中央研究院 原子與分子科學研究所

Structural identification of N-glycan isomers using logically derived sequence tandem mass spectrometry. Commun. Chem. 4, 92 (2021).

In past studies, mistakes in determining the ionization mechanism in matrix assisted laser desorption/ionization (MALDI) were made because an inappropriate ion-to-neutral ratio was used. The ion-to-neutral ratio of the analyte differs substantially from that of the matrix in MALDI. However, these ratios were not carefully distinguished in previous studies. We begin by describing the properties of ion-to-neutral ratios and reviews early experimental measurements. A discussion of the errors committed in previous theoretical studies and a comparison of recent experimental measurements follow. We then describe a thermal proton transfer model and demonstrate how the model appropriately describes ion-to-neutral ratios and the total ion intensity. Arguments raised to challenge thermal ionization are then discussed. We demonstrate how none of the arguments are valid before concluding that thermal proton transfer must play a crucial role in the ionization process of MALDI.

Abstract
RATIONALE: In most of the previous studies, the ratios of desorbed ions and neutrals from matrix-assisted laser desorption ionization (MALDI) were measured outside the common MALDI conditions. In this work, we measured the ratios in common MALDI conditions.
METHODS: Ions were detected using a time-of-flight mass spectrometer in combination with a time-gated ion imaging detector. Mass-resolved desorbed neutral molecules at different angles and velocities were measured using a modified crossed molecular beam apparatus.
RESULTS: The upper limit of ion-to-neutral ratio from pure2,5-dihydroxybenzoic acid is 4´10^-9at laser fluence 40 J/m², it increases to 3´10^-7at laser fluence 250 J/m². The ratios ofmatrix from the mixture of 25DHB and analyte remain in the same order of magnitude as pure 25DHB. However, the ratio ofanalyte depends strongly on the analyte.Values as large as 10^-3-10^-4for bradykinin and as small as <10^-8for glycine were observed at laser fluence ~100 J/m².
CONCLUSION: The ion-to-neutralratios of 25DHB matrix measured in this work are much smaller than some of the values reported in previous work using different methods and/or under different MALDI conditions.

Photoisomerization and photodissociation of aniline and 4-methylpyridine at 193 nm were studied separately using multimass ion imaging techniques. Photofragment translational energy distributions and dissociation rates were measured. Our results demonstrate that more than 23% of the ground electronic state aniline and 10% of 4-methylpyridine produced from the excitation by 193 nm photons after internal conversion isomerize to seven-membered ring isomers, followed by the H atom migration in the seven-membered ring, and then rearomatize to both methylpyridine and aniline prior to dissociation. The significance of this isomerization is that the carbon, nitrogen, and hydrogen atoms belonging to the alkyl or amino groups are involved in the exchange with those atoms in the aromatic ring during the isomerization.