Residual gas analyzer, usually a quadrupole mass spectrometer bolted to the chamber, which ionizes residual gas and sorts the ions by mass-to-charge ratio. A scan shows partial pressures by mass: 2 for hydrogen, 18 for water, 28 for nitrogen and carbon monoxide, 32 for oxygen, 40 for argon, 44 for carbon dioxide. An air leak shows 28 and 32 in roughly a 4 to 1 ratio with argon at 40, while a cluster at 41, 43, 55 and 57 points to hydrocarbons such as pump oil.
RGA shows 28 and 32 at four to one with argon at 40, that's air, we've got a real leak.
An RGA parked on mass 4 makes a usable helium leak detector, which saves breaking vacuum to hook up the leak checker.
Also heard as
- residual gas analyzer
- quad
- mass spec
Related terms
Partial pressure
The share of the total pressure contributed by one gas species in a mixture.
Cracking pattern
Set of fragment ion peaks a molecule produces in a mass spectrometer, used to identify it in an RGA scan.
Helium leak detector
Mass spectrometer tuned to helium that finds and measures leaks by detecting helium drawn into the system.
Chase a leak
To hunt down a leak of unknown location joint by joint, often over hours, until the source is pinned down.