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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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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vince_Darmancier
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Re: RGA Spectrum Interpretation

Post by vince_Darmancier »

I have been messing with this RGA I found, Its a very neat device here are a few pictures..
IMG_7918.JPG
This one is about 20 ish years old but perform beautifully...
As you said it`s not absolutely necessary, another expensive toy for a cool nerdy reward.. BUT
I placed mine on top of this 6 way and added a micro injection valve for a controlled leakage, that way I can "analyze"
whatever gas I allow to leak in the chamber.
IMG_7917.JPG
I like how the filament intensity, extraction voltage, ion and electron energy are all easy to control
(sliders at the bottom left)
water degass MS JPEG.jpg
The water is the largest peak at 18amu.
If this unit was attached to a fusor, I think it may (big emphasis on the may) be possible to see the tritium/He3 peak build up
since the setting can be tunes to favor mono or di-atomic species (kinda)
This would help resolve isobaric interference like the tritium and helium 3 problem (both at mass 3 )
zoom in water MS JPEG.jpg
But this would have to be conducted away from the main chamber and if possible "live"
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Re: RGA Spectrum Interpretation

Post by Jerry Biehler »

RGAa are nice, very nice. I have a couple though only one is working at this point, hardware issue. It makes leak finding so much easier and if you keep an eye out you can get a deal on ebay. You're not going to kill a probe with what most of us run in a vacuum system, heck, these are sometimes used for endpoint detection on processes much nastier.

You might be able to detect tritium, Doug Coulter did with his setup, he had a newer MKS RGA. Wish I could get a company discount on one, lol, I work for a division of MKS.
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vince_Darmancier
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Re: RGA Spectrum Interpretation

Post by vince_Darmancier »

Jerry Biehler wrote: Tue Nov 08, 2022 11:57 pm I work for a division of MKS.
Interesting... So do you have access to older model specifications, manual etc.. ? for example, I know the "D" or the "F" on the front of the unit describe the type of detector F for Faraday only and D for Both.. Also, the software allows you to change th total amu accessible with the correct password. i have increased my range from 100 to 300 amu successfully but the probe does not seem to be able to keep up .. would you be able to trace the quadrupole specifications based on a serial number ?
Thanks !
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Re: RGA Spectrum Interpretation

Post by Jerry Biehler »

No, I dont have any contacts in that business group. We build laser micro machining systems.

hacking the software to kick up the AMUs won't do any good, the mass that the RGA is capable of detecting is based on the physical dimensions of the quadrupoles. You need longer ones to get enough deflection of the ions for the heavier masses. Its not a software limitation like in a Rigol scope.
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vince_Darmancier
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Re: RGA Spectrum Interpretation

Post by vince_Darmancier »

I understand that.
The following picture show 2 of the 3 Quadrupole I have installed and tried,
The smallest is rated for 1 to 80 amu. So I was hoping the other 2 could easily get to 200 (at the very least)
The one I have currently installed is the one I collected the spectrum on the previous post with and is even longer.
Yet, it does not seem to get passed about 105 to 106 amu... The stop is very abrupt by the way.
Where is my mistake ?
Q compare RGA.JPG
Jerry Biehler
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Re: RGA Spectrum Interpretation

Post by Jerry Biehler »

Long one is probably 100AMU, 200 is even longer and if you had a 300 its longer yet.

And you cant expect just sticking another quadrupole on the head is going to work. The heads are tuned for the quadrupole. I am not sure if the software you have has an autotune feature.
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vince_Darmancier
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Re: RGA Spectrum Interpretation

Post by vince_Darmancier »

Ha, Well, as long as I used the correct unit with the correct quadrupole and "tuned" the system, this seemed to work fine.
Here is all the isotope of Mercury easily matched to the natural isotopic concentration from mass 197 to 204.
Snapshot - 45.png

And the full spectrum up to mass 250...
Snapshot - 46.png
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Richard Hull
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Re: RGA Spectrum Interpretation

Post by Richard Hull »

Seems like you really have honed the system to a full valuable working RGA. Nice work.

Richard Hull
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
Retired now...Doing only what I want and not what I should...every day is a saturday.
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