Development Of VUV Photoionization Time Of Flight Mass Spectrometer&Its Applications In Atmospheric Chemistry

Vacuum ultraviolet photoionization mass spectrometry(VUV-PIMS)as one of soft ionization methods is suitable for real time and online species analysis and has already been applied in a great deal of fields such as atmospheric chemistry,physical chemistry and combustion chemistry.In VUV-PIMS,molecules are softly ionized at their ionization energy thresholds after absorbing single VUV photon energy and then molecular information without fragmentation can be obtained directly.It’s significant to develop a sensitive VUV-PIMS and apply to the detection of products during atmospheric chemical reactions for studying the mechanism of reactions and revealing the causes of the formation of atmospheric pollutants.The author will present a home-made VUV photoionization time-of-flight mass spectrometer(VUV-PI-TOFMS)and its initial applications in atmospheric chemistry in this dissertation.A VUV-PI-TOFMS has been designed and preliminarily used for the study of atmospheric chemistry in laboratory coupled with an atmospheric simulation chamber and a flow tube as reactors.Three kinds of inlets are used to sample species into the VUV-PI-TOFMS.A capillary is installed to sample stable species from the simulation chamber at one atmospheric pressure into the photoionization region directly.A molecular beam is adopted for sampling both stable species and reactive intermediates like radicals and especially is combined with the flow tube to study radical reactions at low pressure.In addition,a novel aerodynamic lens is designed for focusing and transmitting ultrafine nanoparticles efficiently into the VUV-PI-TOFMS.More concretely,the various products of the atmospheric reactions in the above reactors have been probed at a molecular level.This dissertation mainly includes the following four parts:1.The principle and the design of the VUV-PI-TOFMS.The VUV-PI-TOFMS has a compact design and is mainly composed of a commercial VUV discharge lamp as photoionization light source and a reflectron time-of-flight mass analyzer to measure the ion mass.Firstly,according to the classical Newton’s laws,the focus points of first and second order acceleration electric field in a mass spectrometer are theoretically calculated and the ion flying paths in the electric field are simulated with asoftware of Simion 8.1.Then,the VUV-PI-TOFMS has been designed as an orthogonal accerelation reflectron time-of-flight mass spectrometer with a compact configuration.The influence of the ion transmission lens and the mental mesh used in acceleration and reflectron on the detection sensitivity has also been analyzed.2.The optimization and the performances of the VUV-PI-TOFMS according to the experimental results.Three kinds of commercial VUV discharge lamp,E-luTM126、PKS106 and L7293,are used in experiments to photoionize a concentration calibrated mixture gases containing benzene,toluene and xylene and then to measure the mass resolution and the limit of detection(LOD)of the VUV-PI-TOFMS.A high sensitivity has been achieved at LOD=0.3 ppbv for benzene.3.Coupled with the fast flow-tube reactor,the VUV-PI-TOFMS has been primarily used in the study of atmospheric reactions of radicals.A method to quantify radicals is proposed and has been utilized to measure the concentration of CH3 radical Combined with the literature self-reaction rate constant of CH3 radical and its’kinetic process detection,the concentration of CH3 radical in the fast flow tube is determined real time.The LOD of this setup for CH3 is obtained to be 1.3 ppbv.Meanwhile,the products of reaction between CH3 radical and O2 were obtained and Cl initiated oxidation of toluene under low pressure(2 Torr)and atmosphere were also investigated.4.The VUV-PI-TOFMS coupled with an aerodynamic lens is used to measure the chemical compositions of ultrafine nanoparticles.A novel aerodynamic lens has been specially designed and coupled with VUV-PI-TOFMS to transmit and focus ultrafine nanaopartcles,and then measure their checmial compositions at a molecular level.Oleic acid(OA)and Dioctyl Phthalate(DOP)are used as representative examples of monodisperse aerosol to test the LOD of the VUV-PI-TOFMS.The LOD of～40μg m-3 for OA is achieved.And the LOD for 40 nm and 60 nm DOP nanoparticles are 15 μg m-3 and 11 μg m-3,respectively.Newly formed ultrafine particles are produced in the nucleation flow tube which adjust the sizes of particle by changing the reaction time.The iodine-containing aerosols generated by the nucleation reaction of methylene iodide and ozone irradiated with UV light.The compositions of I,HI,HIO,IO2 and HIO3 are identified and assgined in the mass spectrum.The organic particles formed by ozonolysis reactions of α-pinene are also studied and their compositions are also probed in experiments.