## RGA Progress

27 09 2010

I tried the RGA today and finally made some progress. Check this out:

Looks like the RGA works, but it still needs calibration.

Seems we have hydrocarbons and water in the chamber. Where did O2 come from (or is that air)

Wonder what mass 26 and mass 30 and 29 are?

Here is partial pressure over time:

I doubt I’m even using it right. There is a lot to learn.

### 13 responses

28 09 2010

You won’t typically find oxygen as a lone molecule. Paired up is the way you’ll normally find it in the air :-)

30 09 2010

you’ll never be able to see O2 with an RGA. needs to be ionized.

28 09 2010

Do you wear nitrile or latex gloves when you are handling the stuff that goes in the tank? The oil from your hands will take a while to out-gas, also any non-vacuum rated plastics or gaskets. Do you wipe down stuff you may have touched with isopropyl? That usually helps take care of inadvertent finger grease.

28 09 2010

I’ve never taken those precautions. The chamber was not particularly clean to begin with!

28 09 2010

Moin

Quote : “Wonder what mass 16 and mass 30 and 29 are?”

It might be the mass of the atoms the analyser found.
A RGA works similar to a mass spectrometer, doesn’t it ?
So it uses the lorenz force on moving charges to force ions on a circular trajectory with a different radius depending on their mass.
But how does that work for O2 ?

Anyway let’s check a periodic table …
A single O-Atom has the mass 15,999u, which might end up as 16.
(u is the Unified atomic mass unit http://en.wikipedia.org/wiki/Unified_atomic_mass_unit)
But on the other hand it says 16 for O2 (not just O) and H2O should have 18 (H = 1u, O = 16u).
There is a oxygen isotope with O15 but it has a radioactive half life of 2 minutes.

So I’d suggest you check the manual of your software, which mass is meant.

Greetings
Sebastian

28 09 2010

I’m quite sure he meant 26, seeing as its already telling him what 16 is, and the other 2 numbers he asked were the other 2 numbers that weren’t labeled

28 09 2010

Good catch!

Indeed I meant 26, I’ve changed the post.

30 09 2010

Based on the ion fragmentation pattern, I’m guessing your RGA is operating in negative mode, i.e. creating negative ions and measuring those.

No, the way the RGA works is that it ionizes the molecule. Thats’ why water is 17 (instead of 18) what your’e seeing is OH-.

Oxygen (16) is the oxygen radical anion, which derives from ionizing oxygen (O2) molecules.

Hydrocarbons “27” is probably (CH=CH2) anion, which will fragment off of terminally unsaturated hydrocarbons.

Maybe 29 is CH2-CH3 ?

As for 26 and 30…

26 could be CN- which is certainly a stable and not uncommon anion – do you have anything containing cyanide/nitrile around?

30 could be NO, but IIRC NO likes to be NO+ (and not NO-).

I really want to say it’s CH2O (formaldehyde) radical anion. But I don’t know if the former exists and I assume the pathway to make it would come from the methanol anion (CH3O) which is incredibly stable, and you’d expect to see that at 31.

30 09 2010

PS keep in mind that as with any mass spectrometry (and especially ones on very chemically distinct species like this) the percentages spit out don’t necessarily reflect the constituent percentages in your chamber atmosphere. It’s just the percentages of ion strikes on the detector. While relative to the same species, the raw number of strikes (first column) will fluctuate more or less linearly with the raw amount of that species in the mix (i.e. if your oxygen doubles that raw number should double) you can’t compare between them.

That’s because detection depends on the chemical reactivity and stability of the anions that are generate. Some of those species may have more or less of a predilection to be formed based on a billion different variables, esp. voltage at the ionizing head, ioniziation technique, internal temperature, etc etc etc. So you may have 100 times as many oxygen molecules as water molecules but if the water is 500 times easier to ionize, you’d expect to see 5 times as many water molecules in the spec.

30 09 2010

update: formaldehyde radical anion exists. And I think it’s readily formed from methanol.

30 09 2010

Urg. apparently, it’s probably operating in positive mode.

Anyways, here’s a pretty comprehensive chart.

http://www.yeagletech.com/techinfo/rgaspectrainterguide.html#notes

30 09 2010

29 Ethane Fragment or Ethanol Fragment or Isopropyl Alcohol Fragment

Previously I used Isopropyl Alcohol as a leak check: