Kinetics of solvent addition on electrosprayed ions in an electrospray source and in a quadrupole ion trap

Benzylpyridinium cations readily fragment in the electrospray source by loss of pyridine to give benzyl cations (M-79). The full-scan spectra obtained with some instruments also show, in addition, an m/z (M-38) peak corresponding to the addition of acetonitrile, being present in the solvent mixture, on the benzyl cations. Here we report that the addition reaction can occur in the source region of electrospray mass spectrometry instruments, and in a quadrupole ion trap. The kinetics of acetonitrile addition was monitored in an ion trap, acetonitrile being provided by leakage from the source, through the heated capillary. For benzyl ions with different substituents, the addition kinetics has been found positively correlated with the Brown parameter sigma+ of the benzyl radical, and therefore with the effective charge density on the alpha-carbon atom of the benzyl ion. This is consistent with the Langevin or average-dipole-orientation (ADO) theory of ion–molecule reaction kinetics.