Laser Pulse Width Dependence and Ionization Mechanism of Matrix-Assisted Laser Desorption/Ionization

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Spectroscopy

Background:

• Matrix-assisted laser desorption/ionization (MALDI) is a crucial technique for analyzing biomolecules and polymers.
• Understanding the fundamental ionization mechanisms in MALDI is essential for optimizing its performance and interpreting results.
• Previous models, including multiphoton ionization and coupled photophysical and chemical dynamics (CPCD), have been proposed to explain MALDI ionization.

Purpose of the Study:

• To investigate the ionization mechanism of MALDI using ultraviolet laser pulses.
• To determine the influence of laser pulse width on ion and neutral desorption in MALDI.
• To compare experimental findings with predictions from different theoretical models.

Main Methods:

• Utilized ultraviolet laser pulses (355 nm) with variable pulse widths (170 ps to 1.5 ns).
• Simultaneously measured mass spectra of desorbed ions and the intensity/velocity distribution of desorbed neutrals.
• Compared experimental data with numerical simulations based on multiphoton ionization, CPCD, and thermally induced proton transfer models.

Main Results:

• Ion and neutral desorption characteristics were independent of laser pulse width.
• Experimental data strongly agreed with the thermally induced proton transfer model.
• The multiphoton ionization model failed to explain the experimental observations.
• CPCD model predictions were inconsistent for CHCA and SA, and only matched DHB data under extreme parameter adjustments.

Conclusions:

• Thermally induced proton transfer is the dominant ionization mechanism in MALDI for the investigated matrices.
• Laser pulse width variation within the studied range does not significantly alter MALDI ionization pathways or desorption products.
• The findings necessitate a re-evaluation of existing MALDI ionization models, favoring thermally driven processes.