MALDI Mechanism of Dihydroxybenzoic Acid Isomers: Desorption of Neutral Matrix and Analyte

Angular resolved velocity distributions of laser desorbed neutral matrices (dihydroxybenzoic acids, DHB) and analytes (tryptophan) embedded in these matrices were investigated at 322 nm by a modified crossed molecular beam apparatus. Desorbed ions generated from MALDI were measured by a time-of-flight mass. spectrometer, Desorprions of neutral Matrix and analyte. from 2,3-DHB, 2,4-DHB, 2,5-DHB, 2,6-DHB, and 3,5-DHB at 322 nm have similar properties, but the ion intensities are in the order 2,3DHB congruent to 2,6-DHB > 2,5-DHB congruent to 2,4-DHB > 3,5-DHB. It indicates that the combination of various parameters related to neutral species, including absorption coefficient, sublimation energy, contact of analyte and matrix in crystal, and plume dynamics of desorbed species are not crucial in the determination of MALDI process for DHB isomers. The difference of matrix activity of DHB isomers at this wavelength must result from the other properties, like the excited state lifetime, proton affinity, gas phase basicity, acidity, ionization energy, or the other properties related to the primary reactions in ion generation.