Hydration of gas-phase ions formed by electrospray ionization

The hydration of gas-phase ions produced by electrospray ionization was inv estigated. Evidence that the hydrated ions are formed by two mechanisms is presented. First, solvent condensation during the expansion inside the elec trospray source clearly occurs. Second, some solvent evaporation from more extensively solvated ions or droplets is apparent. To the extent that these highly solvated ions have solution-phase structures, then the final isolat ed gas-phase structure of the ion will be determined by the solvent evapora tion process. This process was investigated for hydrated gramicidin S in a Fourier-transform mass spectrometer. Unimolecular dissociation rate constan ts of isolated gramicidin S ions with between 2 and 14 associated water mol ecules were measured. These rate constants increased from 16 to 230 s(-1) w ith increasing hydration, with smaller values corresponding to magic number s. (J Am Soc Mass Spectrom 1999, 10, 958-968) (C) 1999 American Society for Mass Spectrometry.