Validation of a multi-residue method for the determination of pesticides

Abstract

The validation of a multi-residue method is presented for the extraction and quantitative determination of some widely used pesticides in wheat, strawberry and apple. Validation was carried out by an interlaboratory trial. The method involved an ethyl acetate I cyclohexane extraction, removal of coextracts by gel permeation chromatography on an SX-3 column and determination of residues by gas liquid chromatography. This method was not only aimed at these three crops but can be extended to a wider variety. It was also used on different classes of pesticides confirming its suitability as a multi-residue method. Chlorpyriphos-methyl, deltamethrin, lindane, permethrin and pirimiphos methyl were used on wheat samples. Bupirimate, chlorpyriphos, dichlofluanid and iprodione were used on the strawberry samples while bromopropylate, captan and phosalone were used on the apple samples. Gas liquid chromatography was used in the separation and quantitation of the pesticide mixtures. An electron capture detector was used in the determination of most pesticides while a thermionic sensitive detector was used in the determination of pirimiphos-methyl in the wheat samples. A split/less injection was used and appropriate temperature gradients enabled efficient separation and hence quantitation. Incurred and spiked samples were analysed according to an analytical protocol supplied by the coordinating laboratory. Varying results were obtained for different pesticides in different matrices. These results were improved by the use of matrix matched calibration solutions which minimised the matrix effects. Wheat turned out to be the most difficult to analyse since low recoveries were obtained for some pesticides like pirimiphos methyl (43 %) and chlorpyriphos-methyl (42 %) in the spiked wheat samples. Better recoveries were however obtained for the strawberry and apple samples in agreement with the results obtained by the other participating laboratories. Good recoveries ranging from 65 to 80 %, were in fact obtained for chlorpyriphos, bupirimate and iprodione in strawberry and for bromopropylate, captan and phosalone in apple.