Experiments on the kinetics of field evaporation of small ions from droplets

Abstract

The phenomenon of ion evaporation from charged liquid surfaces is at the 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 dr of the residue particles left after complete evaporation of the solvent. When the droplets contain small monovalent dissolved ions, the electric field E on the surface of their solid residues is found to be independent of dr. One can thus argue that the source of small ions in electrospray ionization is field-emission, and not other proposed mechanisms such as Dole’s charged residue model. A consequence of the observed independence of E on dr is that the rate of ion ejection is simply related to the rate of solvent evaporation, estimated here as that for a clean surface of pure solvent. The reduction G(E) brought about by the electric field E in the activation energy for ion evaporation has thus been inferred as a function of the measured field E in the range 1.5<E(V/nm)<3.25. It agrees surprisingly well with the so-called Schottky hump from the image potential model (IPM), GIPM=(e3E/4πε0)1/2. This remarkably simple result is paradoxical in view of two major objections raised earlier against the use of the IPM for ion evaporation from liquids. However, the correct mechanism (first introduced by Iribarne and Thomson) leading to an attractive interaction between the liquid surface and the escaping ion is not the creation of an image charge, but the polarization of the dielectric liquid by the ion. In the limit of a large dielectric constant ε≫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 Röllgen. Also, the dielectric nature of the liquid and its strong screening of the net charges near its surface resolves another paradox raised by Fenn regarding the discrete distribution of charges. This screening also introduces a correction in the model proposed by Iribarne and Thomson for G(E), making its predictions virtually indistinguishable from those of GIPM(E). In conclusion, small ions observed in electrospray ionization are produced by field-emission. Measured ionization rates are well represented by results from a ‘‘polarization potential model’’ which appears to be physically sound. These predictions coincide with those from the IPM in the limit ε≫1, the only case studied so far.

Tandem mobility mass spectrometry study of electrosprayed tetraheptyl ammonium bromide clusters

“…The reason why the z ϭ 3, 4, and 5 charge states at 1/Z ϭ 2.02 Vs/cm 2 survive in part in going from the DMA to the MS, yet none of the higher charge states remain in their original state at high Z, can be understood in light of previous work [13,23]. The ion evaporation mechanism from drops is controlled for large drops mainly by the electric field E on their surface.…”

Simultaneous ion and neutral evaporation in aqueous nanodrops: experiment, theory, and molecular dynamics simulations

“…Experimental results are bounded by these two curves. Ion evaporation is expected to lead to a nearly size independent surface electric field strength for drops 4,9,12 , which is not expected for drops close to their Rayleigh limit (drops near their Rayleigh limit additionally attain much higher charge levels, as shown by the orange dashed line). Interestingly, the Rayleigh limit and the simulated and theoretical calculation results are somewhat coincident.…”

Coulomb fission in multiply charged molecular clusters: Experiment and theory

“…[17][18][19][20][21][22][23][24][25] Discussions of the final steps of the ESI process have focused on two mechanisms: a charge residue model (CRM) where highly charged ions, such as proteins, are thought to form as a result of extensive solvent evaporation, during which the ion of interest retains or acquires a significant fraction of the total available charge. 26,27 A second mechanism, the ion evaporation mechanism (IEM), is believed to proceed via the ejection of small solvated ions and appears to be more applicable to the appearance of relatively small residual ions.…”