Charged residue versus ion evaporation for formation of alkali metal halide cluster ions in ESI

Abstract

Alkali metal halides have been used as probes to gain insight into the electrospray ionization (ESI) mechanism. Relative abundances of monatomic alkali cations and halide anions were found to be dependent on the solvation energies of the ions. Results are consistent with predictions based upon two proposed ESI models: the ion evaporation model (IEM) and the charged residue model (CRM). In the study of gas-phase cluster ions originating from equimolar mixtures of two alkali metal halides, a dependence of the relative abundances of the cluster ions on the solvation energies was present in one case, but absent in another. In the first case, where two different ions participated in the droplet charge excess while the counter ion was fixed, the relative abundance of a cluster ion increased with decreasing solvation energy of the cluster ion. In the second case, where the ion constituting the droplet charge excess was held constant and the counter ions varied, the relative abundance of a cluster ion increased with increasing solvation energy. Experimental results were rationalized by an extended charged residue model which proposes that preferential enrichment of the less strongly solvated ions constituting the droplet charge excess can take place during the uneven fission process. Thus, when ions constituting the droplet charge excess are varied, preferential enrichment of the ions of lower solvation energies will lead to the formation of more abundant cluster ions containing these ions. On the other hand, when the ion constituting the droplet charge excess is held constant, the lack of correlation between the solvation energies and the observed relative abundances of cluster ions suggests that gas-phase ions are not formed by ion evaporation as entities, but rather they are formed as a result of several combined events: uneven droplet fission, increased salt concentration due to continuous solvent evaporation, and a tendency to form more stable cluster ions in the later stages of the droplet lifetime.