Determination of residual organophosphorus thioester pesticides in agricultural products by chemical isotope-labelling liquid chromatography-tandem mass spectrometry coupled with in-syringe dispersive solid phase clean-up and in situ cleavage.

Chemical isotope labelling in combination with high-performance liquid chromatography-tandem mass spectrometry (CIL-HPLC-MS/MS) is a powerful method for quantitative profiling of targeted molecules. In the current work, we successfully developed a novel CIL-HPLC-MS/MS method for quantitative profiling of residual organophosphorus thioester pesticides (OPTPs) in agricultural products through the determination of the cleavage products of thiol (CP-thiol) compounds. In this method, we synthesized a novel pair of CIL reagents, i.e., N-(4-(carbazole-9-yl)-phenyl)-N-maleimide (NCPM-d0) and its deuterated analogue NCPM-d2, both of which contain a maleimide moiety as the reactive group and an isotope tag to sensitively label CP-thiol compounds. NCPM-d0 was used to label CP-thiol compounds cleaved from OPTPs in the investigated agricultural product samples, and NCPM-d2 was used to label CP-thiol compounds cleaved from OPTPs in the standard substance-spiked organic agricultural product samples. The heavily labelled derivatives were used as the internal standards (ISs) to compensate for the matrix effects during MS analysis. The NCPM-d0- and NCPM-d2-labelled derivatives generated two characteristic product ions (PIs) at m/z 372.5 and 374.5 under collision induced dissociation, respectively, which are used to establish the multiple reaction monitoring (MRM) mode-based detection. The precursor ions of NCPM-d0 and NCPM-d2 labelled derivatives of CP-thiol compounds were deduced according to the structures of the OPTPs. The peak pairs with a fixed mass difference and similar retention times were assigned as potential CP-thiol candidates for the identification of the corresponding OPTPs. Using the proposed method, we successfully determined seven residual OPTPs in agricultural product samples. Taken together, the presented method was demonstrated to be a promising new technique in the quantitation of OPTPs in agricultural product samples with high reliability.