Chromatography Forum

Hi everybody, we have to move our GC-FID to a new location and we have some questions about the gas supply...
Which purity of gas (He, H2, N2 and Air) do you use for your instruments?
Actually we are using Air from a cylinder but we are thinking about the possibility to use the air from the air compressor (oil-free and water free). Do we need a generator of zero Air for the FID or we could just use the air from the compressor with some traps (hydrocarbon and water)?
Up to now we are using high purity He (5.0), H2 (5.0) and N2 (5.0) without any traps, do you normally use traps before the GCs?

Thanks in advance for the answer,

Have a good day!

Davide

If you have not had problems with your carrier gases then there is no reason to change what you have done previously.

The air compressor will have an intake and you do not know if there may be contaminants in the source air which might affect your detector response.

If you choose the air compressor I would recommend that you install a large capacity trap for hydrocarbons.

Even for cylinders it is not a bad idea because one never knows if the cylinder was clean before it was filled with compressed air or that the compressed air was clean. Hydrocarbons in your FID air supply will reduce the sensitivity of the detector, how much is too much will depend upon the type of analysis you perform.

best wishes,

Rod

As Rod said the type of your analysis matters.

I do mostly ultra-trace analysis where I found the following setup quite fitting:

All connections are made of 316 SS to be diffusion proof.


Air for FID:
- Use a larger-volume oil free compressor. (Mine has 200 liters @ 8 bar). Why so? The large volume serves as a condensate separator which helps you keeping your traps clean.
- The next step is a build-in expansion cooler which purges the air from most of its moisture.
- Then I use a high-capacity molsieve trap to remove water and higher hydrocarbons which may have snuck in.
- After that I have a high capacity charcoal trap to remove most other hydrocarbons.

But even with that setup I sometimes see baseline problems which I'm going to solve with an additional zero-air generator which is feed with my pre-purified compressed-air.


H2 for FID:

5.0 quality gas which is purified over molsieve/charcoal for increased baseline stability. I use this setup after I had a very bad experience with a 5.0 hydrogen cylinder: After falling beyond about 40 bar the baseline was a mess. The cylinder contained huge amounts of hydrocarbons which I removed using the setup outlined above.


N2 as make-up gas:

As I had a surplus Supelco high capacity purifier at hand I just use it to purify my 5.0 N2.


Carrier gas:
- Indicating glas trap with molsieve
- Glas trap with charcoal
- Oxygen scrubber
- Indicating oxygen trap


Why all that effort? My answer is simple: I want to eliminate as many possible sources of errors as possible. Regenerating/changing the traps in regular intervals saves you a lot of headaches and gives you a more reliable analysis.

On the other hand one can argue that all the additional connections allow contaminants to sneak in or that traps may contain dirt so YMMV.

Even for cylinders it is not a bad idea because one never knows if the cylinder was clean before it was filled with compressed air or that the compressed air was clean.

Just a factoid: most instrument/zero grade air in compressed cylinders is not, in fact, simply compressed atmosphere. It is blended from pure nitrogen and oxygen. And when there's pure oxygen around, trust me, everything is quite clean.

Anyway, the biggest difficulty in blending zero air is methane, since methane is typically present in liquid oxygen in the 10-20 ppm range. The primary difference between industrial grade and zero grade compressed air is the methane content. Zero grade air is made from oxygen which has had the methane scrubbed from it.

If you need to use zero grade air for your instrument to work correctly, you will have a tough time replacing the gas cylinder with a compressor.

But, as others have said, it depends on your application.