APPLICATION OF INTERNAL STANDARD FOR DERIVATIVE-SPECTROPHOTOMETRIC DETERMINATION OF AZINPHOS-METHYL IN COMMERCIAL FORMULATIONS

The use of an internal standard is proposed in this work for first der ivative spectrophotometric determination of azinphos in formulations. Generally, the spectrophotometric procedure is simpler and less expens ive than chromatographic techniques recommended for the analysis of pe sticides. However, while determining the pesticide in commercial formu lations the, many-fold dilution required for such sensitive detection is a serious source of analytical error. It is known that an internal standard (IS), if properly chosen, can help eliminate this type of pro blem. A mixture of acetophenone (used as an IS in the high performance liquid chromatography (HPLC) procedure) and the blue dye Erioglaucine (A) was applied for the determination of azinphos-methyl in commercia l formulations. To ensure the best conditions for the zero-crossing te chnique, the composition of the mixture was optimized to obtain the ze ro value of the first derivative absorbance of the IS at a minimum of the azinphos-methyl first derivative absorbance. Also, at the maximum of the first derivative spectrum of the IS, the differential absorptio n signal of the analyte was negligible. Analytical characteristics for the first derivative spectrophotometric procedure proposed were evalu ated (r(2) = 0.9998, detection limit = 0.043 mg, quantification limit = 0.143 mg) and the analytical. results obtained (35.02 +/- 0.20% of a zinphos-methyl in the formulation) were in good agreement with the res ults obtained using the official HPLC method (35.44 +/- 0.32%).