Supercritical Fluid Extraction and Solid-Phase Extraction of AC 263,222 and Imazethapyr from Three Texas Soils P. F. Pace, 1 S. A. Senseman, 2 M. L. Ketchersid, 3 H. T. Cralle 2 1 DeKalb Genetics Corporation, 3100 Sycamore Rd., DeKalb, Illinois 60115, USA 2 Texas A&M University and Texas Agricultural Experiment Station, Department of Soil and Crop Sciences, College Station, Texas 77843-2474, USA 3 Texas A&M University and Texas Agricultural Extension Service, Department of Entomology, College Station, Texas 77843-2488, USA Accepted: 19 April 1999 Abstract. Supercritical fluid extraction (SFE) using CO 2 and solid-phase extraction (SPE) are two technologies recently discussed in the literature as alternatives to Soxhlet and liquid/liquid extraction (LLE). This research compared SFE and SPE extraction efficiency of two imidazolinone herbicides, AC263,222 and imazethapyr, from three soils. Recovery of the herbicides using SFE-CO 2 with a 4.6:1 acetonitrile:acetic acid cosolvent was approximately 80% for Poth and Tremona soils and 60% when extracted from Ships soil with high clay content and high pH. SPE recovery of both herbicides averaged 78% and was not statistically different between soils. Combining SPE disks with a SPE cartridge cleanup procedure provided a faster filtration with cleaner filtrate compared with using SPE C-18 cartridges by themselves. Cleanup was needed after both SFE and SPE disk extraction due to interfering peaks in the chromatography. Soxhlet and liquid/liquid extraction (LLE) methods are widely used for extraction of herbicide residues in soil because they provide low detection limits, high precision and are reproduc- ible. Disadvantages of these methods include lengthy protocols, use of large quantities of organic solvent, and they are labor intensive. Reddy and Locke (1994) state that the ideal method would be rapid and simple, require limited labor, and use small amounts of organic solvents. Because supercritical fluid extrac- tion (SFE) and solid-phase extraction (SPE) possess some of the characteristics of the ideal method they could be considered as alternatives to conventional methods. SFE uses small amounts of organic solvents (Snyder et al. 1993; Alzaga et al. 1995), is automated (Snyder et al. 1993), can extract compounds that cannot be extracted with other methods (Huber and Otto 1983), and utilizes a nontoxic solvent (CO 2 ) (Reddy and Locke 1994). Simple modifications to the SFE extraction method can result in greater recovery. Modifica- tions include the use of a cosolvent modifier (Alzaga et al. 1995; Khan 1995; Koeber and Niessner 1996; Locke 1993; Snyder et al. 1993; Wuchner et al. 1993) or changes in extraction cell temperature (Alzaga et al. 1995; Lagenfield et al. 1993; Locke 1993; Snyder et al. 1993). The newer classes of herbicide chemistry are inherently more difficult to detect and quantify analytically due to lower use rates applied. AC 263,222 (()-2-[4,5-dihydro-4-methyl-4- (1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridine- carboxylic acid) and imazethapyr (()-2-[4,5-dihydro-4-methyl- 4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridine- carboxylic acid) are imidazolinone herbicides that fit into the low-use-rate category. Their chemical structures are very simi- lar in that they only differ by one methyl group (Figure 1). The imidazolinones are a class of herbicides used for weed control in peanuts, soybeans, and corn. These compounds are relatively water soluble, have half-lives ranging from 60 to 120 days, and are weakly adsorbed to soil (Table 1). SFE of imazaquin (2-[4,5-dihydro-4-(1-methyl-4-(1-methy- lethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid), was reported by Reddy and Locke (1994). Imazaquin applied on tissue paper was recovered with over 90% efficiency, but recovery dropped to 55% to 64% from soil (Reddy and Locke 1994). The lower recovery in soil was explained by incomplete contact of supercritical CO 2 with the sorptive sites and/or the inability of supercritical CO 2 to compete with the analytes for the sorptive sites (Hawthorne 1990; Reddy and Locke 1994). Also, Hawthorne (1990) states that supercritical CO 2 is more efficient in extracting nonpolar compounds than polar com- pounds, like the imidazolinones. Solid-phase extraction (SPE) has several of the same advan- tages over conventional extraction methods that SFE possesses (Majors and Raynie 1997). The solid phase is prepared commercially as columns or disks, and herbicides can be extracted from water using both forms of SPE. Several solid phases can be used, including C-2, C-8, C-18, aminopropyl, and cation or anion exchanges (Majors and Raynie 1997). Disks are more desirable than columns for water samples because the flow rate of the sample through the disk is high and samples containing particulates can be processed through filter media that are placed on top of the SPE disk (Markell et al. 1990; Majors and Raynie 1997). SPE cartridges and disks have mainly been used for the extraction of compounds from water samples (Benfenati et al. Correspondence to: S. Senseman Arch. Environ. Contam. Toxicol. 37, 440–444 (1999) ARCHIVESOF E nvironmental Contamination and T oxicology  1999 Springer-Verlag NewYork Inc.