Special Feature: Tutorial
Ionization in matrix-assisted laser desorption/ionization: singly charged molecular ions are the lucky survivors
Version of Record online: 22 FEB 2000
Copyright © 2000 John Wiley & Sons, Ltd.
Journal of Mass Spectrometry
Volume 35, Issue 1, pages 1–12, January 2000
How to Cite
Karas, M., Glückmann, M. and Schäfer, J. (2000), Ionization in matrix-assisted laser desorption/ionization: singly charged molecular ions are the lucky survivors. J. Mass Spectrom., 35: 1–12. doi: 10.1002/(SICI)1096-9888(200001)35:1<1::AID-JMS904>3.0.CO;2-0
- Issue online: 22 FEB 2000
- Version of Record online: 22 FEB 2000
- Manuscript Accepted: 3 OCT 1999
- Manuscript Revised: 24 SEP 1999
- Manuscript Received: 13 JUL 1999
- matrix-assisted laser desorption/ionization;
- singly charged molecular ions
A new model for the ionization processes in UV matrix-assisted laser desorption/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 charged ions are detected. Based on experimental evidence, the formation of singly and multiply charged clusters by a deficiency/excess of ions and also by photoionization and subsequent photochemical processes is pointed out to 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 plume facilitating a high overall ionization yield. On the other hand, these electrons, and also the large excess of protonated matrix ions in the negative ion mode, induce effective ion reneutralization in the plume. These neutralization processes are most effective for the highly charged cluster ions initially formed. Their fragmentation behaviour is evidenced in fast metastable fragmentation characteristics and agrees well with an electron capture dissociation mechanism and the enthalpy transfer upon neutralization forms the rationale for the prominent fragmentation and intense chemical noise accompanying successful MALDI. Within the course of the paper, cross-correlations with other desorption/ionization techniques and with earlier discussions on their mechanisms are drawn. Copyright © 2000 John Wiley & Sons, Ltd.