When determining the phenoxycarboxylic acid 2,4-D and its esters and conjugates, it was necessary to carry out a hydrolysis (validation analysis) in the event of a positive result using the multi-method in order to fulfill the legal residue definition. The hydrolysis of the acid esters and conjugated metabolites required in the residue definition for phenoxycarboxylic acids is not trivial and very matrix-specific. Six laboratories did not perform hydrolysis, which means that the QS and the legal requirements were not properly implemented. In routine analysis, this approach could lead to the failure to collect residue-relevant phenoxycarboxylic acid esters or their conjugates, which could lead to underreporting.
There were increased quantification errors in the active substances cyantraniliprole (multimethod) and chlorate (single method), which have already been tested several times in proficiency tests. Cyantraniliprole analysis is challenging because recoveries are often too high (matrix effect). This was also evident in this test, in which the quantification errors were above 120% of the spiked value in two-thirds of the cases. The highly polar active substance chlorate, which has to be determined using a special method, shows a similar picture. The analysis of such substances is fundamentally demanding and requires laboratories to have the appropriate experience, particularly in dealing with matrix influences on the separation and detection systems used. This is where the absolute necessity of proficiency testing becomes apparent in order to verify the performance of laboratories and to train new laboratories. It is crucial to identify and correctly quantify all active substances, even in complex matrices.
