Simulation on a Novel Method for Low Vacuum Mass Spectrometry

As an advanced product in scientific instruments, mass spectrometer plays an important role in many areas. Recently, a lot of efforts have been put on the miniature ion trap MS system development. However, due to the restraints of vacuum systems, it is difficult to reduce the size of a miniature mass spectrometer furthermore. Performing mass analysis in a low-vacuum environment can avoid the use of a bulky turbo pump, which will significantly reduce the size, weight, cost and power of the instrument. Moreover, this technology can be used to develop next generation hand-portable instruments and bring mass spectrometry to new applications. Quadrupole mass filter is one of the most mature and widely used mass analysers. In general, when using the quadrupole mass filter, the pressure in the chamber needs to be maintained on the order of 10-5 Pa. As background pressure increasing, the ions collide with the background gas molecules more frequently. The energy of the ion's axial motion decreases gradually and finally falls to zero. Thus, mass analysis failed since the ions are unable to reach the detector located at the end of the analyser. To address this issue, a mass analysis method was proposed in this study where the drift gas flow was used to provide the ejection energy for ions. During mass analysis, the sample was ionised in the ion source. Then, the ions shot into the mass analyser and travelled under the combined action of the quadrupole field and the drift gas flow. The drift gas flow provided the kinetic energy for axial ion motion, and the quadrupole field selected ions depending on the mass-to-charge ratio in the cross-sectional direction. Thus, although in low-vacuum environment, ions in stable motion could pass through the mass analyser and hit the ion detector, where spectral information was generated. The simulation platform was built in COMSOL Multiphysics. The simulation results showed that the obtained spectral peak was superior to those without drift gas flow in the intensity and the width of the spectral peak. The proposed method is suitable when a high precision is not required. Besides, as components for low vacuum mass spectrometry, a novel quadrupole mass filter and a novel ion detector were used. For the novel QMF, rectangular electrodes were employed instead of traditional hyperboloid or cylindrical electrodes. To eliminate the turbulence near the ion detector when the drift gas flow through the detector, a novel ion detector with a double layer structure was constructed. The research results lay a theoretical foundation for low vacuum mass spectrometer, and it will promote the development of portability mass spectrometers. In the following work, a prototype will be built to verify the simulation results.