Aldicarb (2-methyl-2(methylthio)propionaldehyde O-(methylcarbamoyl)oxime, marketed as Temik@) is a toxic pesticide widely used in Florida on citrus, potato, and peanut crops to control nematodes. It is highly soluble in water (6000 ppm, w/w'). In soil, it is oxidized to aldicarb sulfoxide and aldicarb sulfone'J. Due to the porous nature of Florida's soils, these compounds have been detected in groundwater. As a result of the heavy usage of this pesticide, we felt it necessary to predict the fate of aldicarb, aldicarb sulfoxide, and aldicarb sulfone in the Floridan Aquifer using groundwater microcosms in the laboratory. Since each of these carbamoyl oximes degrades to the corresponding oximes and nitriles, we required an analytical method that could rapidly identify and quantitate these nine compounds in aqueous samples at low concentrations.
Several methods have been developed for analysis of aldicarb and degradation products in environmental samples but most have been developed for aldicarb, aldicarb sulfoxide, and aldicarb sulfone (often called total toxic residue, TTR)Z-14. Gas chromatogrpahic procedures are difficult because (a) many of these compounds are thermally unstable, (b) samples must be extracted and treated before analysis, and (c) speciation of the TTR requires an additional liquid chromatographic clean-up stepzm7. Trehy et al.5 reported a gas chromatographic-mass spectrometric method that is rapid and sensitive for aldicarb, aldicarb oxime, and aldicarb nitrile, but did not include aldicarb sulfoxide, aldicarb sulfone, and their oximes and nitriles. Thin-layer chromatography is also a very useful technique for monitoring pesticide degradation but usually requires radiolabelled compounds for sensitive detection9.
High-performance liquid chromatography (HPLC) offers a simple and rapid method for determination of aldicarb and its derivatives. HPLC with post-column reaction and fluorescence detection is a widely accepted method for the determination of very low concentrations of aldicarb, aldicarb sulfoxide, and aldicarb sulfone (all carbamoyl oximes)9p10. Although this technique is sensitive and selective for these compounds, it does not respond to the non-carbamoyl oxime derivatives such as the oximes and nitriles. HPLC with mass spectrometric detection is also a sensitive method that provides qualitative as well as quantitative information, but the high cost of this instrumentation precludes its use for routine analysis' l.
HPLC with UV detection has been used for the determination of aldicarb in vegetation but crop interferences limited its usefulness'*. Cochrane et all3