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K. A. Johnson,* F. D. Harper, and C. P. Weisskopf
Abstract
Extraction methods using solid-phase extraction (SPE) cartridges are generally preferred over traditional liquid-liquid extraction methods when they are available. Solid-phase extraction methods require less preparative work, thus increasing sample through-put. A SPE method using a graphite carbon sorbent was developed to extract low concentrations (140 (mu)g/L) of the polar pesticide, aldicarb [2-methyl2-(methylthio)propionaldehyde O-(methylcarbamoyl) oxime], and its oxidative metabolites, aldicarb sulfoxide [2-methyl-2-(methylsulfinayl)propionaldehyde-O-(methylcarbamoyl) oxime] and aldicarb sulfone [2-methyl-2-(methylsulfonyl) propionaldehyde-O-(methylcarbamoyl) oxime] from water samples. The analytes were removed from a 150-mL water sample by the SPE cartridge and eluted with 2 mL of methanol. Recoveries from water for all three compounds exceeded 90% with coefficients of variation ranging from 3 to 9%. An extraction method was also developed to extract aldicarb and its metabolites from soil using water as the extractant. The analytes are then recovered from the water using the same SPE method referred to above. Recoveries from soil for all three compounds ranged from 80 to 95%. Our SPE method also worked well (recoveries >95%) for two other carbamates, methomyl [S-methyl N-[(methylcarbamoyl)oxy]thioacetimidate] and oxamyl [N', N'-dimethyl-N-[(methylcarbamoyl)oxy]-1thiooxamimidate] tested and should work with other polar compounds once they are in the aqueous phase.
PESTICIDE migration from agricultural areas has led to their detection in air, rain, groundwater, and surface waters. Runoff concentrations have ranged from parts per million (mg/L) to low parts per billion ((mu)g/L) (Baker and Richards, 1990; Leonard, 1990). The varying levels can lead to extraction and analysis problems (i.e., sample extracts that are below the limit of detection). Traditionally, liquid-liquid extraction (LLE) methods are used to determine aqueous pesticide concentrations. However, to detect trace quantities, excessively large volumes of water are necessary. Furthermore, LLE methods can require large amounts of hazardous solvents, involve the formation of emulsions, and are time consuming. Extraction methods are needed that are rapid, reproducible, and field adaptable.
Solid-phase extraction methods have gained popularity since the 1970s (Borra et al., 1986; Junk and Richard, 1988; Mangani et al., 1988; Hsu et al.,1991; Molto et al., 1991; Font et al., 1993). Solid-phase extraction involves passing a sample through a cartridge containing a solid sorbent; the analytes of interest partition into the sorbent where they are accumulated. After all...