EFFECT OF SFE FLOW-RATE ON EXTRACTION RATES - CLASSIFYING SAMPLE EXTRACTION BEHAVIOR

The effect of flow rate on SFE extraction rates can be used to determine whether the extraction is limited primarily by analyte solubility and chromatographic retention of analytes on matrix active sites (i.e., the solubility/elution process) or by the kinetics of the initial transport of bound analytes from the matrix into the extraction fluid (i.e., the desorption/kinetic process), The extraction rates of analytes from samples that are controlled primarily by the solubility/elution process (e.g., fat from potato chips, motor oil from a highly contaminated soil) show direct correlation with SFE flow rates (e.g., doubling the now rate doubled the extraction rate), In contrast, extraction rates for samples that are controlled primarily by the kinetics of the initial desorption step (e.g., limonene from lemon peel, alkylbenzenes from polystyrene beads) show little or no change with different SFE now rates, Even similar samples can show different types of behavior, For example, the extraction rates of many PAHs from a highly contaminated soil depend heavily on SFE flow rate and are therefore limited primarily by the solubility/elution step, while the extraction rates of the same PAHs on a less contaminated soil show little or no dependence on flow rate and are therefore limited primarily by the desorption/kinetic step. For samples limited by the solubility/elution step, SFE rates are inversely related to sample size, while samples controlled by the desorption/kinetic step show little effect of sample size on extraction rates, Similarly, samples limited by the solubility/elution step are extracted most efficiently using dynamic (flowing) SFE, while samples limited by the desorption/kinetic step are efficiently extracted using either static or dynamic SFE.