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Rationale: Previously, we have reported a liquid chromatography/tandem mass spectrometry method for the identification and quantification of regulated veterinary drugs in food animals. The method uses three selected transition ions per analyte but structural characterization is also needed. This work is a continuation of two previous publications in which we propose structures of the selected transition ions of 130 veterinary drugs altogether., Methods: In this work, 24 additional veterinary drugs were analyzed by infusion into a high-resolution quadrupole time-of-flight (QTOF) mass spectrometer using electrospray ionization (ESI) in positive or negative mode. The TOF analyzer was calibrated to achieve low error mass accuracy in the MS and MS/MS modes. Also, the MS(2) and MS(3) spectra were obtained by using a Q-Trap mass spectrometer to further determine the possible pathways of ion formation., Results: The low error mass spectrometry analysis allowed the elucidation of the ion formulae of selected transition ions for qualitative identification. The rational interpretation of data including a review of the published literature led to the proposed structures of the MS/MS product ions of 24 compounds covering two classes of regulated veterinary drugs (anthelmintics and thyreostats). In addition, the use of MS(2) and MS(3) experiments led to the establishment of fragmentation patterns., Conclusions: The identification and quantification of veterinary drug residues is helpful information for regulatory monitoring programs in defense of regulatory enforcement actions. Published in 2016. This article is a U.S. Government work and is in the public domain in the USA., (Published in 2016. This article is a U.S. Government work and is in the public domain in the USA.)

A liquid chromatographic-tandem mass spectrometric (LC-MS/MS) multi-residue method for the simultaneous quantification and identification of 38 residues of the most widely used anthelmintic veterinary drugs (including benzimidazoles, macrocyclic lactones, and flukicides) in milk and liver has been developed and validated. For sample preparation, we used a simple modification of the QuEChERS method, which was initially developed for pesticide residue analysis. The method involved extracting sample (10 g) with acetonitrile (10 mL), followed by phase separation from water (salting out) with MgSO(4):NaCl (4:1, w/w). After centrifugation, an aliquot of the extract (1 mL) was purified by dispersive solid-phase extraction with MgSO(4) (150 mg) and C(18) (50mg), prior to LC-MS/MS analysis. Two injections of the same extract were required with the LC-MS/MS instrument to cover the 30 electrospray positive and 8 electrospray negative analytes. The limit of quantitation of the method was 5 microgkg(-1) for 37 analytes (and 10 microgkg(-1) for dichlorvos). The method was successfully validated according to the 2002/657/EC guidelines. Recovery of analytes was typically in the 70-120% range, with repeatabilities and reproducibilities typically <15% in milk and <20% in liver.