Coupling Supercritical CO 2 and Subcritical (Hot) Water for the Determination of Dacthal and Its Acid Metabolites in Soil Jennifer A. Field,* Keith Monohan, and Ralph Reed Department of Agricultural Chemistry, Oregon State University, Corvallis, Oregon 97331 Dacthal and its mono- and diacid metabolites were sequentially extracted from soils by first performing a supercritical carbon dioxide extraction to recover Dacthal, followed by a subcritical (hot) water extraction step to recover metabolites. Dacthal was recovered from soil in 1 5 min by supercritical carbon dioxide at 1 5 0 °C and 4 0 0 bar. The mono- and diacid metabolites were extracted from soil in 1 0 min under the subcritical water conditions of 5 0 °C and 200 bar. The metabolites were trapped in situ on a strong anion-exchange disk placed over the exit frit of the extraction cell. Metabolites are combined with Dacthal by placing the disk into the GC autosampler vial containing the SFE extract. The metabolites then are simultaneously eluted from the disk and derivatized to their ethyl esters by adding 100 μL of ethyl iodide and heating the vial at 1 0 0 °C for 1 h. Using this approach, only a single sample is analyzed, and because the disk- catalyzed alkylation reaction does not transesterify Dacthal, the speciation of Dacthal is maintained. In addition, no sample cleanup steps are required, the use of diaz- omethane for derivatization is avoided, and the method consumes a total of 5 mL of nonchlorinated organic solvent. Dacthal is a widely used preemergent herbicide that is applied to many crops for the control of annual weeds. Dacthal is typically applied to agricultural soils at 6-14 kg/ ha. 1 In the soil environ- ment, Dacthal transforms to mono- and diacid metabolites that are more water soluble than the parent herbicide. 2-4 In eastern Oregon, where Dacthal is applied to onions, the diacid metabolite is the principal form of Dacthal detected in groundwater obtained from domestic wells. 5,6 To assess the fate of Dacthal that is applied to soil, both parent and metabolite forms in water and soil should be considered. While rapid methods exist for the determination of Dacthal and its metabolites in water, 5,6 quantitative and rapid methods are needed to determine Dacthal and its metabolites in soils, since conventional methods require large volumes of solvent and time to process the extract. For example, the conventional method for extracting Dacthal and its metabolites from soil requires 200 mL of 0.4 M HCl/ acetone to extract a 20-g sample and the use of hazardous diazopropane to derivatize the acids to their ester forms. 7 Supercritical fluid extraction (SFE) is an attractive analytical technique for recovering organic compounds from soils and sediments. Carbon dioxide (CO 2 ) is currently the fluid of choice, due to its low toxicity and environmental acceptability. The physicochemical properties of supercritical fluids, including low viscosity, variable solvent strength, and high diffusivity, contribute to faster extractions compared to conventional extraction tech- niques, such as Soxhlet extraction or sonication. Supercritical fluid extraction methods have been successfully developed for nonpolar compounds that exhibit high solubilities in CO 2 , such as PAHs, 8 PCBs, 9 dioxins, 10 and organochlorine pesticides. 11 With the addition of methanol as modifier, supercritical CO 2 becomes more amenable to the extraction of moderately polar pesticides, including triazines, 12 organophosphate insecticides, 13 and sulfo- nylureas. 14 Adding chemical reagents to soil samples prior to SFE has received attention as an alternative for extending supercritical CO 2 toward acidic analytes such as chlorophenoxy acid herbicides; however, the recoveries of acid analytes are variable and depend on the sample matrix. 15-18 * Corresponding author. Fax: (541) 737-0497. E-mail: fieldj@ bcc.orst.edu. (1) The Pesticide Manual: A World Compendium, 7th ed.; Worthing, C. R., Ed.; British Crop Protection Council: London, UK, 1983. (2) Gershon, H.; McClure, G.W. Contrib. Boyce Thompson Inst. 1966 , 23, 291- 294. 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