Applicability of some mass spectrometric criteria for the confirmation of pesticide residues

This study was initiated to demonstrate the impact of tolerance intervals calculated based on experimental data and standard criteria on the capability to detect and identify pesticide residues by GC-MS in plant extract. The tolerance intervals specified by the current guidelines and standards (EU, EPA, ISO, EC) result in a large number of false negative values. The tolerance intervals calculated based on the actually measured ion ratios at 2 different probability levels did not provide a better estimate. Based on experimental findings a 2-phase procedure for confirmation of pesticide residues is recommended. The first screening phase comprises identification of those pesticide residues that are likely to be present avoiding false negatives as far as possible. Relatively large tolerance intervals, +/-30% of absolute ion abundances ratios, have to be used at this point. When 2 or 3 selected ion ratios are within the established tolerance intervals the residue is confirmed. When the ions detected still indicate the possible presence of a residue the result may be reported as tentatively identified. However, when the result would lead to regulatory action, second confirmatory phase has to be undertaken. This can be achieved with the same GC-MS equipment, by injecting matrix-matched standards of the suspected analyte, in order to compensate for matrix influence on ion ratios. Alternatively, methods using different physico-chemical properties of the compound have to be used. The choice of the technique depends on their availability, time and cost.