Shang-Ting Tsai

Dissociation rate of hot benzene

The dissociation rate of benzene and d6-benzene were measured under collision-free condition by multimass ion imaging techniques. The value of 1±0.2×105 s−1 and 5±1×104 s−1 were obtained for benzene and d6-benzene, respectively, with internal energy of 618 kJ/mol. The dissociation rate of benzene with internal energy of 483 kJ/mol was too slow to be measured, and the upper limit of the dissociation rate was estimated to be 3×103 s−1.

Multimass ion imaging detection: Application to photodissociation

A constant momentum mass spectrometer with a two-dimensional ion detector in conjunction with a pulsed vacuum ultraviolet laser is used for the simultaneous measurement of the translational energy distributions of many different fragments. A description of the apparatus and its performance are presented. Preliminary experimental results on the photodissociation of toluene and benzene are given.

Photoionization of methanol dimer using a tunable vacuum ultraviolet laser

Methanol dimer (CD3OH)2 ion–molecule reaction is initialized by VUV (vacuum ultraviolet) laser photoionization. The proton and deuteron transfers are the dominant reactions. The relative probabilities of deuteron transfer from the methyl group and proton from the hydroxyl group were measured as a function of VUV photon energy between 10.91 to 10.49 eV. According to those results, the probability of proton transfer from the hydroxyl group increases with the VUV photon energy. Isotopic scrambling is not complete before dissociation of the ion complex in the photon energy used. In addition, ab initio calculations are performed and four stable structures of the methanol dimer ion are found. One of these structures is an unreported complex, CD3OHD+⋯CD2OH, which has a very unusual type of hydrogen bond. This complex plays a significant role in the deuteron transfer reaction in the range of excitation energies used in this study.

Carbon–carbon bond cleavage in the photoionization of ethanol and 1-propanol clusters

Tunable VUV laser was used to initiate the ion-molecule reactions in the clusters of ethanol and 1-propanol by photoionization in the region between 10.49 to 10.08 eV. Ionic products were detected by the time-of-flight mass spectrometer. In addition to the protonated clusters from proton transfer reactions, the products corresponding to beta carbon-carbon bond cleavage were found to be one of the major products for small sizes of clusters. A comparison with photoionization of methanol clusters and the results of ab initio calculation has been made.

355 nm Multiphoton Dissociation and Ionization of 2, 5-Dihydroxyacetophenone†

Multiphoton dissociation and ionization of 2,5-dihydroxyacetophenone (DHAP), an important matrix compound in UV matrix-assisted laser desorption/ionization (MALDI), is studied in a molecular beam at 355 nm using multimass ion imaging mass spectrometer and time-of-flight mass spectrometry. For laser fluence larger than 130 mJ/cm(2), nearly all of the irradiated molecules absorb at least one photon. The absorption cross section was found to be sigma = 1.3(+/-0.2) x 10(-17)cm(2). Molecules excited by two photons quickly dissociate into fragments. The major channels are (1) C(6)H(3)(OH)(2)COCH(3) --> C(6)H(3)(OH)(2)CO + CH(3) and (2) C(6)H(3)(OH)(2)COCH(3) --> C(6)H(3)(OH)(2) + COCH(3). Molecules absorbing three or more photons become parent ions or crack into smaller ionic fragments. The concentration ratio of ions (parent ions and ionic fragments) to neutral fragments is about 10(-6):1. Changing the molecular beam carrier gas from He at 250 Torr to Ar at 300 Torr results in molecular beam clustering (dimers and trimers). Multiphoton ionization of clusters by a 355 nm laser beam produces only dimer cations, (C(6)H(3)(OH)(2)COCH(3))(2)(+). Protonated clusters or negatively charged ions, observed from a solid sample of DHAP using 355 nm multiphoton ionization, were not found in the molecular beam. The experimental results indicate that the photoionization occurs in the gas phase after DHAP vaporizes from the solid phase may not play an important role in the MALDI process.

Simple Method for De Novo Structural Determination of Underivatised Glucose Oligosaccharides

Carbohydrates have various functions in biological systems. However, the structural analysis of carbohydrates remains challenging. Most of the commonly used methods involve derivatization of carbohydrates or can only identify part of the structure. Here, we report a de novo method for completely structural identification of underivatised oligosaccharides. This method, which can provide assignments of linkages, anomeric configurations, and branch locations, entails low-energy collision-induced dissociation (CID) of sodium ion adducts that enable the cleavage of selective chemical bonds, a logical procedure to identify structurally decisive fragment ions for subsequent CID, and the specially prepared disaccharide CID spectrum databases. This method was first applied to determine the structures of four underivatised glucose oligosaccharides. Then, high-performance liquid chromatography and a mass spectrometer with a built-in logical procedure were established to demonstrate the capability of the in situ CID spectrum measurement and structural determination of the oligosaccharides in chromatogram. This consolidation provides a simple, rapid, sensitive method for the structural determination of glucose oligosaccharides, and applications to oligosaccharides containing hexoses other than glucose can be made provided the corresponding disaccharide databases are available.

Simple Approach for De Novo Structural Identification of Mannose Trisaccharides

AbstractOligosaccharides have diverse functions in biological systems. However, the structural determination of oligosaccharides remains difficult and has created a bottleneck in carbohydrate research. In this study, a new approach for the de novo structural determination of underivatized oligosaccharides is demonstrated. A low-energy collision-induced dissociation (CID) of sodium ion adducts was used to facilitate the cleavage of desired chemical bonds during the dissociation. The selection of fragments for the subsequent CID was guided using a procedure that we built from the understanding of the saccharide dissociation mechanism. The linkages, anomeric configurations, and branch locations of oligosaccharides were determined by comparing the CID spectra of oligosaccharide with the fragmentation patterns based on the dissociation mechanism and our specially prepared disaccharide CID spectrum database. The usefulness of this method was demonstrated to determine the structures of several mannose trisaccharides. This method can also be applied in the structural determination of oligosaccharides larger than trisaccharides and containing hexose other than mannose if authentic standards are available. Graphical Abstract

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

AbstractUltraviolet laser pulses at 355 nm with variable pulse widths in the region from 170 ps to 1.5 ns were used to investigate the ionization mechanism of matrix-assisted laser desorption/ionization (MALDI) for matrices 2,5-dihydroxybenzoic acid (DHB), α-cyano-4-hydroxycinnamic acid (CHCA), and sinapinic acid (SA). The mass spectra of desorbed ions and the intensity and velocity distribution of desorbed neutrals were measured simultaneously for each laser shot. These quantities were found to be independent of the laser pulse width. A comparison of the experimental measurements and numerical simulations according to the multiphoton ionization, coupled photophysical and chemical dynamics (CPCD), and thermally induced proton transfer models showed that the predictions of thermally induced proton transfer model were in agreement with the experimental data, but those of the multiphoton ionization model were not. Moreover, the predictions of the CPCD model based on singlet–singlet energy pooling were inconsistent with the experimental data of CHCA and SA, but were consistent with the experimental data of DHB only when some parameters used in the model were adjusted to extreme values. Graphical Abstractᅟ