MATRIX-ASSISTED LASER-DESORPTION IONIZATION - DEPENDENCE OF THE ION YIELD ON THE LASER-BEAM INCIDENCE ANGLE

Some basic properties of matrix-assisted laser desorption/ionization (MALDI) have been investigated by varying the angle between the pulsed UV-laser beam and the target surface, thus determining its influence on the MALDI process. The minimum deposited laser energy needed for protein ion production (the threshold energy) and the protein-yield dependence on the laser energy have been determined for incidence angles between 35-degrees and 60-degrees with respect to the target surface normal. The measurements demonstrate that neither the threshold energy nor the dependence of the protein yield on the laser energy vary with the incidence angle within the range studied. Hence the corresponding parameters that involve the area of irradiation, namely the threshold laser fluence as well as the ion-yield dependence on fluence have a cosine dependence on the incidence angle. Furthermore, the threshold energy at constant incidence angle scales with the mass of the analyte molecule for proteins in the range from 5700 to 66 000 u.