Ionization of 2,5-dihydroxybenzoic acid (DHB) matrix-assisted laser desorption ionization experiments and theoretical study

The 2,5-dihydroxybenzoic acid (DHB), well known as one of the most efficient matrix-assisted laser desorption ionization (MALDI) matrices, was photoionized at different irradiances of a UV laser. Molecular orbital calculations were performed with a Becke-style 3 parameter using the Lee-Yang-Parr correlation functional density approach to estimate the thermochemical properties of DHB and showed a very good agreement with available experimental data. We report a first estimation of the electron affinity of DHB (54 kJ/mol). In a second step, the calculation method was used to evaluate the thermochemical reliability of several ionization models postulated from MALDI experiments. Among those proposed, we report the reactions, which are thermodynamically the most probable in the solid state and in the gas phase. We evaluated the energy involved in the formation of DHB.+ and DHB.+ molecular ions, DHBH+ and [DHB-H](-) pseudomolecular ions. The phenomenon of reduction of molecular ions was also theoretically studied as well as the role of the DHBH. radical species. Our results on the reactivity of DHBH. radical species show evidence of their very probable implication in molecular ion formation and of the great role played by the hydroxyl substituants in the particular efficiency of DHB as a MALDI matrix. We propose a simple thermochemical diagram showing evidence that MALDI is a very energy demanding process and that differences in efficiency between matrices should be related to their chemical structures rather than to their physical properties. (C) 2001 Elsevier Science B.V.