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Journal of Analytical Methods in Chemistry
Volume 2012, Article ID 161865, 8 pages
Research Article

On the Primary Ionization Mechanism(s) in Matrix-Assisted Laser Desorption Ionization

1National Council of Researches, Institute of Molecular Sciences and Technologies, Corso Stati Uniti 4, I35100 Padova, Italy
2Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
3National Medicines Institute, Chełmska 30/34, 00-725 Warsaw, Poland
4National Centre for Nuclear Research, 05-400 Otwock, Świerk, Poland

Received 25 May 2012; Accepted 19 October 2012

Academic Editor: Giuseppe Ruberto

Copyright © 2012 Laura Molin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


A mechanism is proposed for the first step of ionization occurring in matrix-assisted laser desorption ionization, leading to protonated and deprotonated matrix (Ma) molecules ( and ions). It is based on observation that in solid state, for carboxyl-containing MALDI matrices, the molecules form strong hydrogen bonds and their carboxylic groups can act as both donors and acceptors. This behavior leads to stable dimeric structures. The laser irradiation leads to the cleavage of these hydrogen bonds, and theoretical calculations show that both and ions can be formed through a two-photon absorption process. Alternatively, by the absorption of one photon only, a heterodissociation of one of the O–H bonds can lead to a stable structure containing both cationic and anionic sites. This structure could be considered an intermediate that, through the absorption of a further photon, leads to the formation of matrix ions. Some experiments have been performed to evaluate the role of thermal ionization and indicate that its effect is negligible. Some differences have been observed for different matrices in the formation of analyte molecule (M) ion , , , and , and they have been explained in terms of ionization energies, pKa values, and thermodynamic stability.