title: 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. Part 2: Benzimidazoles, nitromidazoles, phenothiazines, and mectins.
abstract: BACKGROUND
  Analysis for identification and quantification of regulated veterinary drug residues in foods is usually achieved by liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS). The instrumental method requires the selection of characteristic ions, but structural elucidation is seldom performed to help ensure accuracy. This study is a continuation of previous work to characterize selected product ions in support of regulatory monitoring programs.
  
  
  METHODS
  The tandem mass spectra of 28 veterinary drugs from a previously published LC/MS/MS method were acquired with a high-resolution quadrupole time-of-flight (Q-TOF) mass spectrometer using electrospray ionization (ESI) in positive mode. The TOF analyzer was calibrated to achieve a mass accuracy error <5 ppm for the MS and MS/MS modes, and samples were infused for data acquisition.
  
  
  RESULTS
  The high mass accuracy achieved in Q-TOF allowed elucidation of the formulae of the product ions previously selected for qualitative identification. Rational interpretation of results was made and compared with the published literature, and the structure for the MS/MS product ions of four classes of regulated drugs (mectins, benzimidazoles, nitroimidazoles, and phenothiazines), totaling 28 compounds, were examined leading to the report of new structures or confirmation of published structures using low-resolution MS.
  
  
  CONCLUSIONS
  Structural characterization of the product ions selected for identification and quantification of veterinary drug residues is important information for regulatory monitoring programs in defense of regulatory enforcement actions. This study has allowed structural elucidation of 84 MS/MS product ions previously selected for the LC/MS/MS analysis of 28 drug analytes.
authors.affiliation: US Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA 19038, USA.
authors.forename: Alberto
authors.initials: A
authors.lastname: Nuñez
authors.affiliation: US Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA 19038, USA.
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Lehotay
authors.affiliation: US Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA 19038, USA.|Cátedra de Farmacognosia y Productos Naturales, DQO, Facultad de Química, UdelaR, 2124 General Flores, Montevideo12800, Uruguay.
authors.forename: Lucía
authors.initials: L
authors.lastname: Geis-Asteggiante
mesh_terms: D001562:Benzimidazoles / Q000032:analysis; D004350:Drug Residues / Q000032:analysis*; D005504:Food Analysis / Q000379:methods*; D009593:Nitroimidazoles / Q000032:analysis; D010640:Phenothiazines / Q000032:analysis; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry / Q000379:methods; D019155:Veterinary Drugs / Q000032:analysis*
chemical_list: D001562:Benzimidazoles; D009593:Nitroimidazoles; D010640:Phenothiazines; D019155:Veterinary Drugs