Two-photon ionization thresholds of matrix-assisted laser desorption/ionization matrix clusters

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Spectroscopy

Background:

• Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is a key technique for analyzing large molecules.
• Direct two-photon ionization of matrix materials is a proposed primary ionization mechanism in MALDI.
• The ionization thresholds of matrix clusters, not just monomers, are crucial for understanding MALDI plume expansion.

Purpose of the Study:

• To investigate the ionization thresholds of 2,5-dihydroxy benzoic acid (DHB) clusters and mixed clusters with hydroquinone.
• To determine if these clusters can be directly ionized by two-photon absorption of nitrogen laser photons.
• To assess the role of matrix clusters in the MALDI ionization process.

Main Methods:

• Utilized two-color two-photon ionization spectroscopy.
• Studied jet-cooled clusters of 2,5-dihydroxy benzoic acid (DHB) and its thermal decomposition product, hydroquinone.
• Measured ionization thresholds of monomer and cluster species.

Main Results:

• The ionization thresholds of DHB clusters were only slightly reduced compared to monomers, reaching a limit of 7.82 eV for pure DHB clusters.
• None of the investigated DHB or mixed clusters could be directly ionized by two nitrogen laser photons (7.36 eV).
• Observed low ionization efficiency at the measured thresholds.

Conclusions:

• Direct two-photon ionization of DHB clusters by nitrogen laser photons is not a significant primary ionization mechanism in MALDI.
• The high ionization thresholds of matrix clusters limit their direct photoionization.
• Further investigation into alternative MALDI ionization mechanisms is warranted.