芳香族分子在分子束中及在基質輔助雷射脫附游離過程中的光誘發反應

Photo-induced reactions of aromatics in molecular beam and during MALDI process

貝格奇(Arnab Bagchi)

指導教授 ： 倪其焜

國立清華大學/理學院/化學系/博士(2013年)

https://doi.org/10.6843/NTHU.2013.00377

摘要

The first part of this thesis features photodissociation dynamics experiments performed on a choice of molecular systems using multimass ion imaging technique. The desired system was placed at collisionless environment in molecular beam and was expanded inside a vacuum chamber. Photoionization and photodissociation of molecules in the molecular beam were accomplished by excitation with a UV laser beam. The excited photo-fragments were subsequently ionized by vacuum ultraviolet (VUV, 118 nm) laser pulse at a certain delay time. The primary photodissociation and photoionization along with its dynamics in the gas phase have been studied for diverse aromatic molecules at different UV photolysis wavelengths. The unique features of multimass ion imaging technique reveal primary photodissociation channels like –CO2 and –COOH elimination in addition to the usually observed –OH dissociation by other experimental techniques for the aromatic molecule benzoic acid. Studies of photodissociation dynamics for another interesting aromatic compound benzaldehyde indicate at least four processes including –CO and –COH dissociation channels upon UV excitation by various photon energies on benzaldehyde molecular beam. Photodissociation and photoionization of potential MALDI matrix molecules like 2,5-Dihydroxy benzoic acid (2,5-DHB) and 2,5-Dihydroxyacetophenone (2,5-DHAP) expose their photochemistry in gas phase after being excited by UV photon and also acquaint with photon number dependences for the observed photofragments when studied by means of multi-photon ionization. At 355 nm photolysis wavelength 2,5-DHB molecule display H2O elimination as a major dissociation channel from the electronic ground state. For both the MALDI matrix molecules an estimated concentration of neutral fragments outnumbers the ionic fragments by 105-106 to one, when studied at 355 nm photolysis wavelength in molecular beam environment. Multiphoton ionization of matrix clusters generated in molecular beam environment do not show any protonated or negatively charged ions as usually observed in solid state MALDI. The observations indicate protonation or deprotonation reactions usually observed in solid state MALDI experiment must involve some complicated reactions; whereas any possible photoionization processes occurring in the gas phase after the matrix molecules vaporize from the condensed phase, does not seem to play an important role during MALDI process.
The second part of the thesis involves a study of condensed phase to gas phase desorption-ionization process using two-step laser desorption/ionization mass spectrometry. Here a second VUV (118 nm) laser pulse is interrogated to post-ionize the neutral species in desorption plume after they primarily get desorbed into gas phase by initial UV laser pulse. A direct correlation between the instantaneously formed ions generated by initial UV laser pulse and the post-ionized neutral molecules have been performed with an anticipation to characterize overall desorption event by decoupling desorption and ionization processes. Two of the most widely used MALDI matrix compounds 2,5-DHB and α-Cyano-4-hydroxycinnamic acid (CHCA) have been chosen as molecular systems. A better signal correlation between the directly formed ions and post-ionized neutrals have been observed for the matrix molecule CHCA unlike 2,5-DHB. The desorption and ionization events for pure matrix molecules have been studied to address some of the silent features in MALDI and throw light for better understanding of the fundamental material ejection in conventional UV-MALDI phenomenon. The objective of this particular experimental purpose is an attempt towards an improved quantitative analytical applications using MALDI mass spectrometry through a perception of the underlying basic mechanism for matrix-assisted laser desorption/ionization phenomenon.

關鍵字

Photodissociation Dynamics ； Molecular Beam ； Two-step Laser Desorption/Ionization mass spectrometry ； VUV photoionization ； MALDI

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