FACTORS CONTROLLING QUANTITATIVE SUPERCRITICAL-FLUID EXTRACTION OF ENVIRONMENTAL-SAMPLES

The development of quantitative supercritical fluid extraction (SFE) methods for the recovery of organic pollutants from environmental samples requires three steps: quantitative partitioning of the analytes from the sample into the extraction fluid, quantitative removal from the extraction vessel, and quantitative collection of the extracted analytes. While spike recovery studies are an excellent method to develop the final two steps, they are often not valid for determining extraction efficiencies from complex real-world samples such as soils and sediments, exchaust particulates, and sludges. SFE conditions that yield quantitative recoveries of spiked analytes may recover <10% of the same analytes from real-world samples, because spiked pollutants are not exposed to the same active sites as the native pollutants. Because of the heterogeneous nature of environmental samples, the partitioning step may be controlled by analyte solubility in the extraction fluid, kinetic limitations, and/or the ability ot the extraction fluid to interrupt matrix-analyte interactions. While the interactions that control SFE rates from heterogenous environmental samples are not well understood, a generalized scheme for developing quantitative SFE methods is proposed based on interactive considerations of the collection efficiencies after SFE, fluid flow parameters in the extraction cell, analyte solubility, extraction kinetics, and analyte-matrix-extraction fluid interactions. The proposed development scheme includes increasing SFE extraction rates by the use of more polar fluids than CO2 such as CHClF2, the addition of organic modifiers to CO2, and the use of high temperature extractions with pure CO2. Validation of quantitative extractions based on multiple extraction methods (SFE followed by liquid solvent extractions) is also described.