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ITO E RO E K SOP No.026,REV.1 <br /> ASSOCIATES,INC INDOOR AIR AND SUB-SLAB SOIL GAS SAMPLING METHODS <br /> PAGE 16 <br /> 4.0 POTENTIAL INTERFERENCES <br /> A number of factors specific to soil gas can affect the response of a PID. Rapid temperature <br /> fluctuations will cause the PID lamp to fog; high humidity can also cause lamp fogging and <br /> decreased sensitivity. High concentrations of methane can cause a downscale deflection of <br /> the meter. High and low temperature, electrical fields, FM radio transmission, and naturally <br /> occurring compounds will affect instrument response. <br /> In general, concentrations of organic compounds in soil gas can be affected by the physical <br /> and chemical characteristics of the soil, soil moisture, and nature of the target compound. <br /> Important factors to consider include the contaminant's vapor pressure, solubility, and <br /> density. Organic molecules can be tightly adsorbed to the surface of chemically active soil <br /> particles, such as clays; thus, reducing the concentration in the soil interstitial spaces. <br /> Similarly, some organic compounds can be dissolved in the soil water or associated with soil <br /> organic components. <br /> Weather conditions may also present certain limitations on soil vapor sampling. <br /> Condensation in the sample tubing may occur during winter sampling due to lower outdoor <br /> air temperatures. Devices such as tube warmers may be used to address this issue. <br /> Anticipated limitations to indoor air and sub-slab soil sampling should be discussed prior to <br /> the sampling event to ensure appropriate measures can be taken to address potential issues <br /> and produce representative and reliable data. <br />