Is energy pooling necessary in ultraviolet matrix-assisted laser desorption/ionization?

RATIONALEEnergy pooling has been suggested as the key process for generating the primary ions during ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI). In previous studies, decreases in fluorescence quantum yields as laser fluence increased for 2-aminobenzoic acid, 2,5-dihydroxybenzoic acid (2,5-DHB), and 3-hydroxypicolinic acid were used as evidence of energy pooling. This work extends the research to other matrices and addresses whether energy pooling is a universal property in UV-MALDI. METHODSEnergy pooling was investigated in a time-resolved fluorescence experiment by using a short laser pulse (355nm, 20ps pulse width) for excitation and a streak camera (1ps time resolution) for fluorescence detection. RESULTSThe excited-state lifetime of 2,5-DHB decreased with increases in laser fluence. This suggests that a reaction occurs between two excited molecules, and that energy pooling may be one of the possible reactions. However, the excited-state lifetime of 2,4,6-trihydroxyacetophenone (THAP) did not change with increases in laser fluence. The upper limit of the energy pooling rate constant for THAP is estimated to be approximately 100-500 times smaller than that of 2,5-DHB. CONCLUSIONSThe small energy pooling rate constant for THAP indicates that the potential contribution of the energy pooling mechanism to the generation of THAP matrix primary ions should be reconsidered. Copyright (c) 2013 John Wiley & Sons, Ltd.