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

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.