Plume expansion dynamics of matrix-assisted laser desorption ionization.

High-resolution angular and velocity distributions for neutral analytes (tryptophan and poly-tryptophan) and matrix (2,4,6-trihydroxyacetophenon, THAP) are measured by using 355 nm laser desorption. The information suggests that two separate mechanisms dominate the angular and velocity distributions at the beginning and before the end of desorption. A molecular jet-like isentropic expansion dominates the plume expansion at the beginning of desorption. This only occurs at high surface temperature, thus resulting in a large velocity normal to the surface and a very narrow angular distribution. Most of the analytes are produced under these conditions. Before the end of desorption, the surface temperature decreases and the mechanism of thermal desorption at low vapor pressure takes over. The velocities become small and the angular distribution is close to cosθ. Only a very small amount of analytes are generated under these conditions. Compared to tryptophan, poly-tryptophan has a much narrower angular distribution, thereby suggesting that it is only produced at the higher surface temperatures.