The Detection And Analysis Of Trace Gas With Quadrupole Mass Spectrometer Of High Sensitivity

Trace gas analysis is an important part of the gas analysis, which has a very wide range of applications in industrial production and daily life. Generally speaking, the trace gas analysis mainly has two meanings:a lot of trace or ultra-trace analysis of impurities in the total amount of gas to be analyzed and a small amount of the total gas detection.The common method of detecting trace gases such as quadrupole mass spectrometry, gas chromatography, spectroscopy etc. had been described and compared. To the trace gas analysis in a small amount of total gas, the quadrupole mass spectrometer has highest sensitivity. The open ion source and closed ion source of quadrupole mass spectrometer were analyzed, which impact the sensitivity of trace gas. The result shows that the open ion source can not meet the requirement of sensitivity of trace gas. However, the closed ion source has higher pressure difference between inner and outer ionization. So the content of trace gases is improved, which suppresses the background interference. Thus, closed ion source was used for the trace gas analysis. Meanwhile, the non-destructive and destructive measurement of total amount of gas had been analyzed. By comparison, the gauge measurement of destructive measurement as an absolute measurement has highest accuracy.The performance parameters of closed ion source have a significant impact in sensitivity of trace gas analysis. So theoretical research was carried out. In the performance parameters of closed ion source, the electron entrance aperture not only affects the pressure in the ionization space, but also determines the efficiency of incident electrons that are emitted by filament. COMSOL Multiphysics 5.0 and CST DESIGN ENVIRONMENT were respectively used to study the electric field distribution and electron trajectories of closed ion source. Preliminary simulation result shows that with the increase of the electron entrance aperture radius, the pressure in the ionization chamber is decreased, but the efficiency of incidente electrons is increased. When the electron entrance aperture radius is about 1.8mm, the product the efficiency of incident electrons and the pressure within the enclosed space ionization is maximized.Experimental device used dynamic vacuum system that is made up of mechanical pump and molecular pump. The ultimate vacuum of quadrupole mass spectrometry chamber is 3.44×10-4 Pa. The closed ion source was used in the experiment. At the same time, electron multiplier of 17 beryllium copper and 10cm long quadrupole were matched with closed ion source, which can achieve high sensitivity of trace gas(PPM) and high resolution. Experiment analyzed the gas components of the small ball. The diameter of small ball is 531.1 ?m and the wall thickness is 3.96?m, and the experimental data are obtained.