Matrix Effects on Internal Energy in Desorption Ionization

Abstract

Desorption ionization (DI) mass spectrometry encompasses a family of techniques in which energization of a condensed phase leads to ejection of ions into the vacuum with subsequent mass analysis and detection [1]. In contrast to the gas—phase ionization methods of electron, chemical, and photo- ionization, DI techniques are by their nature sensitive to the physical and chemical nature of the matrix from which the ions are ejected. Success in the analysis of nonvolatile and thermally fragile molecules has been enough to thrust these techniques into routine use in just a few years. DI spectra of pure compounds can often be interpreted in terms of known gas—phase ion chemistry. This provides evidence for the desorption, from the surface, of intact ions with some degree of internal excitation; the fragmentation processes undergone are then defined by the nature and the amount of the internal energy. Since the ion is in an isolated state, fragmentation processes should be the same as those undergone by ions of the same structure formed directly in the gas phase by chemical ionization [2]. MS/MS experiments have confirmed this premise for particular DI conditions. Ions are isolated by a first stage of mass analysis, and then activated by collision. The masses and relative abundances of the resultant fragment ions (determined by the second stage of mass analysis) match those of the DI spectra. Metastable ion transitions have also been directly observed in DI spectra.