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Advanced RGA Spectra Interpretation


Now, let us diagnose the problems in an LN2 trapped, diffusion pumped chamber that has been pumping a long time and is in the mid 10-6 torr range.

We've increased the challenge by showing a magnetic sector RGA spectrum, with a non-linear, uncalibrated mass scale. Notice the low masses have excellent separation (high resolution) while high masses are unresolved. The intensity scale is also log form (which, while giving valuable information, does not look “normal.”)

Start with the most intense peak (and remember it is a logarithmic scale). The clue about the long pumping time suggests it is 18 amu (water vapor). That makes the peaks to its immediate right, 17 amu (OH+), 16 amu (O+ and CH4+), 15 amu (CH3+ ion), 14 amu (N+ and CH2+ ions) — all consistent with water, hydrocarbon and maybe nitrogen. This huge partial pressure of water strongly suggests the LN2 trap is empty or at least very low on cryogen.

Look on the high mass side of 18 amu. There are two small peaks at 19 and 20, to which we will return later, then a gap followed by an increasing trio with the third peak being the tallest one above 18 amu. If you “eye-ball” the distance between 18-19, 19-20, and the pairs of the trio, you can estimate that the large peak is actually 28 amu. That is, the trio is 26 amu, 27 amu and 28 amu. Above 28 amu there is a peak at 29 amu about the same intensity as 27, tiny peaks at 30 and 31 amu, and a substantial peak at 32. Although we cannot judge the relative peak intensities for peaks 28 and 32, there is the strong suggestion that these two are mostly nitrogen and oxygen. That is, this system has a leak!

The peaks at 27, 28 and 29 are often given by the hydrocarbons, ethane, and ethylene or maybe by higher mass hydrocarbons giving fragment ions in this region. A tell-tale sign of high mass hydrocarbons is “unzipping,” That is, they lose 14 amu units (the mass of CH2+). They also tend to have pairs of peaks (2 amu apart) at these 14 amu loss groups. For example, in the mass/ion table are the mass pairs: 41/43, 55/57 and 69/71 from hydrocarbons and if we went higher: 83/85, 97/99, 111/113, etc. Almost without counting mass peaks, you can see that there are pairs of peaks at intervals with increasing mass. If you do count (with considerable magnification) from the 41/43 pair, you will find 55/57. Above that the “eye-of-faith” tells you there is serious hydrocarbon contamination in this chamber; possibly the result of oil backstreaming from the diffusion pump since there is no LN2 in the trap.

Finally, we return to the 19 and 20 peak. In normal (low pressure) mass spectrometry used in RGA work, the chances of getting adduct ions (a proton added to H2O to give the H3O ion at 19 amu) is very unlikely. The chances of seeing neon at 20 and 22 amu is also remote (it is too expensive for anyone to use like argon or helium). The strong possibility is that 19 amu is due to fluorine and 20 amu is due to hydrogen fluoride. That is the system has been contaminated with fluorine from some source such as perfluoro kerosene used to identify the mass scale (very unlikely in a short mass range RGA), a low molecular weight fluorocarbon or chlorofluorocarbon used to clean a component or fixture in the chamber, or a PFPE fluid used in a pump. Fluorine, once introduced is very difficult to remove.

In summary, the RGA indicates this system has:

  • Air leak
  • Bad hydrocarbon contamination
  • No LN2 in the trap
  • Fluorine contamination

Not bad information to have, and the whole explanation took just over 500 words. No, this isn't rocket science.

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