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 stable and metastable molecular ions of bovine insulin depends on both the laser energy and the distance between the target and the laser focal plane. 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 model including two fluence thresholds for the stable‐ and metastable‐ion emission has been developed and compared to the experimental data.