Cryogenic Ion Mobility-Mass Spectrometry Captures Hydrated Ions Produced During Electrospray Ionization

Evaporation of water from extensively hydrated protons and peptides formed by electrospray ionization (ESI) has been examined for the first time by cryogenic ion mobility-mass spectrometry (IM-MS). The extent of hydration was controlled using a heated capillary inlet operated between 340 and 391 K. Cold cluster ions formed in the source region were transported into a low temperature (similar to 80 K) IM drift tube using an electrostatic ion guide where they were separated on the basis of size-to-charge via low-energy collisions with helium gas. The eluting IM profile was subsequently pulsed into an orthogonal time-of-flight (TOF) mass spectrometer for mass-to-charge (m/z) identification of the cluster ion species. Key parameters that influence the cluster distributions were critically examined including the inlet temperature, drift tube temperature, and IM field strength. In agreement with previous studies, our findings indicate that water evaporation is largely dependent upon the particular charge-carrying species within the cluster. IM-MS results for protonated water clusters suggest that the special stability of H+(H2O)(n) (n = 21) is attributed to the presence of a compact isomer (assigned to a clathrate cage) that falls below the trendline produced by adjacent clusters in the n = 15 to 35 size range. Peptide studies are also presented in which specific and nonspecific solvation is observed for gramicidin S [GS + 2H](2+)(H2O)(n) (n = 0 to similar to 26) and bradykinin [BK + 2H](2+)(H2O) (n = 0 to similar to 73), respectively.