This study reports on the efforts to ensure homogeneity of certified reference materials. Residues of four pesticides (γ-HCH, fenitrothion, chlorpyrifos, and procymidone) were added to tomato pulp in order to produce reference materials. The two most important properties of such materials are known to be homogeneity and stability. Before being shipped to other laboratories, reference materials must have their homogeneity certified. As part of a preliminary study, hundreds of samples were investigated in order to provide data on the most suitable treatments to minimize analytical variability and ensure the quality of samples intended for use as reference materials as well as to yield estimates of the uncertainty associated to the homogeneity data. In addition, the variance analysis provided accurate information on the bulk variability and the degree of non-homogeneity of contaminated samples. After preparation, the samples were exposed to 2 kGy of gamma radiation and submitted to an interlaboratory certification study. The certified concentrations of γ-HCH, fenithrotion, chlorpyrifos, and procymidone were 0.191 ± 0.047 mg.kg-1; 0.192 ± 0.068 mg.kg-1; 0.225 ± 0.076 mg.kg-1 , and 0.177 ± 0.051 mg.kg-1, respectively.
pesticides; tomato; reference material; homogeneity
