Study On Photoionization Of Two Methyl Esters And Development Of Proton Transfer Reaction Time-of-Flight Mass Spectrometer

Volatile organic compounds(VOCs)are important precursors of the formation of atmospheric aerosols.Detecting and studying the physicochemical properties of VOCs is very important for the study of atmospheric aerosols.Based on the photoionization experiments of methyl methacrylate and methyl crotonate,the possible dissociation channels of the main debris and the transition states and intermediates involved in the dissociation channels are analyzed experimentally and theoretically by combining the quantum chemistry theory calculation.It provides analytical support for the substance’s participation in atmospheric chemical reactions and aerosol formation.In addition,a proton transfer reaction time-of-flight mass spectrometry(PTR-TOFMS)has been established,the structure of the device has been adj usted and optimized,the installation and testing of various systems of the instrument has been carried out,and the performance of the instrument has been debugged in two steps.The mass spectrometry can realize on-line detection by chemical ionization,which provides important support for the research of atmospheric aerosols.The content of this paper is divided into six chapters.In the first chapter,the significance of photoionization of two methyl esters and the background of PTR-MS are summarized.Methyl methacrylate(MMA)and methyl crotonate(MC)have the characteristics of acute toxicity,high volatility and large emission,so it is necessary to clarify their single molecule chemical properties in order to study their effects on air quality.In addition,PTR-MS belongs to chemical ionization mass spectrometry technology,so the ion-molecule reaction and its development in chemical ionization are introduced in detail.Finally,the purpose and conception of the research are given.In Chapter 2,the photoionization of MMA is introduced in detail.The photoionization of MMA was studied in the energy range of 9.0-15.5 eV.The ionization energy of MMA molecule and the appearance energies of main fragment ions were obtained by measuring PIE curves.On the basis of experiments and theoretical calculations,the dissociation channels of major fragment ions are proposed and discussed in detail.H transfer and ring opening are the main processes in the production channels of MMA fragment ions.In Chapter 3,the photoionization and dissociation of MC are introduced in detail.The VUV photoionization and photodissociation of methyl crotonate in the energy range of 9.0-14.5 eV were studied.The photodissociation processes of two methyl esters were also compared.In Chapter 4,the basic principle,structure and application of PTR-MS are introduced in detail.PTR-MS mainly includes three parts:ion source,drift tube and ion detection system.The types of ion sources and the selection of reagent ions,the principle of quantitative analysis of drift tube,the selection of E/N value and the ion detection system are briefly introduced.PTR-MS is widely used in the fields of atmosphere,food and medicine.Chapter 5 introduces the detailed design of each part of the PTR-TOFMS and the installation and testing of each system.The theory and structure design of each part of the instrument are introduced in detail.Then the instrument vacuum,power supply,sample injection and detection system were installed and tested.Chapter 6 introduces the debugging of photoionization source mass spectrometry and PTR-TOFMS.The instrument is debugged in two steps:(1)Mass spectrometry is debugged with a simple VUV lamp ionization source.Firstly,the ability of mass spectrometry to detect the standard samples was examined by MI-TOFMS.Then the influence of the inlet flow rate,pressure and temperature on the signal intensity was tested.Finally,the linear range and sensitivity of the MI-TOFMS instrument were examined,and the essence and quality of the sample toothpaste were tested.(2)Replacement of annular glow discharge ion source and drift tube for PTR-TOFMS debugging and experiment.Firstly,the discharge conditions of annular glow discharge ion source were optimized,and different reactive ions were obtained by feeding different reactive gases into the ion source.Change the working conditions of the drift tube to control the reaction ion and cluster ion of the instrument.Some common organic compounds were detected and studied,and the characteristic product ions of each substance were obtained.Then PTR-TOF MS was used to detect the standard sample gas and to calculate the detection limit and sensitivity of the instrument.PTR-TOF MS for membrane injection was preliminarily debugged.Finally,the toothpaste samples were detected and compared.