Influence of Acceleration Field and Inlet Temperature on Ion Yields of Small Molecules in Atmospheric Pressure MALDI

The optimization of the accelerating electric field near a target plate as well as temperature of the inlet capillary in a mass spectrometer is essential for achieving a high ion yield in atmospheric pressure matrix-assisted laser desorption ionization (AP/MALDI) and atmospheric pressure/laser desorption ionization (AP/LDI). The yields of protonated rhodamine 6G ions, protonated matrix ions (M+H)⁺, matrix radical ions (M•)⁺, as well as protonated reserpine ions were studied as a function of the inlet capillary temperature in Orbitrap and electric field applied across the inlet and MALDI target plate. The findings indicated that 2,5-dihydroxybenzoic acid (DHB) generated predominantly cation radicals (M•)⁺ rather than protonated ions. This trend is not common for most MALDI matrices used for positive-mode MALDI-MS analysis. By changing capillary temperature, it has been found that the temperature trends observed for the (DHB+H)⁺/(DHB•)⁺ ratios in both AP/MALDI and electrospray ionization (ESI) are similar. The ion yield dependence on the electric field strength in AP/MALDI as well as low-pressure MALDI sources proved to be more complex than that suggested in the previously developed model accounting for charge neutralization in the plume mitigated by the field-driven separation of opposite charges. The laser photoionization of the matrix molecules within a thin surface layer of a matrix microcrystal followed by thermally activated proton transfer from the matrix radical cation to adjacent matrix molecules has been discussed.