Spectrophotometric determination of methomyl residues in selected vegetables, grains, and soil.

Methomyl (S-methyl-N-[(methylcarbamoyl) oxy] thioacetimidate) is converted to oxime and hydroxylamine by alkali and acid treatment, respectively. Hydroxylamine is oxidized with iodine in the presence of sulfanilic acid to yield p-diazoniumbenzenesulfonic acid which is coupled with alpha-naphthylamine to form a crimson p-benzenesulfonic acid-azo-alpha-naphthylamine with an absorption maximum at 520 nm. The relationship between absorbance and concentration of methomyl is linear in the range 0.5-10 microgram. The method is sensitive and specific; 0.625 ppm methomyl can be determined in a 40 g sample of selected vegetables, grains, and soil.