Residual Gas Analyser

A mass spectrometer small enough to bolt onto a vacuum flange and sensitive enough to smell a single molecule of water in a room the size of a football field.

Analyses the composition of residual gases in a vacuum system in real time. Engineers use it to diagnose contamination sources (water vapour from a dirty flange, hydrocarbons from pump oil, helium from a leak in a cryostat), verify system cleanliness before sensitive deposition processes, detect virtual leaks, identify outgassing products during bakeout, and confirm that a chamber has reached true UHV rather than just low total pressure dominated by one stubborn species.

Gas molecules effuse from the vacuum chamber into an open cylindrical ioniser. A hot filament (thoriated iridium in most commercial RGAs) emits electrons at typically 70 eV — enough energy to ionise essentially every gas species without preferentially fragmenting them. The resulting positive ions are extracted and injected into a quadrupole mass filter: four parallel metal rods arranged in a square, with opposing rods held at the same potential. A combination of DC voltage and radio-frequency AC voltage is applied; by varying the RF amplitude and DC-to-RF ratio, only ions of a specific mass-to-charge ratio travel the full rod length without being deflected into the rods. All others are lost. The mass-selected ion beam is then measured by a Faraday cup (a simple charge collector) or, for higher sensitivity, a continuous-dynode electron multiplier that amplifies the signal by a factor of up to 10⁸. Scanning through the RF amplitude sweeps from 1 to 100, 200, or 300 amu in seconds, producing a mass spectrum.

In plain terms

Imagine you could weigh individual pieces of air and sort them by how heavy they are. An RGA does exactly that — it grabs stray gas molecules inside a vacuum chamber, fires electrons at them to knock off a charge, and then uses electric fields to sort them by mass, like a very fast pinball machine that routes heavy balls one way and light balls another. The result is a bar chart showing exactly which gases are present and how much of each.

Also called: RGA, quadrupole mass spectrometer, vacuum mass spectrometer.

Why it matters

Total pressure gauges cannot distinguish between 10⁻⁸ mbar of hydrogen (harmless) and 10⁻⁸ mbar of oxygen (catastrophic for a fresh superconducting film) — only a mass spectrum can tell them apart.
Virtual leaks — trapped volumes of gas under a bolt head or inside a blind hole that outgas slowly — produce a characteristic water and nitrogen/oxygen fingerprint that an RGA reveals within minutes.
Semiconductor fabs rely on RGAs for in-situ process gas monitoring: detecting a 1 ppm water contamination in a sputtering argon atmosphere requires partial-pressure sensitivity below 10⁻¹⁰ Torr, which only an electron-multiplier RGA provides.
Helium leak detection: even without a dedicated helium mass spectrometer, an RGA tuned to mass 4 (⁴He) can detect a sniff-test helium leak at better than 10⁻¹⁰ mbar·L/s sensitivity.

Where it lives

Semiconductor thin-film deposition chambers (CVD, PVD, ALD)
Synchrotron and free-electron laser beam-line vacuum systems
Spacecraft leak-test facilities (NASA, ESA)
Dilution refrigerator vacuum jacket leak checking
Nuclear fusion research chambers (JET, ITER beam-injection systems)
Surface-science instruments (XPS, LEED, AES, SIMS)

Key numbers

Mass range: 1 – 100 / 200 / 300 amu (Standard SRS RGA100/200/300; some instruments to 5,000 amu)
Operating pressure (max): 1 × 10⁻⁴ Torr (Above this, ion–ion collisions degrade resolution)
Detection limit (Faraday): 5 × 10⁻¹¹ Torr (SRS RGA with Faraday cup; KJLC Element series similar)
Detection limit (multiplier): 5 × 10⁻¹⁴ Torr (SRS RGA with continuous-dynode electron multiplier)
Ionisation energy: 70 eV (standard) (Programmable 25–105 eV; 70 eV matches NIST fragmentation tables)
Filament material: Thoriated iridium (dual) (Field-replaceable; firmware over-current protection)
Resolution: < 0.5 amu (@ 10 % peak height) (Per AVS standard 2.3; unit mass throughout range)

Surprising facts

With an electron-multiplier detector, an RGA can measure a partial pressure of 5 × 10⁻¹⁴ Torr — this corresponds to roughly 1,300 molecules per cubic centimetre, or about one molecule in every 40 billion air molecules at sea level.
The quadrupole mass filter has no moving parts and no magnetic poles: mass separation is achieved purely by the ratio of DC to AC voltage applied to four metal rods, an idea first demonstrated by Wolfgang Paul in the early 1950s for which he shared the 1989 Nobel Prize in Physics.
A baked-out, well-conditioned UHV chamber at 10⁻¹⁰ Torr still contains residual gas that is typically 80–90 % hydrogen — not air — because hydrogen diffuses through and out of stainless steel walls at a rate that exceeds every other species by orders of magnitude.
The filament in an RGA must reach above 1,300 °C to emit electrons, yet sits inside a chamber that may be at 10⁻¹¹ Torr and will be baked to 300 °C during conditioning — the filament itself is a significant outgassing source if not properly degassed.

A short history

Mass spectrometry dates to J. J. Thomson's parabola spectrograph (1913) and A. J. Dempster's and F. W. Aston's sector-magnet instruments of the 1920s. The quadrupole mass filter — the basis of every modern RGA — was invented by Wolfgang Paul and Helmut Steinwedel at the University of Bonn in the early 1950s and first published in 1953. Paul received the Nobel Prize in Physics in 1989 for this work. The quadrupole's key advantage over sector-magnet instruments is that mass selection depends only on voltages — no heavy, fixed magnets — making the filter compact and scannable at high speed. By the 1970s–1980s, quadrupole RGAs were small enough to mount directly on a 2.75-inch ConFlat flange and connect to a desktop computer, displacing magnetic-sector residual gas instruments in almost all vacuum-systems work. Stanford Research Systems (SRS) introduced the RGA100 as a low-cost, high-performance benchtop instrument in the 1990s, setting the format that most commercial RGAs still follow.

Sources

Stanford Research Systems — RGA Catalog (thinksrs.com) (https://www.thinksrs.com/downloads/pdfs/catalog/RGAc.pdf)
Stanford Research Systems — Choosing a Quadrupole Gas Analyzer (application note) (https://www.thinksrs.com/downloads/pdfs/applicationnotes/cis.pdf)
Nobel Prize in Physics 1989 — Wolfgang Paul and Hans Dehmelt (ion trap technique) (https://www.nobelprize.org/prizes/physics/1989/paul/facts/)
NIST / Hiden Analytical — Quadrupole Mass Spectrometers for Residual Gas Analysis (https://www.nist.gov/document/hal201rcmassspectrometer)
Kurt J. Lesker Company — KJLC Element RGA 100 amu datasheet (https://www.lesker.com/residual-gas-analyzers-rga/kjlc-residual-gas-analyzers-rga/part/ele1mb100)
Hiden Analytical — RGA Minimum Detectable Partial Pressure (Vacuum Expo 2012) (https://www.hidenanalytical.com/wp-content/uploads/2020/05/RGA-Minimum-Detectable-Partial-Pressure-9mm-3F-301-PIC-Vacuum-Expo-2012.pdf)