Research Of A Digital Ion Trap Mass Spectrometer And A Novel Ion Excitation Method

Mass spectrometry has achieved a wide application owing to its capability of the determination of mass to charge ratio and the identification of molecular formula through the information of fragment ions. Currently, mass spectrometer has become an indispensable analytical instrument in many areas, which has played more and more vital role in the high-end analysis field such as life science, aerospace and defence technology, and sensitive hot-spot fields such as environmental protection and food safety. In the area of environmental emergencies monitoring, traditional gas chromatography-mass spectrometry(MS) is hard to be truly applied to emergency monitoring since the longer analysis time and the need to qualitative and quantitative analysis of air pollutants. Based on digital rectangular waveform driving divided ion trap, and coupled with the technology of membrane introduction and single photon soft ionization, this thesis developed a portable mass spectrometer which adopted digital ion trap technology for the first time, and used to rapid emergency monitoring of environmental pollutants. The introduction of a membrane sampling technique realized a rapid response(second level) of the instrument without sample processing, while the sensitivity of the instrument has been increased to pptv level because of membrane pre-concentration effect. In this design, a single photon ultraviolet lamp is used as the ionization source to produce molecular ion peaks and rapid and qualitative analysis of samples is accomplished in combination with the advantage of tandem MS. The application range of this portable instrument is greatly increased because the use of frequency scanning at medium and low fixed voltage magnitude(±100 V) in this ion trap could overcome the traditional discharge risk with sine wave scanning voltage of several thousand volts. The portable mass spectrometer has a weight of 18 kg and a size of 49 cm × 39 cm × 16 cm, the battery can last up to 4 hours, the average membrane response time is 9 seconds, the mass stability is less than 0.1 Th within 48 hours, the limit of detection is less than 1 ppbv, the instrument has reached advanced domestic level and can satisfy the monitoring need of rapid, in-situ, qualitative and quantitative.I have participated in the preliminary evaluation, theoretical study and design of the protype of the portable digital linear ion trap mass spectrometer during my graduate years. During the doctoral period, the prototype was assembled and debugged, and the performance verification of the instrument was evaluated. Most importantly, a more comprehensive technical improvement was implemented. In this improvement of the technology, non-integer dipole excitation phenomenon was discovered and theoretically explanation was given through the synchronized waving ejection effect. At the same time, the digital rectangular driven ion trap technique is applied to other types of mass spectrometers with initial results. In order to keep the integrity of the thesis, the theory and design of the portable digital linear ion trap are described in detail in the thesis.1. History of mass spectrometersThis chapter introduced ion sources and mass analyzers mainly used in mass spectrometers, while the performance indicators to evaluate the instrument such as resolution, sensitivity and mass range were also discussed. The working principle and developments of quadrupole mass spectrometers were analyzed.2. Theoretical calculation and simulation of ion traps Various frequency components of the ion motion were obtained by fast fourier transform(FFT) of the ion motion in the ion trap which decides the resonance frequency of ion resonance excitation signals and other higher order frequency components. For the design of a flat plate divided electrode ion trap, least square method was used to calculate the electric field components in the ion trap with different divided potential ratio(DPR), and the high-order field proportion under different DPR and the optimum DPR range were obtained. Moreover, system simulations were thoroughly launched on repulsion focusing electrodes of the ionization chamber in order to increase the ion extraction efficiency, and the position of the vacuum ultraviolet lamp, mass resolution and the trapping efficiency of ions under different pressures, at last the optimum position of vacuum ultraviolet lamp and the optimum pressure were obtained, which is a good guidance for the follow-up instrument design.3. Implementation of the whole instrument This chapter described the design of the vacuum system, the ionization source, the membrane sampling and heating module, the circuit design of high-voltage high-frequency module, further optimization of high-frequency high-voltage power supply modules, the design and performance testing of the dipole excitation module, RS485 firmware and USB2.0 driver development of the main control board, the circuit design and module improvement of direct current amplifier, respectively. The performance of the instrument was tested using the mixed gas standard(benzene, toluene, xylene and monochlorobenzene), including mass resolution, limit of detection, dynamic range and the membrane response time. MS2 was achieved by methyl salicylate through the special tandem MS method of the digital ion trap. A quick qualitative analysis method was established which uses molecular ion peak produced by single photon ionization together with tandem MS.4. Research of a novel ion excitation method The goal was to study a new excitation method which can be used to improve sensitivity and resolution of the instrument. During the experiments, it is found that non-integer dipole excitation could improve both mass resolution and sensitivity. An attempt was made to analyze this method theoretically. It is concluded that non-integer dipole excitation may be explained by the theory of surge excitation. By calculating appropriate scanning speed and phase difference between the dipole excitation waveform and the main RF waveform, the improvements of mass resolution and sensitivity of the instrument were achieved simultaneously.5. Coupling of digital driven rectangular wave round rod ion trap with time-of-flight mass spectrometer(TOF-MS)From the point of view of the application of digital driven rectangular wave technology, the digital rectangular wave was applied to the drive of the round rod ion trap, which was combined with the TOF-MS. I am mainly responsible for the control of the round rod ion trap module. According to the difference of the round rod ion trap and the flat plate divided electrode ion trap, the drive circuit is improved to achieve desired objectives. The performance of the ion trap modules was investigated in the transmission mode, the ion accumulation mode, the ion isolation mode, MS2 analysis mode and MS3 analysis mode.6. Summary and prospective. The results obtained in this paper were summarized. Meanwhile, the shortcomings of this study and the future works were discussed.