On the Distributions of Ion/Neutral Molecule Clusters in Electrospray and Laser Spray—A Cluster Division Model for the Electrospray Process

The clustering of a medium-sized, involatile, neutral molecule, octyl β-D-glucopyranoside (OG), with Na+, Ca2+, and Yb3+ (Mz+) ions in electrospray (ESI) was investigated using laser spray (LSI). Extensive distributions of [(Mz+)i (OG)a]n+-clusters, extending beyond 50 kDa, were observed. The distributions were highly stable and reproducible and changed only marginally when concentrations of electrolyte or neutral compound were varied by orders of magnitude. Compared with ESI, laser spray yielded superior intensities, particularly of the larger clusters. The cluster distributions demonstrated a range of remarkable features. In particular, the Yb3+/OG cluster distribution was unusual. For example, no clusters with 35–52 or with 110–116 OG molecules were observed. The distribution pattern revealed that the clusters were formed as a result of cluster dissociations, such as [(Yb3+)3(OG) ∼110W]9+ → [(Yb3+)2(OG)∼90W]6+ + [(Yb3+)1(OG) ∼20W]3+, where W represents the water content at the time of dissociation. Based on this study, a cluster division model for electrospray of aqueous solutions of strongly solvated ions is proposed: the Rayleigh droplet disintegration process, which is well-established for the initial stages of electrospray, maintains its general character as it proceeds through a final regime of multiply charged cluster dissociations to the singly and multiply charged ions in mass spectrometry. In the dissociation of multiply charged clusters, the size of each daughter cluster is roughly proportional to the square of the cluster charge. Observed cluster distributions are consistent with a mixture of symmetric and asymmetric cluster dissociations.