Role of photoionization and photochemistry in ionization processes of organic molecules and relevance for matrix‐assisted laser desorption lonization mass spectrometry

Abstract

AbstractThe ionization in ultraviolet laser desorption was investigated for a large number of small polar organic molecules which have a strong resonance absorption at the laser wavelength. In many cases, both positive‐ and/or negative‐ion mass spectra show strong signals of ion species which deviate from the simple scheme of even‐electron quasimolecular and fragment ion formation commonly expected for desorption techniques. These are radical cations and ion species formed by single and multiple hydrogen cleavage or addition. A model is proposed and discussed which explains these features assuming photoionization as the common initial ionization step followed by ion‐molecule reactions to the final product ions. The mass spectra of all compounds proved to function well in matrix‐assisted ultraviolet laser desorption/ionization show characteristic features indicative of their photochemical reactivity. This observation substantiates the hypothesis of tbe essential role of the matrix in analyte ionization.