Desorption dynamics of neutral molecules in matrix-assisted laser desorption/ionization

This work studies the expansion dynamics of molecules ejected from sample crystals in matrix-assisted laser desorption/ionization (MALDI) reaction. Measurements were made in two steps - sample desorption using a 355 nm pulsed laser beam and gas-phase postionization using a 266 nm laser at various delays. The estimated ion production yield is approximately 104 times better than that obtained using conventional MALDI methods. A model system of co-crystallized tryptophan (Trp) analyte and 2,4,6-trihydroxyacetophenone (THAP) matrix yielded overlapping time-of-arrival profiles. This result suggests that translational behaviours of gas-phase Trp and THAP are thermalized during expansion. The observed plume velocity depends strongly on the matrix and analyte composition: the most probable velocity is 214m/s for pure Trp and 450m/s for pure THAP. The incorporation of Trp into THAP makes both their velocities fall between the two extreme cases. Multiple half-space Maxwellian curves adequately describe the observed desorption velocity profile, implying that MALDI consists of multiple desorption components.