Differentiation of regioisomers of sulfobenzoic acid by traveling-wave ion mobility mass spectrometry

An ion mobility mass spectrometry (IM-MS) investigation using a Synapt G2 mass spectrometer was conducted to separate anions generated from the three regioisomers of sulfobenzoic acid. The results revealed that the differences in arrival time distributions (ATDs) were inadequate to differentiate the isomers unambiguously. However, the ATD profiles of the product ions, generated by fragmenting the respective mass-selected m/z 201 precursor ions in the Trap region of the three-compartment traveling-wave ion guide of the Synapt G2 mass spectrometer, were distinctly different, enabling definitive differentiation of the isomers. An arrival-time peak for an ion of m/z 157 resulting from the loss of CO₂ from the respective precursors was common to all three mobilograms. However, only the profile recorded from the para-isomer exhibited a unique arrival-time peak for an ion of m/z 137, originating from an SO₂ loss. Such a peak corresponding to an SO₂ loss was absent in the ATD profiles of the ortho- and meta-isomers. Additionally, the mobilogram of the meta-isomer displayed a unique peak at 3.42 ms. Based on its product ion spectrum, this peak was attributed to the bisulfite anion (m/z 81; HSO₃ˉ). Previously, this meta-isomer specific m/z 81 ion had been proposed to originate from a two-step process involving the intermediacy of an m/z 157 ion formed by CO₂ loss. However, our detailed tandem mass spectrometric experiments suggest that the m/z 81 is not a secondary product but rather an ion that originated from a direct elimination of a benzyne derivative from the m/z 201 precursor ion.