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

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Mass Spectrometry

Background:

• Matrix-assisted laser desorption ionization (MALDI) is a crucial technique for analyzing biomolecules.
• The initial ionization steps in MALDI, particularly for matrix molecules, are not fully elucidated.
• Carboxyl-containing MALDI matrices form stable dimeric structures via hydrogen bonds in the solid state.

Purpose of the Study:

• To propose and investigate a mechanism for the initial ionization of matrix molecules in MALDI.
• To understand the formation pathways of protonated and deprotonated matrix ions ([Ma + H](+) and [Ma - H](-)).
• To evaluate the contribution of thermal ionization and explain matrix-dependent analyte ionization differences.

Main Methods:

• Theoretical calculations to explore ion formation pathways.
• Experimental investigations to assess the role of thermal ionization.
• Analysis of matrix properties such as ionization energies, pKa values, and thermodynamic stability.

Main Results:

• A proposed mechanism involving two-photon absorption leading to [Ma + H](+) and [Ma - H](-) ions.
• An alternative pathway via a stable intermediate formed by single-photon heterodissociation.
• Experimental evidence suggests negligible contribution from thermal ionization.
• Observed differences in analyte ionization are explained by matrix properties.

Conclusions:

• The proposed mechanism provides insight into the fundamental ionization processes in MALDI.
• Both direct two-photon absorption and a single-photon intermediate pathway contribute to matrix ion formation.
• Matrix properties significantly influence analyte ionization efficiency in MALDI.