Mechanisms of electrospray ionization of singly and multiply charged salt clusters

Electrosprays of solutions of tetraheptyl ammonium bromide (A(+)B(-)) in formamide and propanol are investigated with a differential mobility analyzer (DMA) and a particle size magnifier (PSM) detector able to count single ions and to infer their charge state. The DMA can analyze all the charged species present, covering a vast range of masses and charge states, from (A(+))(z)(A(+)B(-))(n) clusters (z = 1, 2, 3...; n = 0, 1, 2,..., 18,...), up to highly charged salt nanoparticles with diameters of tens of nanometers. Each charge state z appears only in a limited range of aggregation n, with n(max)(z-1) similar to n(min)(z), which leads to a regular pattern of ordered modulations in the mobility spectra of the neutralized clusters (z reduced to 1). Coulomb explosions of the electrosprayed drops can be suppressed at will in solvents with electrical conductivities K higher than 1 S/m, as confirmed independently via energy analysis of the spray drops in a vacuum. The mobility spectra then change drastically, showing only singly charged clusters with n = 0, 1,...,5 (evidently field-evaporated) and large residues, with no traces of doubly or triply charged clusters in between. Coulomb explosions are therefore, responsible for the formation of all multiply charged clusters of intermediate sizes, covering the vast range from (A(+))(2)(A(+)B(-))(4) up to relatively large nanoparticles. But ion evaporation determines the charge state of all these residues. The resulting curves n(max)(z) and n(min)(z) hence carry key quantitative information on the kinetics of ion evaporation. This includes the dependence of the solvation energy Delta(R, z) on drop radius R and charge, which is also modelled theoretically in the Appendix A. (C)2000 Elsevier Science B.V. All rights reserved.