H-atom transfer following analyte photoionization in matrix-assisted laser desorption/ionization processes

As recently shown in Fourier transform ion cyclotron resonance mass spectrometry studies (Rapid Commun. Mass Spectrom. 11, 1602, 1997), photochromic systems can be used as molecular probe for the study of matrix-assisted laser desorption ionization (MALDI) processes inducing the typical formation of the triply hydrogenated molecules [M + 2H + H]⁺ simultaneously with the color change. In this paper, a novel photochromic system blocked in its open form allows the proposal of a possible mechanism for protonated ions. The results suggest that H-atom transfer from a matrix molecule to an analyte plays an important role in the ionization step. The transferring H-atom may be derived from electronically excited states of matrix and analyte molecules via the triplet states. Among several ionization processes described in the literature, we observe that H-atom transfer following analyte photoionization is a possible ionization mechanism in MALDI.