Dependence of Collision-Induced Mass Spectra of Protonated Michler's Ketone on the Nature of LC-MS Mobile Phase.

Michler's ketone (MK) is a dimethylamino ketone that undergoes facile protonation under electrospray-ionization conditions to produce an ion of / 269. Initial LC-MS results showed that the collision-induced dissociation (CID) spectra of the / 269 ion depend heavily on the composition of the chromatographic mobile phase. Subsequent ion-mobility separation of the mass-selected / 269 ion revealed that protonated MK exists as two tautomeric forms. Moreover, the relative population of the two protomeric forms in the ion ensemble depends on the nature of the ambient molecules present in the atmospheric pressure ion source. For example, the ion-mobility arrival-time profile acquired from the mass-selected / 269 ion generated from an acetonitrile solution showed two peaks of near equal intensity. The peak with the shorter arrival time represented the -protomer and that with the longer arrival time represented the -protomer. However, when methanol or ammonia vapors were introduced to the ambient-pressure ion source, the intensity of the -protomer peak decreased rapidly and that of the -protomer signal soared until it became the dominant peak. When the introduction of methanol (or ammonia) vapors was stopped, the mobilogram signals gradually reverted back to their initial intensities. To rationalize this observation, we propose that the -protomer of MK in the presence of methanol vapor undergoes transformation to the -protomer by a Grotthuss-type mechanism via a methanol-based solvent bridge.