Structural characterization of fragment ions by electrospray ionization and Q-TOF mass spectrometry to support regulatory analysis of veterinary drug residues in foods

Authors: Nunez, Alberto; GEIS-ASTEGGIANTE, LUCÍA - University Of Maryland; Lehotay, Steven; Lightfield, Alan

Submitted to: Rapid Communications in Mass Spectrometry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2014
Publication Date: 5/30/2014
Publication URL: http://handle.nal.usda.gov/10113/58696
Citation: Nunez, A., Geis-Asteggiante, L., Lehotay, S.J., Lightfield, A.R. 2014. Structural characterization of fragment ions by electrospray ionization and Q-TOF mass spectrometry to support regulatory analysis of veterinary drug residues in foods. Rapid Communications in Mass Spectrometry. 28(10):1061-1081.

Interpretive Summary: Veterinary drugs are frequently used in animal production, but misuse can lead to illegal residues in food. Countries have set laws to limit residue levels, which have become a major trade issue involving billions of dollars. In the USA, the USDA Food Safety and Inspection Service (FSIS) is responsible for surveillance and enforcement monitoring of veterinary drug residues in meat and poultry products. A common method for monitoring drug residues is by the use of ion fragments produced by mass spectrometry in association with high-pressure liquid chromatography. However ion fragments been monitored are usually selected due to signal intensities instead of being correctly selected on their unique structural characteristics, potentially leading to false positives. In this work modern high-resolution mass spectrometry has been used to elucidate the structure of ions for 62 veterinary drugs that are of concern and are routinely use for detection of residues in food. The use of this technology has allowed the precise determination of molecular formulas of the ion fragments permitting the elaboration of molecular structures; consequently, more effective methods can be developed for the identification of veterinary drug residues in food.

Technical Abstract: 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.