Singlet‐singlet annihilation in ultraviolet matrix‐assisted laser desorption/ionization studied by fluorescence spectroscopy

Laser‐induced fluorescence spectroscopy was carried out on microcrystalline samples of three typical matrices under conditions of matrix‐assisted laser desorption/ionization (MALDI). The emitted fluorescence intensity was determined as a function of incident laser fluence and a sublinear increase of the fluorescence intensity with laser fluence was found. A very good fit was obtained when the experimental fluorescence vs. fluence data were compared with a numerical model assuming that under typical MALDI fluence conditions a large fraction of molecules in the excited singlet state undergoes singlet‐singlet annihilation. Throughout the fluence range relevant for MALDI, however, the experimental data could not be fit well to a model assuming resonant two‐photon absorption as the process depopulating the singlet state. In a separate set of experiments, the singlet lifetimes of several typical crystalline MALDI matrices were determined and found to be considerably shorter than previously reported. While both singlet‐singlet annihilation and resonant two‐photon absorption have been discussed in the literature as candidates for pathways to primary matrix ion generation in MALDI, the data presented here suggest that singlet‐singlet annihilation is the dominant mechanism for depopulating the singlet state in a matrix crystal excited at typical MALDI fluences. Copyright © 2002 John Wiley & Sons, Ltd.