Residual gas analyzers (also known as RGAs) are both small spectrometers and the name of the technique that effectively measure the chemical composition of a gas, and are commonly used in industrial processes to check for contamination. In order to gauge what comprises a gas, residual gas analyzers ionize separate components to create a charge, and then determine the mass-to-charge ratios. The process can take place inside a vacuum, where quality is easier to monitor and impurities and inconsistencies are easier to detect because of the low pressure. Setups such as this can be found inside accelerators and scanning microscopes.
There are three main components in both types of RGAs: an ion source, a mass analyzer, and a detector. First, the gas is moved through an ion source, which turns molecules into ions. Then, the ions are sorted out according to mass by the mass analyzer, which does so by employing electric and magnetic fields. Lastly, the detector calculates the mass-to-charge ratio. The final display of data is referred to as a mass scan or a mass spectrum.
The ionizer, which turns the molecules of gas into ions, does so using electron impact ionization wherein an electron beam ionizes atoms of gas. A hot emission filament is responsible for creating the beam, but a magnetic field can destroy the filament and disrupt the beam. Because reactive gases, such as oxygen, can disrupt the electron flow, RGAs work best at low pressures.
After ionization takes place, the ions are sorted according to mass by a mass analyzer. Although there are numerous ways of analyzing ion mass, it is common for RGAs to depend on an RF quadrupole, which prevents ions with the incorrect mass-to-charge ratio for the given frequency from passing on to the ion collector. Depending on the sensitivity of the frequency range, either a Faraday cup or an electron multiplier may be used as an RGA ion detector.
The final data, or RGA mass spectrum, can be displayed as a chart of mass-to-charge ratio and relative intensity. Previous knowledge of how different molecules of gas with the same mass can have different mass-to-charge ratios can be helpful in identifying the gases.
When it comes to ionization, there are currently two types of RGA systems that work well: open source RGAs and closed source RGAs.
Open Ion Source RGAs
It is not uncommon for vacuum systems used in production to operate at two distinct pressure ranges. Base pressure is often used to clean the vacuum and subsequent parts. Process pressure occurs at a higher range and when specific gases are added for a given process. Base pressure, if less than 1E-4 Torr (a unit of pressure that is approximately 1/1760 of an atmosphere), can make use of an open ion source RGA ionizer. Because open ion source RGAs can only handle a maximum pressure of 1E-4 Torr, and base pressure tends to fall below this figure, they can usually be attached directly to the vacuum chamber. They measure the gas present without changing the gas composition or altering the vacuum environment.
Closed Ion Source RGAs
If pressure is between 1E-4 and 1E-3 Torr, then using a closed ion source RGA can help reduce overall process gas. A closed ion source RGA, which is a small ionizer, attaches to a quadrupole filter and has a tube with two openings: one for the electrons to enter and one for the electrons to exit. Alumina rings seal the tube, and the majority of the quadrupole is comprised of electrodes. As soon as the process begins and electron contact is intitated, the ions are formed. The actual ionization occurs at the same level of pressure as the process, meaning the pressure is the same in the rest of the chamber as it is where ionization occurs. However, the rest of the mass analyzer is under high pressure. Generally speaking, closed ion source RGAs operate between 1E-2 and 1E-11 Torr.
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