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Simple RGA Spectra Interpretation

Before plowing through the nitty-gritty here, please accept two important points:

  • An RGA spectrum is the only way to truly diagnose vacuum problems
  • Interpreting an RGA spectrum is easy! This is not rocket science.

Simple Example


The gas remaining in a chamber under vacuum — the residual gas — is analyzed using an RGA.

The typical quadrupole RGA spectrum (Figure 1) has a linear mass scale (x-axis) and a log or linear peak height or intensity scale (y-axis) indicating the amount present at that mass.

The left spectrum was taken just after the pressure reach the RGA's working range. The major peak is below 30 amu.

The right spectrum was taken later (at lower pressure). All peaks have lower intensities but the major peak is now below 20 amu.

Parent Gas Relative Mass Ions Formed amu
Hydrogen 2 H2 2
  H 1
Helium 4 He 4
Methane 16 CH4 16
  CH3 15
  CH2 14
Ammonia 17 NH3 17
  NH2 16
Water 18 H2O 18
  OH 17
  O 16
Fluorine 19 F 19
20 HF 20
F 19
Neon 20, 22 Ne 20, 22
Nitrogen 28 N2 28
  N 14
Ethylene 28 C2H4 28
  C2H3 27
  CH3 15
  CH2 14
28 CO 28
  O 16
  C 12
Ethane 30 C2H5 29
  C2H4 28
  C2H3 27
  CH3 15
  CH2 14
Oxygen 32 O2 32
  O 16
Argon 40 Ar 40
36 Ar 36
Hydrocarbons various HC various
  C5H11 71
  C5H9 69
  C4H9 57
  C4H7 55
  C3H7 43
  C3H5 41

To interpret these two spectra, we follow this logic.

  • What do we expect when first pumping out a chamber? Air
  • Major components of air? Nitrogen and oxygen

Looking at the mass/ion table (left), we see

  • N2 appears at 28 amu
  • O2 at 32 amu
  • Ratio for air is 78% to 21% (3.7 to 1)

Left-Hand Spectrum

  • The left-hand spectrum has peaks at 28 and 32
  • Rough ratio is 7 to 2 (3.5 to 1)
  • That means, the residual gas is mostly air

But what about the large peaks at 18 amu and 14 amu? From the mass/ion table we learn 18 is H2O+. An immediate question arises, why is the water peak more intense than the oxygen peak? Gases like nitrogen, oxygen, and argon are not strongly bound to the surfaces. This means they easily enter the gas phase and are easily pumped away. Water vapor, by contrast, clings to every surface, many molecular layers thick. As the pressure is reduced, water vapor molecules enter the gas phase but when they hit another surface they are again strongly bound. This makes it very difficult to pump away and it eventually becomes the dominant residual gas.

Looking at the mass/ion table, we see 14 is attributed to nitrogen. When 70 eV electrons from the ionizer hit neutral nitrogen molecules, not only is a valency electron kicked out (forming a molecular ion N2+) but enough electronic and vibrational energy is dumped into each ion that some will split into N + N, one of which claims the positive charge. Given the high intensity of the 28 peak, it is no surprise to find this so-called fragment ion (N+).

[To make a complete story I should mention: if a pure gas sample is ionized and the spectrum of molecular and fragments ions examined, the relative peak intensities and various masses are determined by molecular statistics, kinetics, thermodynamics and instrument factors. Have no fear, that's the last time those subjects are mentioned.]

Novices often think life would be wonderful if RGAs gave only molecular ions. Old hands know fragment ions help the feeble mass resolution of their RGAs. They just wish nature would let us build small, very low-cost, high-mass resolution RGAs so they could ignore fragment ions.

Indeed, this spectrum is a good example of using fragment ions. Look at the mass/ion table and you'll see 28 amu can come from carbon monoxide (CO), ethylene (C2H6), or nitrogen (N2). Why choose nitrogen? Well, ask yourself, what fragment ions are expected from them?

CO gives 12 and 16 → there is little 12 in the spectrum.
C2H6 gives 27, 15, and 14 → 27 is present but much smaller than 14.

So, a large 14 peak associated with an intense 28 peak, means N2.

Right-Hand Spectrum

The most intense peak is 18 (water vapor), with its attendant gaggle of fragment ions at 17 and 16. The second most intense peak, 28 amu, could be N2, CO, or C2H6 or a mixture. While there is something at 14, the ratio 14 to 28 doesn't equal that seen in the left-hand spectrum.

Also, while there is a 27 peak, the 27/28 ratio is smaller than in the left-hand spectrum. This suggests the 28 peak is mostly carbon monoxide. As partial confirmation, look for a CO2+ at 44 and CO+ at 28. They are frequently found in the RGAs of diffusion pumped chambers. The large peak at 2 amu is H2.

Initially, most chambers show strong nitrogen, oxygen and water vapor peaks. After long pumping, the RGA shows strong water vapor, carbon monoxide, carbon dioxide and a little hydrogen. These spectra are consistent with the typical chamber behavior.

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