Photoionization pathways and free electrons in UV-MALDI

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Mass Spectrometry

Background:

• The existing model for UV-MALDI ion formation is extended to address photoionization mechanisms.
• Understanding electron and negative ion production is crucial for optimizing MALDI performance.

Purpose of the Study:

• To refine the UV-MALDI model by incorporating two-photon matrix ionization and analyzing electron emission phenomena.
• To investigate the influence of sample thickness, substrate type, and laser intensity on ion formation and electron emission.

Main Methods:

• Application of a modified UV-MALDI ion formation model.
• Analysis of experimental data on electron emission versus laser intensity.
• Molecular dynamics calculations to study surface charging effects.

Main Results:

• The modified model accurately predicts electron emission data, especially for thin samples on metal substrates.
• Matrix exciton pooling remains the dominant ionization pathway in most cases.
• Electron capture and negative ion formation are significant below a 10 nm depth due to short electron mean free paths.
• Surface emission of electrons is limited by surface charging at high laser intensities.

Conclusions:

• The enhanced UV-MALDI model provides a comprehensive understanding of ion formation, electron emission, and negative ion production.
• The model's predictions are validated by experimental observations, offering insights into MALDI processes.
• Surface charging effects play a critical role in limiting electron emission at high laser fluences.