of between 100 and 200 milliliters per minute,a step thought to prevent ambient air intrusion as a result of taking the sample too quickly. Our manifold has a"built-in"flow restrictor; a frit of stainless steel tubing between the two gauges that is calibrated to 167 milliliters per minute. The first gauge, located prior to the flow restrictor, Is a vacuum gauge that informs the sampler if sufficient vapor is being collected from the soil or if the substrate is too compacted. Because this is not a flow meter but a measure of pressure/vacuum, the gauge should read at zero if there is sufficient flow from the soil. If the gauge begins to read a vacuum,then the flow is being restricted.The second gauge,in line after the flow controller and prior to the purge canister, is a vacuum gauge that indicates to the sampler whether or not the canister is filling properly at the expected rate. This setup enables the sampler to evaluate the lithologic conditions at the site and determine if a valid soil gas sample is being taken. Finally,when duplicate samples are required, the manifold can be used as a duplicate sampling"T"by simply replacing the purge canister with another sample canister. T r T Filter Gauge Rvarlctor Gauge F-Manifold Valve Sample f—Pwge CAnisler Canister -4 23