Solvent Molecules Undergo Homolytic Cleavage and Radical Recombination Processes during Negative-Mode Electrospray Ionization: Adduct Formation with Antimony(III)-Tartrate Dianion

Negative-ionization mode electrospray ionization-mass spectrometry (ESI-MS) analysis of antimony(III)-tartrate in frequently used solvent systems, ACN/H2O and MeOH/H2O, revealed that the antimony(III)-tartrate dianion associates to solvent reaction products generated by radical formation and their subsequent recombination during the negative-mode electrospray process. A systematic increase and decrease in negative spray capillary voltage (SCV) from normal operational voltage ranges of a conventional quadrupole ion trap instrument during these analyses showed initially unobserved adduct ions to correspondingly increase and diminish in relative ion intensity. The identity of the adducted species, including products such as H2O2, NCCH2CH2CN, and CH2(OH)(2), were confirmed by performing similar experiments with deuterated and nondeuterated solvent mixtures. Relative intensity dependence of these adducted ions was monitored as the volume composition of each solvent system was changed. It was clearly observed that the relative intensity of {[Sb-2-tar(2)][H-O-O-H]}(2-) and {[Sb-2-tar(2)][NC-CH2-CH2-CN]}(2-) adduct ions increased with the volume percent of H2O and CH3CN, respectively. Similarly, an increase in volume percent of CH3OH increased the relative intensity of {[Sb-2-tar(2)][H-O-CH2-O-H]}(2-) adducted ions. On the basis of this evidence, it was proposed that homolytic cleavage of C-H bonds for CH3CN and CH3OH molecules, and O-H bonds for H2O molecules, produces a series of radicals during negative-ionization mode ESI, and subsequent self-recombination or cross-recombination of these radicals then occurs to form the neutral solvent products, which are observed in the mass spectra as [Sb-2-tar(2)]-adducted ionic species. These findings provide new insight into processes, which are relevant to understanding the mechanism of electrospray ionization, a widely used technique.