“Meta Elimination,” a Diagnostic Fragmentation in Mass Spectrometry

Abstract

The diagnostic value of the "ortho effect" for unknown identification by mass spectrometry is well known. Here, we report the existence of a novel "meta effect," which adds to the repertoire of useful mass spectrometric fragmentation mechanisms. For example, the meta-specific elimination pathway described in this report enables unequivocal identification of meta isomers from ortho and para isomers of carboxyanilides. The reaction follows a specific path to eliminate a molecule of meta-benzyne, from the anion produced after the initial decarboxylation of the precursor. Consequently, in the CID spectra of carboxyanilides, a peak for the (R-CO-NH) -anion is observed only for the meta isomers. For example, the peaks observed at m/z 58, 86, 120, 128, and 170 from acetamido-, butamido-, benzamido, heptamido-, and decanamido-benzoates, respectively, were specific only to the spectra of meta isomers.

Fragmentation of Protonated N-(3-Aminophenyl)Benzamide and Its Derivatives in Gas Phase

“…In those cases, the loss of nitroso (nitrogen and oxygen exchange occurred before the loss of nitroso) was observed and the fragmentation may also be enhanced by the so-called Bortho effect^ [41]. An observation of Bmeta effectô n fragmentation of carboxyanilides was made by Attygalle and co-workers in negative CID spectra [2]. It was found that a peak for the (R-CO-NH) -anion was only observed for the meta isomers; however, no rearrangement was involved in this fragmentation.…”

“…Tremendous efforts have been made with MS techniques for the identification of positional aromatic isomers [1][2][3][4][5][6][7][8]. Unlike radical ions generated with electron impact (EI) ionization [9], most of the modern atmospheric ionization (API) methods produce even-electron gaseous ions.…”

“…In some cases, the generation of one or more fragments may be diagnostic to one of the positional isomers. Experimental examples of Bortho effect [ 2,6,[10][11][12][13][14][15][16][17][18][19][20][21] and Bmeta effect^ [2,7,22,23] of positional aromatic compounds have been recognized with different mass spectrometry techniques.…”