The complementary development of mass spectrometry and vacuum

Why do mass spectrometers need high vacuum?
Mass spectrometry is a very important analytical method in the field of chemistry. It is a technique to measure the mass and strength of ions. By measuring the molecular mass and the corresponding ionic charge ratio, the analysis of the molecular structure in the sample to be measured is completed.

The vacuum system is the main component of the mass spectrometer because it involves the working principle of many components. Por exemplo, the filament that emits electrons in the ion source (such as E source, CI source, etc.) will burn out in the case of more oxygen, so the filament must work in a high vacuum state.

Por exemplo, high air pressure will interfere with the normal regulation of the electron beam in the ion source, and will also cause ion accelerated voltage discharge up to several dry volts, resulting in damage to the ionization chamber and high-voltage power supply lines. Além disso, high vacuum is also conducive to sample volatilization, reduce background interference, avoid molecule-ion reaction in the ionization chamber, and reduce the complexity of the spectrum. The generated ions must also have a certain molecular free path in their movement, otherwise the interaction between ions will cause ions to deflect and not reach the collector.

In short, in the mass spectrometry process, wherever there are sample molecules and ions passing through and present, a high vacuum must be pumped. The pressure requirement is 10-4~10-6Pa. The vacuum system generally includes low vacuum pre-pump (mechanical pump), vacuum pump (diffusion pump and turbomolecular pump are more commonly used), vacuum measuring instruments and vacuum valves, pipelines, etc., in order to obtain the high vacuum required by the instrument.

It can be imagined that if the interior of the cavity is not a vacuum environment, ions will collide during flight, resulting in changes in the flight path of ions or fragmentation, and the mass spectrometer will not be able to analyze and detect ions.

Portanto, the vacuum environment has become a necessary condition for mass spectrometry analysis, and the progress of the two is complementary to each other, the progress of vacuum technology has promoted the development of mass spectrometry, and the development of mass spectrometry has promoted the progress of vacuum technology.

The vacuum requirements of different levels of cavity are different, and the vacuum degree greatly affects the transmission efficiency of ions. The lower the vacuum degree, the smaller the interference and collision of ions in the transmission, and the sensitivity of the instrument will also rise.

Portanto, when designing a convenient and reliable vacuum system for mass spectrometer, the choice of pump is a key issue.

Usually molecular pump selection of need to be considered limiting vacuum of vacuum chamber needed to achieve when idle, the limit of the molecular pump vacuum is higher than the limit of vacuum in vacuum chamber, according to the required working pressure in vacuum chamber will choose molecular pump, the former mechanical pump is also want to make the appropriate selection according to the molecular pump, usually based on molecular pump performance test show that the backing pump molecular pump is pumping speed of different gases (N2, H2, He) 0.02 para 0.1 times the nominal pumping speed. Por exemplo, the pumping speed of a molecular pump is 10L/s, then the pumping speed of the front pump in 1L/s to 0.20L /s can basically meet the requirements, and when the front pump is opened, the molecular pump can work normally when the work reaches the molecular pump starting pressure.