In an interlaboratory study to determine the concentration of petroleum hydrocarbons (PHCs) in an above ground bioreactor at a refinery site in Montreal we found that the EPA Method # 418.1 for the analysis of semivolatile hydrocarbons gave concentrations up two fold higher than expected. The silica mixing step used to clean-up the crude Soxhlet extract and the inclusion of chlorobenzene or benzene in the composition of the IR quantitative standard were later implicated as major sources of bias in the Method. In another comparative study we have determined water effect on the recovery of PHCs from wood preserving soil using the Soxhlet EPA Method # 3450 (with Freon-113) and the SFE Method # 3560 (with CO2). In the Soxhlet method water did not seem to affect the recovery of PHCs since variations in PHC concentrations did not exceed RSD values, i. e., #xB1; 2 to #xB1; 8. While in SFE, an optimal extraction efficiency occurred when the water content in the soil was around the 20 limit, similar to what has been described in the EPA Method # 3560. In the case of a bioslurry both SFE and Soxhlet showed that water removal enhanced the recovery of PHCs, i. e., a reduction in the water content by a factor of two resulted in an increase in the recovery of PHCs by roughly a factor of two.
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