Sample Matrix Interference in Immunoassays for Organochlorine Residues in Plant-derived Foods and Some Strategies for Their Removal

Abstract

Immunoassays for two groups of organochlorine insecticides, cyclodienes (endosulfan and heptachlor) and DDT were applied to the analysis of a diverse range of plant-derived foods. Water-miscible solvent extracts of high-moisture, low-fat foods such as cauliflower, cabbage, green and red blue grapes and tomato caused little or no interference with the assays, enabling methanol or acetonitrile extracts of the foods to be analysed directly by immunoassay, after dilution in assay buffer. Reasonable recoveries of spikes of these pesticides were obtained by direct analysis of extracts of spiked commodities, with reliable detection down to 0.025 mg kg−1 heptachlor or endosulfan and 0.1 mg kg−1 DDT in the commodities. Acetonitrile extracts of milk could also be analysed directly for DDT. In contrast, extracts of low moisture, non-fatty (rice) and fatty (cottonseed) food commodities interfered appreciably with the assays, reducing assay colour and detection sensitivity. Some simple cleanup methods were developed to remove interference and enable detection of spiked organochlorines in these foods. Extracts of coloured foods, such as tea, coffee and spinach caused similarly major interference in the assays, and a number of simple clean-up methods were ineffective in removing interference. However, use of an immunoaffinity chromatography method for cyclodienes enabled quantitative recoveries to be obtained in extracts of several of these foods when analysed by either ELISA or gas chromatography. Direct analysis was suited for screening purposes but immunoaffinity chromatography results were more quantitative. These results indicate that ELISAs can be applied under developing country conditions to a range of diverse foods, but that cleanup strategies need to be tailored to different types of foods.