Probing solution-phase species and chemistry in the gas phase

Electrospray ionization not only is a powerful soft ionization technique for biological mass spectrometry but also provides an unique interface between the solution phase and the gas phase. An experimental apparatus combining electrospray with photodetachment photoelectron spectroscopy has been developed for the investigation of multiply charged anions and solution-phase species in the gas phase. In this article, the principles of this technique and our recent progress are presented and discussed. Photoelectron spectroscopy is ideal to probe free multiply charged anions and has allowed the repulsive Coulomb barrier existing universally in multiply charged anions to be directly observed and investigated. Solvation effects, solvent and counter-ion stabilization of a common inorganic doubly charged anion, SO42-, have been extensively investigated. The minimum number of water molecules needed to stabilize SO42- and its solvation behaviour have been addressed. Large hydrated SO42- clusters are found to exhibit properties of bulk aqueous solutions. We also show that the electronic structures of many inorganic metal complexes, in particular redox species, can be investigated in the gas phase using electrospray and photoelectron spectroscopy.