VUV Photoionization Mass Spectrometric And Dissociative Dynamics Studies Of Some Organic Molecules

In this dissertation, the IR laser desorption/synchrotron-based VUV photoionization mass spectrometry was used to study the photoionization and photodissociation processes of some organic compounds. With help of quantum chemical calculations with Gaussian software, the detailed photon-induced fragmentation pathways of these molecules were studied and the possibly dissociation channels were proposed. Also the typical photoionization mass spectra of these molecules, the ionization energies (IEs) and appearance energies (AEs) of dominant fragment ions were determinedby measuring the photoionization efficiency (PIE) curves. These data are helpful for orgainic mass spectrometric analysis, computation of formation heats and bond dissociation energies.In the1st chapter, the brief history of mass spectrometeric development was summarized and the principal of mass spectrometers was introduced, including the principals of different kinds of mass analyzers and some mostly-used ionization sources. Among the different ionization methods, single-photon ionization (SPI) technique is a potential soft ionization method, which can avoid the complicated ionization processes, such as multi-charge and multiphoton ionization in electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). Hence, the SPI was used to study the photoionization mass spectrometric behaviours and fragmentation patterns of these compounds. At last, the principal and design of the time-of-flight (TOF) mass analyzer used in this work were briefly introduced.The2nd chapter introduces the experimental facility and theoretical calculation methods used in this dissertation. Firstly, the synchrotron vacuum ultraviolet light sources from U10and U14C beamlines at National Synchrotron Radiation Laboratory (Hefei, China) were presented, and some optical parameters such as photon energy range, energy resolution were provided. Secondly, the equipment of IR laser desorption/VUV photoionization mass spectrometry (IR LD/VUV PIMS) was described. Compared to other desorption methods, for instrance thermal evaporation and ion beam desorption, this desorption source used in this work can efficiently desorb the non-volatile and thermally labile compounds and avoid the direct generation of ions. Eventually the computation methods were presented.In the3rd chapter, with the help of theoretical calculations, the photoionization and photon-induced dissociation of sarcosine were investigated. The photoionization mass spectra of sarcosine at different photon energies were obtained experimentally, and some fragment ions such as CH2COOH2+(m/z60), CH3NH2CH2+(m/z45), CH3NHCH2+(m/z44), CH2NHCH2+(m/z43), CH2NCH2+(m/z42) and CH2NH2+(m/z30) were observed. The theoretical calculations were combined to discuss the generation pathways of these fragment ions, among which fragment ion CH2COOH2+derives from CH2NH-elimination of the molecular ion, involving two-step intramolecular hydrogen transfers. However, the fragment ions CH3NH2CH2+and CH3NHCH2+were generated from molecular ion losing CO2and COOH, respectively. Additionally, the IE of sarcosine was determined to be8.42±0.1eV by measuring the PIE curve of the molecular ion.In the4th chapter, the IR LD/VUV PIMS was used to study the photoionization/dissociation processes of some small-molecule drugs, including metronidazole, paracetamol, isoniazid, aspirin and salicylic acid. The photoionization mass spectra of these molecules at various energies were obtained. Compared to electron impact (EI) ionization, the photoionization can exclusively generate the molecular ions (M+) of these molecules without any fragment ions. This is helpful for mass spectrometric analysis and related studies. In the ionization of metronidazole, the fragment ions of C6H8N2O+(m/z124), C6H9N2O+(m/z125), C4H4N3O2+(m/z126) and C4H5N3O2+(m/z127) were measured experimentally. With help of theoretical computation, it is proposed that C6H8N2O+and C6H9N2O+were generated from molecular ion (M+) losing side-chain nitryl group of imidazole. However C4H4N3O2+and C4H5NsO2+were derived from side-chain aminoethyl elimination. In the photoionization studies of paracetamol and aspirin, ketene-elimination was proposed to be the dominat dissociation pathway of these two molecular ions. The fragment ions C6H4N2O+(m/z120), C6H4NO+(m/z106), C5H5N+(m/z79) and C5H4N+(m/z78) generated from NH3-loss, H2NNH-loss and side-chain elimination, respectively, were observed in the photoionization mass spectra of isoniazid. In the photoionizaotn of salicylic acid, the fragment ions of C7H4O2+(m/z120) and C6H4O+(m/z92) were observed, which were proposed to derive from the dehydration and secondary CO-elimination of salicylic acid parent ion.Besides, the IEs of metronidazole, paracetamol, isoniazid, aspirin and salicylic acid were measured to be8.84±0.1eV,7.70±0.05eV,9.14±0.05eV,9.02±0.05eV and8.52±0.05eV, respectively.In the5th chapter, the photoionization and dissociation processes of four neurotransmitter molecules including tyramine, dopamine, histamine and tryptamine were investigated. By selecting the appropriate photon energy, only molecular ions for these four molecules were obtained. Compared to the El ionization generating a lot of fragments, it is obvious that this method used in this work is very convenient for the gas-phase chemical study of small biomolecules. In photoionization processes of tyramine and dopamine, the fragment ions C7H8O2+(m/z124), C7HgO+(m/z108) and CH2NH2+(m/z30) were observed. Furthermore, C7H8O2+and C7H8O+were proposed to derive from the McLafferty rearrangements of the corresponding molecular ions. With help of theoretical calculations, the detailed rearrangement reactions of molecular ions were investigated, and other dissocation pathways were discussed also. Similarly, the McLafferty rearrangement was found to exist in the photoionization process of histamine and tryptamine. Additionally, the IEs of these four molecules were determined to be7.98,7.67,8.00and7.34eV with experimental uncertainty of±0.05eV.In the6th chapter, the ionization processes of a series of steroid molecules, including synthetic anabolic hormones, estrogen and progestogen steroids, were studied with photoionization mass spectrometry. The typical photoionization mass spectra of these molecules were obtained. Especially, only molecular ions were observed without any fragment ions by virtue of near-threshold single-photon ionization technique, which indicates that it is a potential ionization method in mass spectrometric detection of steroid hormones. While increasing photon energy, numbers of fragment ions for each molecule were observed. According to previous reports by other groups and fragmentation rules, the possible photon-induced fragmentation pathways for the ionization processes of these molecules were presented. Besides, the IEs of them were obtained by measuring the PIE curves of the corresponding molecular ions, which are helpful for understanding gasous ion thermal chemistry and ion-molecule reactions.