title: Unexpected peaks in tandem mass spectra due to reaction of product ions with residual water in mass spectrometer collision cells.
abstract: BACKGROUND
  Certain product ions in electrospray ionization tandem mass spectrometry are found to react with residual water in the collision cell. This reaction often leads to the formation of ions that cannot be formed directly from the precursor ions, and this complicates the mass spectra and may distort MRM (multiple reaction monitoring) results.
  
  
  METHODS
  Various drugs, pesticides, metabolites, and other compounds were dissolved in acetonitrile/water/formic acid and studied by electrospray ionization mass spectrometry to record their MS(2) and MS(n) spectra in several mass spectrometers (QqQ, QTOF, IT, and Orbitrap HCD). Certain product ions were found to react with residual water in collision cells. The reaction was confirmed by MS(n) studies and the rate of reaction was determined in the IT instrument using zero collision energy and variable activation times.
  
  
  RESULTS
  Examples of product ions reacting with water include phenyl and certain substituted phenyl cations, benzoyl-type cations formed from protonated folic acid and similar compounds by loss of the glutamate moiety, product ions formed from protonated cyclic siloxanes by loss of methane, product ions formed from organic phosphates, and certain negative ions. The reactions of product ions with residual water varied greatly in their rate constant and in the extent of reaction (due to isomerization).
  
  
  CONCLUSIONS
  Various types of product ions react with residual water in mass spectrometer collision cells. As a result, tandem mass spectra may contain unexplained peaks and MRM results may be distorted by the occurrence of such reactions. These often unavoidable reactions must be taken into account when annotating peaks in tandem mass spectra and when interpreting MRM results. Published in 2014. This article is a U.S. Government work and is in the public domain in the USA.
authors.affiliation: Biomolecular Measurement Division, National Institute of Standards and Technology, Gaithersburg, MD, 20899, USA.
authors.forename: Pedatsur
authors.initials: P
authors.lastname: Neta
authors.forename: Mahnaz
authors.initials: M
authors.lastname: Farahani
authors.forename: Yamil
authors.initials: Y
authors.lastname: Simón-Manso
authors.forename: Yuxue
authors.initials: Y
authors.lastname: Liang
authors.forename: Xiaoyu
authors.initials: X
authors.lastname: Yang
authors.forename: Stephen E
authors.initials: SE
authors.lastname: Stein
mesh_terms: D002412:Cations / Q000737:chemistry; D005492:Folic Acid / Q000031:analogs & derivatives / Q000737:chemistry*; D015203:Reproducibility of Results; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods; D053719:Tandem Mass Spectrometry / Q000379:methods*; D014867:Water / Q000737:chemistry*
chemical_list: D002412:Cations; D014867:Water; D005492:Folic Acid