Sensitive Immunoassays for Methyl-Parathion and Parathion and Their Application to Residues in Foodstuffs

Immunoassays, capable of detecting 0.05 g l⁻¹ and 0.5 g l⁻¹, respectively, have been developed to detect the organophosphate pesticides, methyl-parathion and parathion. Using haptens based on derivatization of the phosphate ester of the methyl and ethyl forms in the target compounds, there was selectivity in detection of methyl-parathion and parathion, respectively, using the two assays. Antisera to methyl-parathion detected parathion with 25-20% cross-reaction, while the parathion antisera detected methyl-parathion with 30-40% cross-reaction in water. The only other commonly-used agrochemical that cross-reacted in the assays was fenitrothion, the 3-methyl derivative of methyl-parathion. The assays were applied to the analysis of residues of these pesticides in water and several food matrices representative of different classes following the extraction of residues using simple procedures. Methanol extracts of most fruits and vegetables tested (high-moisture, low fat foods), including green and blue grapes, cauliflower and cabbage, could be analysed directly in the methyl-parathion assay, as could rice and basmati rice (low-moisture, low fat foods). Methanol extracts of butter and milk (high-fat foods) provided interference, but this was overcome by either further dilution or using acetonitrile as the extractant. A coagulating reagent was used to remove matrix interference from strongly coloured foods (tea and spinach). The parathion assay was subject to greater matrix interference, so it was preferable to analyse parathion in samples on plates coated with methyl-parathion antibody. With these foods, and with water samples, near-quantitative recoveries of spiked methyl-parathion or parathion were usually obtained, while with high-fat foods (milk and butter) and strongly coloured foods recoveries were poorer.