Exciton mobility and trapping in a MALDI matrix.

Energy transfer (ET) from excited matrix to fluorescent traps is used to probe the mobility of excitations in the matrix-assisted laser desorption/ionization (MALDI) matrix material 2,5-dihydroxybenzoic acid. The dependence of host and guest fluorescence on excitation density (laser intensity) and trap concentration gives clear evidence for long-range energy transport in this matrix. This conclusion is further supported by time-resolved emission data showing a 2 ns delay between matrix and trap emission. Rate equation and random walker models give good agreement with the data, allowing determination of hopping, collision, and trapping parameters. Long-range energy transfer contributes to the pooling reactions which can lead to primary ions in MALDI. The results validate the pooling aspect of the prior quantitative MALDI ionization model (J. Mass Spectrom. 2002, 37, 867-877). It is shown that exciton trapping can decrease MALDI ion yield, even at low trap concentration.