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

The 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.

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