Ultraviolet spectrophotometric and thin-layer chromatographic determination of pirimiphos-ethyl and pirimiphos-methyl in insecticide formulations

Abstract

Two methods are described for determining pirimiphos-ethyl and pirimiphosmethyl with an error within ±2%. In the ultraviolet spectrophotometric method, a solution of the sample in chloroform is purified by column chromatography on Florisil. The insecticides are then hydrolysed in boiling hydrochloric acid to 2-diethylamino-4-hydroxy-6-methylpyrimidine, which is extracted from the aqueous solution, buffered at pH 6.5, into chloroform and determined absorptiometrically at 298 nm in methanol. The thin-layer chromatographic method consists of development of the chromatogram on silica gel GF₂₅₄ with hexane-acetone (17 + 3). The isolated insecticide is extracted into methanol with a vacuum extractor and the absorbance measured at 248 nm. Results for pirimiphos-ethyl and -methyl obtained on technical samples and emulsifiable concentrates by the proposed methods compare favourably with those obtained by gas chromatography, but the thin-layer and gas-chromatographic methods, which are more specific than the ultraviolet spectrophotometry, are preferred for application to samples that have been stored under adverse conditions. Both proposed methods are applicable to other formulations, such as powder seed dressings and granules. Pirimiphos-ethyl and -methyl can be separated either by thin-layer chromatography or by gas chromatography.