Impact of Ambient Vapors on Spectra of 4-Nitroaniline Recorded under Atmospheric Solids Analysis Probe (ASAP) Mass Spectrometric Conditions

Ishira Samarasinghe, Athula B. Attygalle

Research output: Contribution to journalArticlepeer-review

3 Scopus citations

Abstract

Thermally desorbed 4-nitroaniline (4-NA), upon atmospheric pressure chemical ionization (APCI), generates gaseous ions for its protonated species. The APCI mass spectrum recorded under mild in-source ion-activating conditions from 4-NA showed a peak at m/z 139, whereas that acquired under high ion-activating conditions showed two additional peaks at m/z 122 (OH loss) and 92 (NO loss). The spectrum changed instantaneously when acetonitrile vapor was introduced to the source. In the new spectrum, both m/z 122 and 92 peaks were absent, while a new peak appeared at m/z 93. Ion-mobility separation carried out with the m/z 139 ion revealed that the initial ion represented the thermodynamically favored nitro-protonated tautomer. The ion population changed to an ensemble dominated by the less-favored amino-protomer when acetonitrile vapor was introduced to the ion source. The amino-protomer, upon collisional activation, loses NO2 to generate an m/z 93 ion, which was confirmed to be the 4-dehydroanilinium ion. Ion mobility provided a practical way to monitor the changes secured by acetonitrile vapor because the two protomers showed different arrival times. Under spray-ionization conditions, the formation of the thermodynamically less favored protomer has been attributed to kinetic trapping. Our study demonstrated that the less favored amino-protomer could be generated by introducing acetonitrile vapor under nonspray conditions. Apparently, under APCI conditions, protonated water vapor attaches to the nitro group to generate a proton-bound heterodimer, which upon activation dissociates to yield the nitro-protomer. In contrast, protonated acetonitrile makes a tighter complex preferentially with the amino group, which upon activation breaks to the amino-protomer.

Original languageEnglish
Pages (from-to)205-217
Number of pages13
JournalJournal of the American Society for Mass Spectrometry
Volume34
Issue number2
DOIs
StatePublished - 1 Feb 2023

Keywords

  • 4-nitroaniline
  • APCI
  • ASAP
  • acetonitrile
  • amino-protomer
  • binding affinity
  • kinetic trapping
  • nitro-protomer
  • proton-bound heterodimer

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