Measurement of organochlorine pesticides in drinking water: laboratory technical proficiency testing in Mexico

An interlaboratory comparison exercise was performed to assess the technical proficiency of laboratories in measuring the organochlorine pesticides in drinking water. The homogeneity and stability of the samples to be used in the test were evaluated, the assigned value and the standard deviation were determined according to ISO Guide 35 and ISO standard 17043, and the results showed that the samples were homogeneous and stable for the study period. The test samples (previously verified, as pesticide-free) and the fortifying solution were distributed to nine laboratories in Mexico and analyzed using their own established analytical methods. The pesticides in the fortifying solution were aldrin, beta-endosulfan, heptachlor, lindane (gamma-HCH), and p,p '-DDE. The assigned values and reference standard deviations were compared with those obtained by each laboratory to determine the z-score for each pesticide and laboratory, and the results are shown graphically for each pesticide. The acceptable z-scores obtained by the participating laboratories were 20 %, 20 %, 20 %, 40 %, and 16.7 % for aldrin, beta-endosulfan, heptachlor, lindane, and p,p '-DDE, respectively. This study also provides a summary of the sample preparation and chromatographic techniques used by the participating laboratories, which highlights the technical problems revealed by the interlaboratory exercise.