Selective supercritical fluid extraction of organochlorine pesticides and herbicides from aqueous samples

Supercritical fluid extraction (SFE) of organochlorine pesticides and two classes of herbicides is achieved from a water matrix using solid-phase extraction prior to SFE. This technique allows for the removal of water prior to supercritical elution with carbon dioxide. Selectivity of extraction between these two groups of compounds is demonstrated by adding 10% methanol modifier to the CO2 after an initial extraction with CO2 only. The first extraction (CO2 only) is used to preferentially remove organochlorine pesticides (approximately 90%) with minimal extraction of the herbicides (less than 5%). Modified CO2 is then used to extract the herbicides, and approximately 90% is recovered. The extracts are analyzed by either gas chromatography with mass selective detection for organochlorine pesticides or reversed-phase high-performance liquid chromatography for the herbicides. This SFE selective extraction may prove useful in segregating pesticides from herbicides prior to analysis. In addition, chromatographic selectivity is also achieved because organochlorine pesticides cannot be determined using reversed-phase high-performance liquid chromatography, and the herbicides are not directly amenable to gas chromatographic separation.