EXPERIMENTS ON THE KINETICS OF FIELD EVAPORATION OF SMALL IONS FROM DROPLETS

The phenomenon of ion evaporation from charged liquid surfaces is at t he basis of electrospray ionization; a source of a stunning variety of gas phase ions. It is studied here by producing a monodisperse cloud of charged droplets and measuring the charge q and diameter d(r) of th e residue particles left after complete evaporation of the solvent. Wh en the droplets contain small monovalent dissolved ions, the electric field E on the surface of their solid residues is found to be independ ent of d(r). One can thus argue that the source of small ions in elect rospray ionization is field-emission, and not other proposed mechanism s such as Dole's charged residue model. A consequence of the observed independence of E on d(r) is that the rate of ion ejection is simply r elated to the rate of solvent; evaporation, estimated here as that for a clean surface of pure solvent. The reduction G(E) brought about by t he electric field E in the activation energy for ion evaporation has t hus been inferred asa function of the measured field E in the range 1. 5<E(V/nm)<3.25. It agrees surprisingly well with the so-called Schottk y hump from the image potential model (IPM), G(IPM)=(e(3)E/4 pi epsilo n(0))(1/2)). This remarkably simple result is paradoxical in view of t wo major objections raised earlier against the use of-the IPM for ion evaporation from liquids. However, the correct mechanism (first introd uced by Iribarne and Thomson) leading to an attractive interaction bet ween the liquid surface and the escaping ion is not the creation of an image charge, but the polarization of the dielectric liquid by the io n. In the limit of a large dielectric constant epsilon much greater th an 1, the image force and the polarization force coincide numerically, though the later sets in much faster and is apparently free from the paradox raised by Rollgen. Also, the dielectric nature of the liquid a nd its strong screening of the net charges near its surface resolves a nother paradox raised by Fenn regarding the discrete distribution of c harges. This screening also introduces a correction in the model propo sed by Iribarne and Thomson for G(E), making its predictions virtually indistinguishable from those of G(IPM)(E) In conclusion, small ions o bserved in electrospray ionization are produced by field-emission. Mea sured ionization rates are well represented by results from a ''polari zation potential model'' which appears to he physically sound. These p redictions coincide with those from the IPM in the limit epsilon much greater than 1, the only case studied so far. (C) 1995 American Instit ute of Physics.