Difference between Consensus Value of Participants' Results and Isotope-Dilution Mass Spectrometric Results in Proficiency Testing for Pesticide Residues in Husked Wheat

Abstract

Proficiency testing was organized by the National Metrology Institute of Japan (NMIJ) as a measure of analytical competency in the quantification of pesticide residues in husked wheat powder. Seventy-one participants submitted analytical concentrations of the target pesticides (diazinon, fenitrothion, malathion, and etofenprox) along with details of the analytical method employed. Two types of assigned values were obtained for each target pesticide, i.e., the participants' analytical results and the results obtained by NMIJ based on isotope-dilution mass spectrometry (IDMS). The former values were lower than the latter due to the incomplete recovery yield of the target pesticides during the pretreatment process. The discrepancy between the two assigned values was particularly pronounced for malathion because of the longer duration of water-soaking used for the test samples. Two corresponding types of z-scores were then calculated to evaluate the analytical performance of the participants, where the z1-score indicates the performance of a participant relative to all participants, and the z2-score indicates the relative deviation of the analytical results of the participant from the IDMS value.

Proficiency testing by the National Metrology Institute of Japan for quantification of pesticide residues in grain samples from 2012 to 2018

“…With regard to sample homogeneity, no statistically significant differences in pesticide concentrations between bottles were observed for any PT samples. 1,[8][9][10] For every PT, the results of the stability assessment indicated that the differences in concentrations before and after distribution of the samples to participants were not significant. 1,[8][9][10] Our samples were beneficial for participants wishing to evaluate their method performance, including extraction efficiency.…”

“…This can be mainly attributed to the different quantification method used. 1,[8][9][10] Most participants used an external or internal standard method to determine the concentration values, and the recovery yields of the target compounds will influence the results if not adequately corrected for. This is distinct from the IDMS method used by the NMIJ.…”

“…7) The National Metrology Institute of Japan (NMIJ) has been organizing PT since 2012 for the quantification of pesticide residues in food. 1,[8][9][10] The NMIJ PT has been carried out seven times so far, and Table 1 summarizes these PT sessions. These target pesticides described in Table 1 were selected because they are widely used in Japan and most of their stabilities have been confirmed.…”

“…Performance is evaluated using two types of z-scores, one based on consensus values calculated from the analytical results reported by participants and one based on reference values obtained by the NMIJ. 1,8,10) 3. A follow-up lecture is held for information exchange about PT results, quality control, the analytical method, and so on.…”

“…In this technical report, an overview of the NMIJ PT results is given, specifically, the availability of test samples, comparisons of consensus and NMIJ reference values, factors potentially influencing the analytical results, and the effects of participation in this PT program. 1,[8][9][10] The raw grain materials (soybean, brown rice, and husked wheat) used for PT samples were grown and sprayed with various target pesticides (the conditions of pesticide spraying are shown in the supplemental information). Only fenitrothion was included in all PT samples, so as to observe the changes in the analytical results of participants.…”