Mechanisms of electrospray ionization of singly and multiply charged salt clusters

Electrosprays of solutions of tetraheptyl ammonium bromide (A(+)B(-)) in fo rmamide and propanol are investigated with a differential mobility analyzer (DMA) and a particle size magnifier (PSM) detector able to count single io ns and to infer their charge state. The DMA can analyze all the charged spe cies 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. Ea ch 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 co nfirmed independently via energy analysis of the spray drops in a vacuum. T he mobility spectra then change drastically, showing only singly charged cl usters with n = 0, 1,...,5 (evidently field-evaporated) and large residues, with no traces of doubly or triply charged clusters in between. Coulomb ex plosions are therefore, responsible for the formation of all multiply charg ed clusters of intermediate sizes, covering the vast range from (A(+))(2)(A (+)B(-))(4) up to relatively large nanoparticles. But ion evaporation deter mines 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 Delt a(R, z) on drop radius R and charge, which is also modelled theoretically i n the Appendix A. (C)2000 Elsevier Science B.V. All rights reserved.