Residual Gas Analyzers

Residual gas analyzers (RGAs) are spectrometers that are used to identify the gases present in vacuum environments.  Residual gas analyzers perform this function by producing ionizing molecules of the gas sample, separating the resulting mixture of ions according to their charge-to-mass ratios, providing an output signal, which is a measure of the relative species present, and counting the rate at which the ions hit the detector. The spectrometers within residual gas analyzers analyze the process by which a mixture of ionic species is separated according to the mass-to-charge ratio (m/z). The analysis may be qualitative and/or quantitative.  There are two common styles of spectrometric analysis used, quadrupole and magnetic sector analysis, although there are specialized and custom designs available.

Quadrupole mass spectrometers consist of an ion source, ion optics to accelerate and focus the ions through an aperture into the quadrupole filter, the quadrupole filter itself with control voltage supplies, an exit aperture, an ion detector, detection electronics, and a high-vacuum system.  The mass spectrometer operates only in a high vacuum whereas the gas it is being used to analyze is often at atmospheric pressure. To reduce the pressure the gas is sampled twice using a "double sniffer" configuration.

Magnetic sector residual gas analyzers use one of two methods to read and identify samples.  Single focusing analyzers focus a circular beam in a path of 180, 90, or 60 degree. A variety of forces influence the particles and separate the ions due to their different mass-to-charge ratios. In double focusing analyzers, an electrostatic analyzer is added to separate particles with difference in kinetic energies.

Residual gas analyzers ionize their samples using a variety of technologies and methods including electron impact ionization (EI), field ionization (FI), and chemical ionization (CI).  EI uses an electron beam, usually generated from a tungsten filament, to ionize gas-phase atoms or molecules. An electron from the beam knocks an electron off of analyte atoms or molecules to create ions.  FI removes the electrons from any species by interaction with an intense electrical field. CI forms new ionized species when gaseous molecules interact with ions. The process may involve transfer of an electron, a proton or other charged species between the reactants.

Finally, there are two distinct detector types that can be found within residual gas analyzers. Electron multiplier tubes are similar in design to photomultiplier-tubes. They consist of a series of biased dynodes that eject secondary electrons when they are struck by an ion. They therefore multiply the ion current and can be used in analog or digital mode.

Faraday cup residual gas analyzers use a metal cup that is placed in the path of the ion beam. It is attached to an electrometer, which measures the ion-beam current. Since a Faraday cup can only be used in an analog mode it is less sensitive than other detectors that are capable of operating in pulse-counting mode.

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