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 determi ne whether the extraction is limited primarily by analyte solubility a nd chromatographic retention of analytes on matrix active sites (i.e., the solubility/elution process) or by the kinetics of the initial tra nsport 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 p rocess (e.g., fat from potato chips, motor oil from a highly contamina ted soil) show direct correlation with SFE flow rates (e.g., doubling the now rate doubled the extraction rate), In contrast, extraction rat es for samples that are controlled primarily by the kinetics of the in itial desorption step (e.g., limonene from lemon peel, alkylbenzenes f rom 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 primar ily 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/kineti c 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 extractio n 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 usin g either static or dynamic SFE.