Ionization in matrix-assisted laser desorption/ionization: singly charged molecular ions are the lucky survivors

A new model for the ionization processes in UV matrix-assisted laser desorp tion/ionization (MALDI) which accounts for the major phenomena observed is presented and discussed. The model retains elements of earlier approaches, such as photoionization and photochemical reactions, but it redefines these in the light of new working questions, most importantly why only singly ch arged ions are detected, Based on experimental evidence, the formation of s ingly and multiply charged clusters by a deficiency/excess of ions and also by photoionization and subsequent photochemical processes is pointed out t o be the major ionization processes, which typically occur in parallel. The generation of electrons and their partial loss into the surrounding vacuum and solid, on the one hand, results in a positively charged ion-neutral pl ume facilitating a high overall ionization yield. On the other hand, these electrons, and also the large excess of protonated matrix ions in the negat ive ion mode, induce effective ion reneutralization in the plume. These neu tralization processes are most effective for the highly charged cluster ion s initially formed. Their fragmentation behaviour is evidenced in fast meta stable fragmentation characteristics and agrees well with an electron captu re dissociation mechanism and the enthalpy transfer upon neutralization for ms the rationale for the prominent fragmentation and intense chemical noise accompanying successful MALDI. Within the course of the paper, cross-corre lations with other desorption/ionization techniques and with earlier discus sions on their mechanisms are drawn. Copyright (C) 2000 John Wiley & Sons, Ltd.