DO SINGLE MATRIX MOLECULES GENERATE PRIMARY IONS IN ULTRAVIOLET MATRIX-ASSISTED-LASER-DESORPTION IONIZATION/

The ionization mechanisms in matrix-assisted laser desorption/ionizati on (MALDI) remain poorly understood. We have begun a program of study aimed at determining the properties of matrix molecules which make the m suitable for MALDI, Initial results are presented here for one of th e most widely used matrix materials, 2,5-dihydroxybenzoic acid (DHB). Spectroscopy of free DHB molecules in a molecular beam shows that the photoionization energy is much lower than expected from semiempirical calculations, only 8.05 eV, yet still not accessible with two nitrogen laser photons (7.36 eV). No evidence is found for labile protons in t he first excited state. This is in spite of structural similarity with salicylates where excited state proton transfer from the 2-hydroxy gr oup occurs. Conventional excited state proton transfer from single DHB molecules to analytes is thus deemed unlikely. On the other hand, a t wo-step reaction taking place via decarboxylated DHB (hydroquinone) is shown to be a potential analyte protonation mechanism. The conclusion is reached that single matrix molecules are probably not the primary ion generators in UV MALDI with this matrix. This is consistent with p roposed models which require dimers or larger aggregates for ion gener ation. The photo/thermal combined ionization model of Allwood, Dyer an d Dreyfus (Rapid Commun. Mass Spectrom. 11, 499 (1997)) is updated wit h measured physical parameters for DHB, and extended to include 2-cent er energy pooling mechanisms. (C) 1998 John Wiley & Sons, Ltd.