Equilibrium and rate study of analyte-matrix interactions in supercritical fluid extraction

Variation in analyte ex-traction efficiencies from environmental samples using supercritical carbon dioxide (SC CO2) have often been attributed to ''matrix effects'' including strong matrix-analyte binding. To examine this hypothesis, the equilibrium distribution of phenanthrene between SC CO2 and five well-characterized natural materials was measured as a function of temperature, pressure, and methanol modifier concentration. Dynamic extraction efficiencies and rates with and without methanol modifier were measured from the same materials. Energetic heterogeneity of active sites on dry soils was suggested by nonlinear isotherms displaying significant capacity differences across soil types. Removal efficiencies were highest from low organic matter sorbents, and these materials exhibited the lowest phenanthrene capacities and most linear isotherms. Modifier addition decreased the sorbent's equilibrium capacity for phenanthrene sorption by as much as an order of magnitude, resulting in linear isotherms and offering strong evidence that methanol acts by competitively displacing analyte from high-energy sites. Temperature increases were more effective than pressure increases in stimulating desorption from soils, consistent with previous observed temperature effects in supercritical fluid extraction. A nonlinear local equilibrium model including no mass transfer limitations was employed to illustrate the impact of variations in isotherm linearity and capacity on the extraction process.