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Structural characterization of product ions by electrospray ionization and quadrupole time-of-flight mass spectrometry to support regulatory analysis of veterinary drug residues in foods

Geis-Asteggiante, Lucia, Nuñez, Alberto, Lehotay, Steven J., Lightfield, Alan R.
Rapid Communications in Mass Spectrometry 2014 v.28 no.10 pp. 1061-1081
adrenal cortex hormones, beta-lactams, drug residues, fluoroquinolones, foods, ionization, ions, liquid chromatography, macrolides, mass spectrometry, monitoring, nonsteroidal anti-inflammatory agents, sulfonamides, tetracyclines, veterinary drugs
RATIONALE: Monitoring of veterinary drug residues in foods is often conducted using liquid chromatography – tandem mass spectrometry (LC-MS/MS). Results have high economic stakes for producers, but the ions monitored are usually selected due to signal intensities without structural interpretation. In this study, the ion transitions were characterized by high-resolution mass spectrometry METHOD: The 62 veterinary drugs from the LC-MS/MS method consisted of sulfonamides, beta-lactams, phenicols, macrolides, tetracyclines, fluoroquinolones, NSAIDs, and corticosteroids. They were individually infused into a time-of-flight quadrupole mass spectrometer (Q-TOF) using electrospray ionization (ESI) operated in the positive mode. The collision-induced dissociation MS and MS/MS spectra for each analyte were obtained for structural elucidation. The Q-TOF instrument was calibrated to obtain a mass accuracy error <5 ppm for the MS and MS/MS spectra. RESULTS: The use of high resolution ESI-Q-TOF for the generation of the MS/MS fragments allowed for the determination of molecular formulas for the analytes, some of which led to new findings. Assigned structures were based on rational interpretation of the most stable possible products with comparison to the scientific literature. In difficult cases, isotopically-labeled drugs or H/D exchange experiments were used help confirm the structures of the product ions. CONCLUSIONS: The use of Q-TOF in this study has allowed structure elucidation of 186 MS/MS ion fragments previously selected for LC-MS/MS analysis of 62 veterinary drugs. This serves to reduce chances for false positives and negatives in the monitoring program, and provides justification and defense in regulatory enforcement actions.