Abstract
Atmospheric pressure spray (APS) ionization can produce quasi-molecular ions of biological molecules from their aqueous solution, and has therefore been developed as an interface for LC/MS. In this article, we first outline models of the ion production mechanisms developed for spray ionization methods such as thermospray and electrospray. Then, the experimental results using APS are presented. We investigate whether the quasi-molecular ions are produced by ion-molecule reactions in the gas phase, by ion evaporation (in which ions are produced by the spray), or by both mechanisms. It is concluded that a volatile molecule such as ammonia is produced by the spray and protonated by an ion-molecule reaction in the gas phase. For nonvolatile molecules such as sucrose and amino acids, however, their quasi-molecular ions are produced partly by ion evaporation and partly by the gas phase reaction. It is also shown that the relative intensities of the protonated amino-acid molecules produced by ion evaporation are not necessarily determined by the equilibrium in the bulk liquid. A qualitative explanation is proposed on the basis of the Gibbs energy of hydration.
