Formation of the bisulfite anion (HSO 3 – , m/z 81) upon collision‐induced dissociation of anions derived from organic sulfonic acids

In the negative‐ion collision‐induced dissociation mass spectra of most organic sulfonates, the base peak is observed at m/z 80 for the sulfur trioxide radical anion (SO 3 –· ). In contrast, the product‐ion spectra of a few sulfonates, such as cysteic acid, aminomethanesulfonate, and 2‐phenylethanesulfonate, show the base peak at m/z 81 for the bisulfite anion (HSO 3 – ). An investigation with an extensive variety of sulfonates revealed that the presence of a hydrogen atom at the β ‐position relative to the sulfur atom is a prerequisite for the formation of the bisulfite anion. The formation of HSO 3 – is highly favored when the atom at the β ‐position is nitrogen, or the leaving neutral species is a highly conjugated molecule such as styrene or acrylic acid. Deuterium‐exchange experiments with aminomethanesulfonate demonstrated that the hydrogen for HSO 3 – formation is transferred from the β ‐position. The presence of a peak at m/z 80 in the spectrum of 2‐sulfoacetic acid, in contrast to a peak at m/z 81 in that of 3‐sulfopropanoic acid, corroborated the proposed hydrogen transfer mechanism. For diacidic compounds, such as 4‐sulfobutanoic acid and cysteic acid, the m/z 81 ion can be formed by an alternative mechanism, in which the negative charge of the carboxylate moiety attacks the α ‐carbon relative to the sulfur atom. Experiments conducted with deuterium‐exchanged and deuterium‐labeled analogs of sulfocarboxylic acids demonstrated that the formation of the bisulfite anion resulted either from a hydrogen transfer from the β ‐carbon, or from a direct attack by the carboxylate moiety on the α ‐carbon. Copyright © 2012 John Wiley & Sons, Ltd.