Research on Key Techniques of Cylindrical Ion Trap Mass Spectrometer and Parameters Optimization

Mass spectrometry is widely used analytical technique due to its high specificity for analyte identification as well as its high sensitivity. A custom mass spectrometer instrument based on a cylindrical ion trap (CIT) was built and its working parameters were optimized. An electron ionization (EI) source was chosen for experimental simplicity and availability of standard EI spectral libraries. The custom designed CIT was consisted of a pair of endcap electrodes and a ring electrode (r₀=10 mm, z₀=8.97 mm). A high voltage RF signal which needed to be applied directly to the ring electrode to produce appropriate quadrupole was initially created by an RF generator at low voltage. A power amplifier and a voltage amplifier circuit were used to amplify to an appropriate power level. The boundary ejection method was used. An electron multiplier, a current amplifier and NI PCIe data acquire card made up the ion detection and data acquisition system. The EI source, CIT and electron multiplier (EM) were mounted in a rail which ensured they were aligned precisely. The software designed in LabVIEW controlled the timing of the whole system. Several MATLAB programs were written to perform post-processing operations on saved MS data files. In order to obtain the optimal working parameters, different values of filament current, bias voltage of EI, background pressure, voltage on the EM detector and mass scan rate were tested in the vacuum chamber. The trends of spectral peak were obtained: 1) Signal intensity increased with the heating current supplied to the EI source due to the increased electron emission at higher temperature; 2) The bias voltage of EI affected the kinetic energy of ions. With the increasing of the voltage, the signal intensity increased. When the voltage increased to a certain range, the intensity fluctuated. In most common EI sources, a kinetic energy of 70 V was used. So the bias voltage was set as 70 V; 3) Both signal intensity and resolution increased when background pressure was increased from 1×10^-4 to 6×10^-4 Pa. When the pressure further increased, the performance decreased; 4) Signal intensity was proportional to the voltage on the EM detector. Considering the signal intensity and the lifetime of EM, a voltage of 1.75 kV was used; 5) Mass scan rate had influence on the signal intensity and resolution. When the scan rate was too fast, the various ions bunched together and increased the signal intensity but reduced the resolution. Slowing the scan rate caused the peaks to spread apart resulting in increased resolution. By using the optimized parameters, several experiments were carried out. The results showed that the mass spectrum was accurate and the resolution was about 300. The MS platform performed well which laid a foundation of further research. To further improve resolution, axial RF will be applied to the endcap electrode to perform resonance ejection in the following work and based on this platform the operation method for high-pressure MS will be explored.