Determination of residues of methomyl and oxamyl and their oximes in crops by gas-liquid chromatography of oxime trimethylsilyl ethers.
The gas-liquid chromatographic behavior of methomyl (methyl N-[(methyl-carbamoyl)oxy]thioacetimidate), oxamyl (methyl N',N'-dimethyl-N-[(methylcarbamoyl)oxyl]-1-thiooxamimidate), their respective oxime hydrolysis products and the trimethylsilyl (TMS) ethers of the oximes on 5% OV-1 was studied under isothermal conditions using a flame-photometric detector in the sulfur-selective mode. In contrast to the behavior of the parent carbamates and underivatized oximes, the oxime-TMS ethers readily produced symmetrical peaks of consistent size. Quantities of derivative equivalent to at least 0.25 ng of oxime were easily measurable. Derivative formation was reproducible for standards over the range 10.0 to 0.25 microgram/ml in benzene and at 10.0 and 0.50 microgram/ml in the presence of extractives from tomato, carrot and celery at concentrations equivalent to 10 g/ml of crop. Derivative yields from crop extract fotifications were 89% or better in most cases. Both the carbamates and oximes were simply and consistently recovered in high yield from crops fortified at 1.00 and 0.05 ppm using the procedures described. The inclusion of a second analytical step provided separate analysis for oximes and carbamates. The application of these observations to the analyses of residues in crops is discussed.