Proficiency testing for determination of pesticide residues in soybean: comparison of assigned values from participants' results and isotope-dilution mass spectrometric determination.

Proficiency testing (PT) for the determination of pesticide residues in soybean samples was organized by the National Metrology Institute of Japan (NMIJ). The candidate certified reference material, NMIJ CRM 7509-a, that was prepared from the raw soybeans containing target pesticides (diazinon, fenitrothion, chlorpyrifos, and permethrin) was used as the test sample. Forty participants submitted two sets of analytical results along with the details of the analytical method and conditions they applied. Two types of assigned values were established for each target pesticide: one was derived from the analytical results of the participants, and the other was provided from the analytical results by isotope-dilution mass spectrometry (IDMS). The latter values were 7.4-16% higher than the former values, plausibly because the analytical values from the IDMS measurements were not affected by the recovery ratio of the target pesticides during the analytical process. Thus, two kinds of z-scores were calculated for individual participants using the corresponding assigned values: one (z1-score) showed the relative performance score for the present PT and the other (z2-score) could be used for evaluation of the trueness of their analytical methods.