Reasons for using helium as leak detection gas in helium mass spectrometry
Helium leak detector is the common name of helium mass spectrometer leak detector. The instrument made by mass spectrum principle is called mass spectrometer or mass spectrometer. Through the mass spectrometer's core component, the mass spectrometer turns gases of different mass into ions and moves in a certain field. Ions with different mass to charge ratios are separated from each other in the field, while ions with the same mass to charge ratio are gathered together in the field. If the receiver is placed at an appropriate position to receive all these ions, the mass spectrum map arranged in sequence according to the mass to charge ratio will be obtained Mass spectrometry.
The mass spectrometer used for leak detection is called mass spectrometer leak detector. All mass spectrometers for measuring gas partial pressure, such as quadrupole mass spectrometer, radio frequency mass spectrometer, time of flight mass spectrometer, cyclotron mass spectrometer, etc., can be used for leak detection.
The specially designed mass spectrometer with helium as tracer gas is called helium mass spectrometer leak detector. In addition to its high sensitivity, the instrument has the advantages of wide application range, accurate positioning and quantitative, non-toxic, safe and fast reaction speed. The most commonly used helium mass spectrometer is the 90 ° and 180 ° magnetic bias transition mass spectrometer.
In helium mass spectrometer leak detector system, helium is usually used as tracer gas in mass spectrometer leak detector system
1. The content of helium in the air and the residual gas of vacuum system is very small (about 5.2ppm in the air) and little in the material outgassing, so the background pressure is small and the background current output is small. Because of the small background, the fluctuation of the background is caused by some reasons, that is, the background noise is also small, so the small leakage rate can be reflected and the sensitivity is high.
2. Helium has a small mass (relative molecular weight of 4) and is easy to pass through the leak hole. In this way, the leakage rate of helium through the same leak hole is larger than that of other gases except hydrogen, which is easy to detect and has high sensitivity.
3. Helium is an inert gas. It does not react with the wall of the tested object, and it will not pollute the object. It is safe to use.
4. On both sides of helium are hydrogen (mass to charge ratio is 2) and double charged atomic carbon (mass to charge ratio is 6). In this way, the deflection radii of the analyzers are different greatly, and they are easy to be separated. When the helium peak is calibrated, it is not easy to be interfered by other ions. Therefore, the requirements for the manufacturing accuracy of the analyzer are reduced and it is easy to process. At the same time, the gap between the outlet electrode of the analyzer and the accelerating electrode of the ion source can be enlarged, so that more helium ions can pass through and the sensitivity of the instrument is improved.
5. Helium is not easy to be adsorbed in the sample and vacuum system. In this way, the helium signal will disappear quickly so that the leak detection can be continued and the leak detection efficiency of the instrument can be improved.
6. Some properties of hydrogen (such as small mass and easy to pass through the leak) are better than helium. However, hydrogen is rarely used because it is explosive on the one hand and produces a large amount of carbon and hydrogen in the oil diffusion pump due to thermal cracking of oil, which makes the hydrogen background extremely high and fluctuates greatly, thus greatly reducing the sensitivity.