title: Signal suppression can bias selected reaction monitoring ratios. Implications for the confirmation of positive findings in residue testing.
abstract: BACKGROUND
  Selected reaction monitoring (SRM) ratios based on two or more transitions are commonly used to confirm the identity of a suspected finding in residue testing. International norms like the EU directive commission decision 2002/657/EC (CD) require the use of such ratios to prove the unequivocal identification of a particular compound detected at trace level (confirmation of a suspected residue).
  
  
  METHODS
  In this study, the relative abundances of different precursor ions (e.g. protonated molecule, solvent adducts, characteristic fragment ions or multiply charged ions) derived from the same analyte molecule were found to be differently (asymmetrically) affected by matrix-related signal suppression effects. This observation was made when analyzing veterinary drug residues (colistin and amoxicillin) in animal tissue extracts.
  
  
  RESULTS
  The term 'asymmetric signal suppression' was coined since different ionic species produced from the same analyte molecule are differently (asymmetrically) suppressed by co-eluting matrix compounds. In the case of the colistin assay, the extent of asymmetric signal suppression is such that the measured SRM ratios lie beyond the defined (CD) tolerances. Hence, the compound present in a sample cannot be confirmed.
  
  
  CONCLUSIONS
  The [M+H](+) ion may be the most commonly used precursor ion in liquid chromatography coupled to electrospray operated in the positive ionization mode. However, the absence of a sufficiently intensive confirmation transition frequently leads to the selection of another precursor ion to be utilized for the confirmation transition. The SRM ratio derived from such transitions should not be compared to the SRM ratio derived from pure standard solutions but spiked blank matrix extracts.
authors.affiliation: Official Food Control Authority, Fehrenstrasse 15, 8032, Zürich, Switzerland.
authors.forename: Anton
authors.initials: A
authors.lastname: Kaufmann
authors.forename: Miryam
authors.initials: M
authors.lastname: Widmer
authors.forename: Kathryn
authors.initials: K
authors.lastname: Maden
mesh_terms: D002853:Chromatography, Liquid / Q000379:methods*; D004350:Drug Residues / Q000032:analysis* / Q000737:chemistry / Q000302:isolation & purification; D007477:Ions; D013058:Mass Spectrometry / Q000379:methods*; D008956:Models, Chemical
chemical_list: D007477:Ions