Oxidative Ionization Under Certain Negative-Ion Mass Spectrometric Conditions

Isra Hassan, Julius Pavlov, Ramu Errabelli, Athula B. Attygalle

Research output: Contribution to journalArticlepeer-review

18 Scopus citations

Abstract

1,4-Hydroquinone and several other phenolic compounds generate (M – 2) –• radical-anions, rather than deprotonated molecules, under certain negative-ion mass spectrometric conditions. In fact, spectra generated under helium-plasma ionization (HePI) conditions from 1,4-hydroquinone and 1,4-benzoquinone (by electron capture) were practically indistinguishable. Because this process involves a net loss of H and H + , it can be termed oxidative ionization. The superoxide radical-anion (O 2 –• ), known to be present in many atmospheric-pressure plasma ion sources operated in the negative mode, plays a critical role in the oxidative ionization process. The presence of a small peak at m/z 142 in the spectrum of 1,4-hydroquinone, but not in that of 1,4-benzoquinone, indicated that the initial step in the oxidative ionization process is the formation of an O 2 –• adduct. On the other hand, under bona fide electrospray ionization (ESI) conditions, 1,4-hydroquinone generates predominantly an (M – 1) ion. It is known that at sufficiently high capillary voltages, corona discharges begin to occur even in an ESI source. At lower ESI capillary voltages, deprotonation predominates; as the capillary voltage is raised, the abundance of O 2 –• present in the plasma increases, and the source in turn increasingly behaves as a composite ESI/APCI source. While maintaining post-ionization ion activation to a minimum (to prevent fragmentation), and monitoring the relative intensities of the m/z 109 (due to deprotonation) and 108 (oxidative ionization) peaks recorded from 1,4-hydroquinone, a semiquantitative estimation of the APCI contribution to the overall ion-generation process can be obtained. [Figure not available: see fulltext.]

Original languageEnglish
Pages (from-to)270-277
Number of pages8
JournalJournal of the American Society for Mass Spectrometry
Volume28
Issue number2
DOIs
StatePublished - 1 Feb 2017

Keywords

  • APCI
  • Ambient ionization
  • ESI
  • HePI
  • Hydroquinone
  • Oxidative ionization
  • Thermometer ions

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