Ion formation from charged droplets: Roles of geometry, energy, and time

The formation of ions from the charged droplets produced in the several spray ionization techniques is viewed as an activated rate process involving field-assisted desorption, in accord with the ideas first set forth by Iribame and Thomson. The novel features of the present treatment are particularly relevant to the unique ability of electrospray ionization to transform large molecules in solution to free ions in the gas phase, with extensive multiple charging. These new features stem mainly from the realization that the spacing of charges on a desorbed ion must relate to the spacing of charges on the surface of the droplet whence it came. The consequences of this "rule" can account for the existence of maxima and minima in the number of charges on the ions of a particular species as well as the nature of the distribution of ions among the intervening charge states. They also explain the dependence of charge state on the configuration in solution of the parent molecule of the desorbed ion. In addition, they provide insight into the sequence in time at which ions in the various charge states leave an evaporating droplet.