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.
Wellman Laboratories of Photomedicine, Harvard Medical School, Massachusetts General Hospital, Boston, MA 02114, USA.
Hans-Christian
HC
Lüdemann
Robert W
RW
Redmond
Franz
F
Hillenkamp
D004583:Electrons; D004735:Energy Transfer; D010777:Photochemistry; D017785:Photons; D013050:Spectrometry, Fluorescence; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization / Q000379:methods*