Comparison of external and internal standard methods in pesticide residue determinations

Abstract

A qualitative and quantitative gas-chromatographic pesticide-residue analysis method for 21 pesticides sensitive to the nitrogen–phosphorus detector was evaluated. The two methods of quantitative determination that were compared were the external standard (ES) method and the internal standard (IS) method. The methods were evaluated for the precision and linearity of the detector response. Precision was assessed by examining repeatability and reproducibility of the detector response. The RSD values found were lower than 20%. A better repeatability and reproducibility were observed with the IS method. The linearity of the detector was found to be satisfactory; by comparing the correlation coefficients found by the two methods, no significant difference was observed. The results were evaluated for the effect of the following factors: solvent of the final extract and time of determination. From a statistical comparison of the results of the IS and the ES methods, it was found that with the ES method, both factors were significant for the determination, and there was correlation between the two factors. With the IS method, only the solvent of the final extract had a significant effect on the results and no correlation was observed in most cases between the two factors.