Determination of Organophosphorus Pesticide Residues in Onion and Welsh Onion by Gas Chromatography with Pulsed Flame Photometric Detector

A rapid gas chromatographic method for determining organophosphorus pesticide residues in allium such as onion and welsh onion containing high levels of sulfur-matrices was studied. A sample was extracted with acetonitrile and the acetonitrile layer separated by salting-out. The extract was cleaned up with gel permeation chromatography, and then with a tandem silica-gel/PSA mini-column. The test solution was subjected to gas chromatography with a pulsed flame photometric detector. Organophosphorus pesticide residues in such sulfur-rich matrices were determined without any serious interfering peaks on the chromatograms by diluting the extracts 8-fold (0.25 g/ml of sample). No additional pretreatment to deactivate enzymes which caused interference was necessary. The rate of recovery of 36 organophosphorus pesticides from fortified onion and welsh onion ranged from 61 to 105% with the RSD usually < 10% for five experiments. The detection limits of these pesticides were good (0.002-0.01 mg/kg) for monitoring organophosphorus pesticide residues in agricultural products including allium, except for degradable trichlorfon. The method was applied to onion and welsh onion to demonstrate its use in routine analysis.