INFLUENCE OF LASER FOCUSING IN MATRIX-ASSISTED LASER-DESORPTION IONIZATION

The influence of laser focusing on the target in matrix-assisted laser desorption/ionization (MALDI) has been studied. The emission of stabl e and metastable molecular ions of bovine insulin depends on both the laser energy and the distance between the target and the laser focal p lane. The results clearly show that the laser fluence is the important parameter in MALDI. The fluence threshold of metastable-ion emission is higher than that of stable-ion emission. Beyond this threshold, the emission of stable ions reaches a plateau. This is due to the spatial inhomogeneity of the laser energy distribution. A phenomenological mo del including two fluence thresholds for the stable- and metastable-io n emission has been developed and compared to the experimental data.