Study on Miniature Magnetic Sector Mass Spectrometer

Miniaturization is one of the most important development directions of mass spectrometers as an analytical technique for in situ field analysis and space applications. However, miniaturization is often associated with performance limitations, such as smaller mass range, lower resolving power and sensitivity as compared to full laboratory and desktop systems. In our work, a theoretical model of the electric and magnetic field distribution of the ion optical system was established. Both theoretical simulation and experimental verification methods were used. Based on the methods, in-depth research was carried out on the interaction between miniaturization and high performance, the changes in ion transmission efficiency, mass discrimination effect, and ion motion trajectory of the instrument under the coupling of multiple parameters was revealed. According to the ion focusing principle, the physical parameters and structure dimensions of the ion optical system, ion source and mass analyzer were optimized, and a new miniature magnetic sector mass spectrometer was developed at the Lanzhou Institute of Physics. The compact instrument can be roughly divided into two parts of the physical unit and the electronic unit. The physical unit comprises an ion source, magnetic mass analyzer and two ion detectors. The total weight of this instrument is 4.3 kg, its dimensions are approximately （220×164×162） mm3, and the power consumption is 25 W. After optimization, the performance indicators have been improved compared with the instrument developed in 2012, including the sensitivity, resolving power, mass range and so on. The instrument key performance indexes were tested, and the research result showed that mass spectrometer can record mass range from 1 u to 134 u with two ion channels, and provide resolving power of about 56 and 13 for the large and small channel at full width half maximum (FWHM), and demonstrate a sensitivity of 1.63×10^-4, 1.17×10^-4, 2.3×10^-5 A/Pa for argon, nitrogen and helium in the secondary electron multiplier (SEM) operating voltage of -1 800 V. The minimum detectable leak rate is measured to be 1.24×10^-10 Pa·m³/s, and the instrument stability is about 2.5% within 3.5 h, which needs further improvement. Magnetic sector mass spectrometer has good mass spectrum peak shape, easy quantitative analysis, insensitive to pollution, simple structure, convenient manufacturing, and especially suitable for light ion detection. These inherent characteristics play a vital role in nuclear industry, geological sciences, space isotope measurement and vacuum leak detection.