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C <br /> �.I �1VSGLRBAL <br /> A;09 <br /> r,,, _.[[,�,., a 3SSE +a11 ({ T ENVIRomMEWAL <br /> GEOCHEMISTRY vVr. <br /> Ms. Mary Meays November 29, 1993 <br /> Public Health Service <br /> P.O. Box 2009 <br /> Stockton, CA 95201 <br /> Ms. Meays: <br /> My apologies for the delay in providing this information to you. As you requested in our phone <br /> discussion last month, this letter provides information documenting the use of liquid-solid <br /> extraction and direct injection for the detection of volatile aromatic hydrocarbons (BTEX) at a <br /> detection level of 5 ug/kg (ppb by weight). You raised 2 primary questions in our discussion <br /> which you requested me to answer: <br /> 1. Can a 5 ug/kg MDL be achieved by direct injection? <br /> 2. Are any volatiles lost in the extraction process using freon? <br /> Achievable MDL <br /> I asked TEG Sacramento to perform an MDL study to answer this question with data, rather <br /> than verbage. The data from their test are shown on the next page. You can observe the <br /> following in these data: <br /> The % RSD is very good and well within EPA required continuing calibration limits. <br /> The MDL's for each compound , computed as 3 standard deviations of the calculated <br /> concentrations (0.015/area), are 0.005 or less. <br /> Volatile Loss Durin Extraction <br /> As I explained, the Henry's law constant gives the equilibrium ratio of molecules of a <br /> compound in the vapor phase to molecules in the liquid phase (i.e., the inverse of the aqueous <br /> solubility). Typically, the liquid is water. For the aromatic hydrocarbons, the Henry's law <br /> constants range from 0.1 to 0.2 as shown in the accompanying table. What this means is that at <br /> equilibrium, 5 to 10 molecules reside in the liquid for every 1 that resides in the vapor. In other <br /> words, 10% to 20% partition into the vapor. This is for water. Freon-113 is less polar than <br /> water, so it's affinity for the BTEX components is much higher than water. Thus, the <br /> partitioning into the vapor will be much less than the amount given by Henry's law. So, <br /> theoretically, the loss into the headspace in the VOA vial should be significantly less than 10%. <br /> To prove the theory, we measured the BTEX in the headspace of a sample and in the soil, and <br /> calculated the total mass in each phase. A summary of the calculation for benzene, toluene, and <br /> xylene was as follows: <br /> Headspace (ng) Soil (ng) Ratio (%) <br /> Benzene: 38 25241 0.15 <br /> Toluene: 1 1783 0.04 <br /> Xylene: 1 5528 0.004 <br /> Mobile and LaboratoryAnalytical Services EnvironmentaISubconsulting GeochemicalR&D SoiIVapor Surveys AirMonitoring <br /> 432 N. Cedros Ave., Solana Beach, CA 92075 Ph: (619) 793-0401 Fax: (619) 793-0404 <br />