Numerical Simulation Of Ion Trap Mass Spectrometry

The rectilinear ion trap (RIT) and electrostatic ion beam trap (EIBT) studied in the thesis meet the scientific and technological progress and market needs in mass spectrometry which is directed to miniaturized desktop with high sensitivity and high resolution..As a newly-developed miniaturized mass analyzer, RIT takes the advantages of linear ion trap of two-dimensional storage by using a simple rectangular structure, which greatly improves the ion storage capacity and storage efficiency, and thus with higher sensitivity, greater mass range and better mass resolution. Based on setups of actual rectilinear ion trap, the whole working process of RIT is simulated fully considering the structural factors of electrode structure. Finite Difference Method was employed to calculate the electric field and Runge-Kutta fourth order integration was used to calculate ion's motion. The motion and ejected distribution of ions in a rectilinear ion trap mass spectrometer are studied and mass spectrum with various waveforms and scanning speeds of RF voltage are investigated. The simulation results is shown as follows,1. Due to the symmetry of the structure, the electric field inside the rectilinear ion trap is mainly shown as quadrupole field, with octupole component overlapped mainly near the electrodes;2. By voltage scanning or frequency scanning, RIT realizes the function of mass analyzer and distinguish the ions according to their mass to charge ratio;3. The Mathieu stability diagram of the RIT with flat electrodes is different to the diagram of ideal quadrupole electric field. The superimposed octupole field shrinks the stability region.4. Stretching the rectilinear ion trap along the x direction makes more ions ejected along the x-axis direction, which could improve both sensitivity and mass resolution of the analyzer.5. The ions will eject according to the mass as voltage increasing, and decreasing scan rate can be used to improve the mass resolution of rectilinear ion trap.6. By the simulation of mass spectrometry with different scanning waveform, it is found that the mass resolution is improved and the voltage needed is reduced when using the square wave. EIBT recently invented is a combination of time of flight mass spectrometry and Fourier transform ion cyclotron resonance mass spectrometry, which could store and analyze ions only with electrostatic field. Since the trap employs electrostatic fields, its operation is not mass limited and therefore its high mass resolving power can be sustained for large mass molecule. Dual-cylinder metal detector is studied to get the induced current. Compared to single cylinder detector, the detected signal is greatly enhanced. Numerical calculation is used to simulation the induced charge distribution on the detector, and boundary effect of the detector is fully taking into account. The induced charge distribution on the detector, and induced current influenced by the radius and length of dual-cylinder detector are studied respectively.1. Compared to single cylinder detector, the induced current on the inner tube of the detector is increased significantly and that of the outer tube is equivalent to that of the single detector due to the interaction between the two tubes.2. The current peaks on the two tubes are almost at the same position with opposite sign. After combining these two currents on the two tubes, the combined signal is increased again which is three times than that on the single detector with the same size of the inner one.3. Decreasing the radius of the inner tube will enhance the strength of combined current signal significantly. When the radius of the inner tube is halved, the combined current signal will be doubled.4. The outer tube radius has little effect on the combined signal strength, which variation causes a slight change of induced current on the outer tube, thus the combined current has a minor change.5. Increasing the length of outer tube enhances combined current. However, when the length of outer tube exceeds twice of the inner one, its effect becomes more and more weak.