Residual Gas Analyzer

Residual Gas Analysers are small mass spectrometers generally measuring masses up to 200amu.   Their name implies use as a vacuum diagnostic tool with measurement of partial pressures within the vacuum system.   Also extensivley used for process monitoring.   RGA's typically employ a quadrupole filter, but any mass spectrometer type that is rugged and low cost fits in the category.

A Residual Gas Analyzer (RGA) is an instrument for measuring the partial pressures of gas species in a vacuum system.   It is really a MASS SPECTROMETER characterised by being small, robust and low cost.

Frequently Asked Questions

THEORY
The origins of RGA date back to the Manhattan Project circa 1942, initially using a magnetic sector field to separate ions of differing charge-to-mass ratio, but nowadays it is the quadrupole mass spectrometer that is usually at the heart of an RGA.

An RGA consists of three modules:

  • An ion source, which can convert gas phase sample molecules into ions.
  • A mass analyzer, which sorts the ions by their masses by applying electromagnetic fields.
  • A detector, which measures the current of ions (rate of flow of charged particles).

The quadrupole mass filter is four rods (or hyperbolic electrodes) with the trajectory of ions along the central z-axis described by the Mathieu equations:

Quadrupole Mass Spectrometer Schematic
RGA Mathieu equations

By applying RF + DC voltages to the rods, ions of one mass-to-charge ratio successfully find their way from the source (ionization) to the detector.  Sweeping the RF/DC amplitude produces a mass spectrum which provides a 'fingerprint' of the ions remaining in the vacuum environment.   The figure below diagrammatically shows the ion paths when the quadrupole filter is tuned to measure mass 28 ions (typically nitrogen).

Ion trajectories through a quadrupole mass filter

RGA Benefits & Features

  • Low cost mass spectrometer — a very powerful diagnostic tool and process monitor
  • Leak Detector
  • Vacuum Troubleshooter
  • Process characterisation
  • Understand your vacuum system:  water, outgassing, leaks (real & virtual), foreign objects
  • Partial pressure analysis reveals actual contributions to Total Pressure
  • Beware: RGA artefacts — Electron source, hot surfaces, ESD spurious peaks