Fab mass spectra of nitrogen heterocycles are the protonated molecular ions predominant peaks?

✍ Scribed by Qingmei Zha; Michel J. Bertrand

• Publisher: John Wiley and Sons
• Year: 1990
• Tongue: English
• Weight: 162 KB
• Volume: 25
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/oms.1210250811

✦ Synopsis

Recently, considerable interest has been focused on the observations that the predominant quasimolecular ions observed are [M-H]+ rather than [M+H]+ in positively charged FAB mass ~p e c t r o m e t r y . l -~ Several reaction pathways have been proposed to determine the origin of the predominant [M-H]+ ions. It was suggested that this phenomenon is a result of the transfer of redox equivalents between neutral molecular species during the bombardment desorption event, namely, the net loss of H. would result in [M-HI+ formation' or of the hydride abstraction similar to that in gas-phase chemical ionization,* or of the protonation induced rearrangement in FAB.3 Although further studies are needed to finally elucidate the reaction mechanism, it is clear that reported predominant [M-H]+ ions are observed from organic compounds containing oxygen and/or sulphur as heteroatoms. In order to find out what class of organic compounds would selectively produce predominant [M-H]+ ions in FAB mass spectrometry, we came across an early report5 that some nitrogen heterocycles do not produce protonated molecular ions as the predominant peaks, but that the most intense peaks in the molecular ion region correspond to [M-HI+, or to M+* and [M+2]+-. It is worth reexamining these nitrogen heterocycles to avoid misinterpreting FAB mass spectra of unknown compounds in future.