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n Air, Water, & Hazardous V asle Sanipiing, Analysis, & Consultation State Certified Hazardous Waste, Chemistry, & Bacteriology Laboratories 141 Suburban Road, Suite C-4 San Luis Obispo, Ca 93401 Fax (805) 543-2685 (805) 543-2553 6483-D Calle Real Goleta, CA 93117 Fax (805) 976-4386 (805) 964-7838 July 19, 1988 Keoni Murphy Emcon Associates 1921 Ringwood Ave. San Jose, CA 95131 Dear Keoni: I have reviewed the data from GC/MS analyses of ambient air samples AA -48 and AA -49 (our lab numbers E-4468 and E-4469) which we analyzed in conjunction with the Monitoring Project #711-01.13. These samples had methylene chloride concentrations of approximately 90 ppbv, substantially higher than the 1-2 ppbv found in other samples collected from the site at the some time. The mass spectra, copies of which are attached, confirm the presence of methylene chloride in the samples. Note that samples AA -48 and AA -49 were received with very little sample volume (less than 100 mL). In such cases, sample contamination may occur due to infiltration of methylene chloride from laboratory air to the sample bag during the long time (up to 20 min) that it takes to load the sample. It is also possible that the Tedlar bags themselves are contaminated with small amounts of methylene chloride and that the concentrations in the samples are augmented due to the small sample volumes involved. It is not possible to determine precisely the extent to which each of these factors contributes to the observed concentrations without additional measurements. We do know, however, that the methylene chloride concentration in our laboratory air is less than 50 ppbv, and we have observed methylene chloride as an impurity in Tedlar bags. I hope this helps explain some of the analytical limitationG imposed when extremely small sample volumes are analyzed. Please call me at (805) 543-2553 if you have any questions. Sincerely yours, CENT AL COAST ANALYTICAL SERVICES, INC. Laurence R. Hilpert, Ph.D. Vice President enclosure Central Coast Analytical Services n Air, Water, & Hazardous V asle Sanipiing, Analysis, & Consultation State Certified Hazardous Waste, Chemistry, & Bacteriology Laboratories 141 Suburban Road, Suite C-4 San Luis Obispo, Ca 93401 Fax (805) 543-2685 (805) 543-2553 6483-D Calle Real Goleta, CA 93117 Fax (805) 976-4386 (805) 964-7838 July 19, 1988 Keoni Murphy Emcon Associates 1921 Ringwood Ave. San Jose, CA 95131 Dear Keoni: I have reviewed the data from GC/MS analyses of ambient air samples AA -48 and AA -49 (our lab numbers E-4468 and E-4469) which we analyzed in conjunction with the Monitoring Project #711-01.13. These samples had methylene chloride concentrations of approximately 90 ppbv, substantially higher than the 1-2 ppbv found in other samples collected from the site at the some time. The mass spectra, copies of which are attached, confirm the presence of methylene chloride in the samples. Note that samples AA -48 and AA -49 were received with very little sample volume (less than 100 mL). In such cases, sample contamination may occur due to infiltration of methylene chloride from laboratory air to the sample bag during the long time (up to 20 min) that it takes to load the sample. It is also possible that the Tedlar bags themselves are contaminated with small amounts of methylene chloride and that the concentrations in the samples are augmented due to the small sample volumes involved. It is not possible to determine precisely the extent to which each of these factors contributes to the observed concentrations without additional measurements. We do know, however, that the methylene chloride concentration in our laboratory air is less than 50 ppbv, and we have observed methylene chloride as an impurity in Tedlar bags. I hope this helps explain some of the analytical limitationG imposed when extremely small sample volumes are analyzed. Please call me at (805) 543-2553 if you have any questions. Sincerely yours, CENT AL COAST ANALYTICAL SERVICES, INC. Laurence R. Hilpert, Ph.D. Vice President enclosure