INFLUENCE OF THE LASER INTENSITY AND SPOT SIZE ON THE DESORPTION OF MOLECULES AND IONS IN MATRIX-ASSISTED LASER-DESORPTION IONIZATION WITH A UNIFORM BEAM PROFILE

The dependence of the number of desorbed particles on laser fluence ha s been investigated for matrix-assisted laser desorption/ionization (M ALDI) of analyte and matrix ions as well as for (photoionized) neutral matrix molecules using a homogeneous ''flat-top'' laser profile. Lase r spot diameters ranging from 10 to 200 mum in size have been used. 2, 5-Dihydroxybenzoic acid (DHB) and 3,5-dimethoxy-4-hydroxycinnamic acid (sinapic acid) have been tested as matrices. The threshold (for ion d etection) is higher and the dependence of the ion signal upon higher-t han-threshold fluences is stronger for directly desorbed ions than for photoionized neutral molecules. Directly desorbed analyte ions exhibi t the same dependence on fluence as the matrix ions with only minor di fferences between the two matrices tested, so both have approximately the same detection threshold. For both ions and photoionized neutral m olecules, the fluence threshold increases with decreasing spot size wh ile the slope of the intensity/fluence curves decreases. A quasi-therm al, sublimation/desportion model was found to describe the experimenta l results with excellent precision. For a complete explanation, non-eq uilibrium effects had to be taken into account.