RGA Spectrum Interpretation

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Liam David
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RGA Spectrum Interpretation

Post by Liam David »

With everything cold the chamber hits 4e-9 torr, and with the RGA (filament) on, it rises to ~1.5e-7 torr. This is expected due to outgassing, and I haven't left the filament on for long, so I'm not too concerned. It also improved by an order of magnitude after another bake.

Does anyone have input on this annotated mass spectrum?


spectrum annotated.jpg



I pulled peak data from http://ytionline.com/technical-informat ... tion-guide and the two attached files.

I think 28 is CO instead of N2 for a few reasons: the former is produced by hot filaments, there appears to be CO2, and the O2 peak is very small, meaning no air leak. The oil peaks likely correspond to turbo pump oil. I think some others are IPA as I used it to clean and dry all the vacuum hardware. The water and hydrogen peaks are as one would expect. For the other peaks and all the smaller junk in between, I just don't know.
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Richard Hull
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Re: RGA Spectrum Interpretation

Post by Richard Hull »

I am so glad that I do not have an RGA. If I did, I would probably "throw in the towel" on fusion here. I would find more crud than deuterium, most likely. Besides, I doubt if I could operate the system in a vacuum that would not foul the RGA probe.

You have a very clean system for that vacuum level, in spite of all those seemingly ugly peaks.

Richard Hull


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Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Rich Feldman
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Re: RGA Spectrum Interpretation

Post by Rich Feldman »

Very interesting, for example your comment that CO+ and N2+ have practically the same mass/charge ratio.

I remember reading about similar ambiguosity, for example H2+ matches atomic D+ and He++. Or was it about HD vs helium-3?

Harold Urey's team's 1932 discovery of deuterium was by high-resolution optical spectroscopy.
The familiar hydrogen-alpha line has a very faint spur on one side, at predicted tiny offset.
It was substantially intensified by two different methods of isotopic enrichment (LH2 distillation and water electrolysis).

[edit] Applause for mobile phone picture of green-screen instrument display, first making sure your fingernails are clean. I've done that with an old RF spectrum analyzer at work; it has a slot for some kind of removable digital media that became obsolete in the last century. (not a floppy disk)
All models are wrong; some models are useful. -- George Box
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Liam David
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Re: RGA Spectrum Interpretation

Post by Liam David »

Sure, an RGA isn't essential to do fusion by any means, but I'd argue that it's always better to be "in the know" for these things. At the very least you have a better idea of what's going on, even if you don't do much with that information.

Usually, the RGA spectrum interpretation is pretty straightforward for things like D+, H2+, He++, and other equal-mass/charge peaks for simple molecules. Using that example, D+ really only shows up in fusion systems and He++ is very energetically expensive, so if you get a peak at 2 it's almost certainly H2+. The peaks at 17 and 18 are always water. Anything higher than 32 is usually a hydrocarbon. Occasionally you can get funkier things like D3+, as I observed several years ago in a previous post. Things get complicated when complex molecules are involved, though much headway can be made by looking at the fractioning patterns and their relative intensities. Unfortunately, RGAs can't have built-in chromatographs.

I have written code to pull data off the RGA via RS-232, but that has disappeared into the void...
Alex Aitken
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Re: RGA Spectrum Interpretation

Post by Alex Aitken »

Your 36:38 peaks are almost 3:1. And you cleaned the RCA parts in HCl? Could they be HCl?
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Liam David
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Re: RGA Spectrum Interpretation

Post by Liam David »

Yep, that would make sense... kind of a duh moment on my part.

Cl = 35 (isotope 35)
HCl = 36 (isotope 35)
Cl = 37 (isotope 37)
HCl = 38 (isotope 37)
Cl2 = 70 (isotope 35)

The peak at 15 is much too large to be 15N, and I don't have HF or Ne for 20. The peaks are well-defined in analog scan mode so I don't think the other peaks are issues with the analyzer. Some of the others are probably strange hydrocarbon fragments.
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