Solvation studies of electrospray ions—method and early results

Experiments are described in which bare (completely desolvated) ions are separated from their source gas and mixed with a bath gas of any desired temperature and precisely controlled composition. The resulting mixture of ions and bath gas is transported by free jet expansion into a vacuum system for mass analysis to reveal the extent to which the ions have formed adducts with other species in the bath gas. Because the ions are separated from their source gas or vapor before they are mixed with the bath gas, the compositions of the bath gas and source gas are completely independent. Thus, one can examine solvation or adduction by any desired species contained in the bath gas without any interference from other species that might be present in the source gas. Reported here are results obtained on the solvation of electrospray ions of a small peptide, leucine-enkephalin, by water and several alkanols. One provocative finding is that a substantial fraction of ions thought to be singly charged monomers turned out to be doubly charged dimers.