Estimation of measurement uncertainty in pesticide residue analysis

Proficiency test results from 5 countries involving 61 separate interlabora tory proficiency tests for pesticide residues were examined in this study. A total of 24 different matrixes and 869 relative standard deviations of th e mean (or median) pesticide residue concentration were statistically evalu ated in relation to the Horwitz function. The aim was to determine whether or not the concentration-dependent relationship described by Horwitz would hold for the much narrower range of chemicals and concentrations covered in routine pesticide residue analysis. Although for fatty (animal-derived) ma trixes the variability increased as the concentration decreased in line wit h the Horwitz equation, the between-laboratories relative standard deviatio ns for nonfatty matrixes (fruit, vegetables, and grain) remained at 25% ove r the entire concentration range of 1 mug/kg to 10 mg/kg for the pesticides studied. Given these findings, the Horwitz equation remains valid for calc ulating uncertainties involving pesticide residues in fatty matrixes. Howev er, for pesticide residue analyses involving nonfatty matrixes, a constant relative standard deviation of 25% is more appropriate for calculating unce rtainties, particularly when a reported result is assessed against a regula tory limit.