Evaluation of drying agents for off-line supercritical fluid extraction

Of 21 potential drying agents investigated, five (anhydrous and monohydrated magnesium sulfate, molecular sieves 3A and 5A and Hydromatrix) were able successfully to prevent restrictor plugging by water during off-line supercritical fluid extraction (e.g., 400 atm CO2 at 60°C) by retaining the majority of the water (but generally not the analytes of interest) in the extraction cell. Increasing the extraction temperature (e.g., to 150°C) or adding a polar modifier [10% (v/v) methanol] to the CO2 extraction fluid greatly reduced the amount of water the drying agents retained. However, when 10% (v/v) toluene was used for the extraction, the drying agents were able to retain the majority of the water (ca. 80% w/w). Polar and non-polar pollutants were quantitatively extracted from the wet drying agents (i.e., water present), but nearly all of the drying agents selectively retained at least one of the polar analytes if used dry (i.e., no water present), thus demonstrating the need for a spike recovery study to determine the potential for analyte loss. The successful drying agents eliminated restrictor plugging when used with moderately wet [ca. 20% (w/w) water at a 1:1 reagent-to-sample ratio] and very wet [ca. 90% (w/w) water at 4:1 reagent-to-sample ratio] samples without the need to heat the restrictor or the collection solvent.