Optical Studies of Electrohydrodynamic Disintegration of Liquids in EH and ES Mass Spectrometry

Abstract

The formation of gaseous ions amenable to mass spectrometric analysis by field induced disintegration of liquids has been achieved for conducting inorganic and organic liquids by basically four different methods:

• 1. Liquid metal ion sources (LMIS). In this method a high potential is applied to a liquid metal tip forming a sharp cone by the field stress at the onset of ion emission [1]. The ions are emitted predominantly by field evaporation from a transient state of a protrusion at the apex of the cone [2,3]. The competing process is a disintegration of the protrusion into charged droplets. The stability of the cone surface up to rather high field strengths near the apex is due to the high surface tension of liquid metals.

• 2 Field desorption mass spectrometry (FD MS). In FD MS preformed ions are extracted from surface protrusions generated by the field stress on a highly viscous liquid [4,5]. Again, ion emission occurs from a transient state of a disintegrating liquid.

• 3. Electrohydrodynamic mass spectrometry (EH MS). The EH MS and the electrospray MS (see below) methods have been developed for the desolvation of ions from liquids of low viscosity and low surface tension. A faster mass flow is involved in the disintegration of such liquids and accordingly capillaries are employed for a continuous supply of the sample solution. In EH MS the disintegration of sample solutions is caused by a high electric field applied to the end of the capillary under vacuum conditions. Since the desolvated ions thus formed are sampled by mass spectrometry at potentials close to that of the capillary [6], they can only emerge from electrohydrodynamic disintegration processes at the end of the capillary.