Reaction of arylium ions with the collision gas N-2 in electrospray ionization mass spectrometry

RATIONALE: The tandem mass spectra of many compounds contained peaks which could not have arisen from the precursor ion. Such peaks were found to be due to reaction of arylium ions with N-2 in the collision cell. Therefore, this reaction was studied in detail with representative compounds. METHODS: Various classes of compounds were dissolved in acetonitrile/water/formic acid and studied by electrospray ionization mass spectrometry to record their MS2 and pseudo-MS3 spectra in a QqQ mass spectrometer and their accurate m/z values in an Orbitrap Elite instrument. Arylium ions were found to react with N-2 in the collision cell. The reaction was confirmed by pseudo-MS3 studies, by comparison with authentic diazonium ions, and by the pressure dependence of the product ion survival yield. RESULTS: Reactions of arylium ions with N-2 were observed with p-toluenesulfonic acid, o-toluenesulfonamide, phenylphosphonic acid, phenol, aniline, aminonaphthalenes, benzoic acid, benzophenone, and other compounds. By using a QqQ mass spectrometer, we observed that the protonated compounds produce arylium ions, which then react with N-2 to form diazonium ions. The diazonium ion was produced with N-2 but not with Ar in the collision cell, and its abundance increased with increasing N-2 pressure. CONCLUSIONS: Arylium ions generated from a wide variety of compounds in electrospray ionization tandem mass spectrometry may react with N-2 to form diazonium ions. The abundance of the diazonium ions is affected by collision energy and N-2 pressure. This reaction should be considered when annotating peaks in MS/MS libraries. Published in 2015. This article is a U.S. Government work and is in the public domain in the USA.