A comparison of the external standard and standard addition methods for the quantitative chromatographic determination of pesticide concentrations in plant samples

Model samples containing known amounts of pesticides in various plant materials were analyzed, and the accuracies of the external standard and standard addition methods were compared based on a criterion such as the relative analyte concentration. It was found that traditionally recommended procedures for the solvent extraction of pesticides from samples of this kind in combination with quantitative analysis by the external standard method determine only 10–70% of the true concentrations of analytes. This is most likely due to analyte adsorption on matrices and/or incomplete extraction. In almost all cases, the use of the standard addition method compensates pesticide losses at the stages of sample preparation and thereby considerably decreases the systematic error of determination. The extrapolation of analyte concentration to the zero amount of the added reference sample was proposed to additionally control the accuracy of results obtained by the standard addition method. Moreover, it was found that solid-phase extraction as the second stage of sample preparation has no considerable advantages over back solvent extraction in terms of the above criterion. This is because the main analyte losses were observed at the first stage of extraction from test samples.