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