Hydrogen detection with a TDC

[vaidostradas]

Hi,

I'm new at chromatography and I have a problem using my CP-4900 Micro-GC. I'm working in a gasification project. We can analyze de gas composition of the gasifier which consists in H2, CO, CO2, CH4 and some CnHm.

To do this, we have a TCD detector and two columns: Molsieve 5A and PPQ. We are using He as carrier gas. With this configuration we cannot see the H2 composition because the use of a TCD and He (more or less the same thermal conductivity as H2). We think that we should use Ar or N2 as carrier gas. However we don't know which is better to analyze the hole gas using only one carrier gas because we only can use one, He, Ar or N2.

I would like to know which problems could appear and if we can analyze all the components with good sensibility or of we can see H2 but we loss other component changing the carrier gas.

Thank you very much

[gserrano_84]

I've heard that in those case, you can still use He to detect hydrogen, but you'll have to increase the detector temperature 'til you get a good H2 peak

If that's not the solution, i'll probably go for Nitrogen. You'll loss some sensitivity, but again, you can always increase the detector temperature a little bit to increase sensitivity.

Gustavo/Supelco

[chromatographer1]

If you use He carrier your hydrogen peak WILL NOT BE linear and any accurate measurement is quite dubious. (And two different concentrations can give the same area response. Crazy but true.)

If you use Argon carrier you will have excellent sensitivity for H2 and He.

The bad news is CO2 response is VERY LOW.

CO, methane, C2s have about 5% of the sensitivity of H2.

So if your levels of H2 are 0.1% range and the other gases are percent levels you should be able to make a good measurement with a calibration mix and an adequate amount of sample injected.

CO2 will be a problem as it has about 1% the response of H2.

Otherwise, use N2 as carrier but EVERYTHING but He and H2 will almost be invisible. Only methane will have about 10% of the response of He and H2. The others, nil.

Good luck,

Rod

[AICMM]

vaidostradas,

Do you have two channel micro or do you have a single channel with a Q and a sieve (backflush to vent)? If two channels, then you need to run channel one with argon and you will get low response for O2, N2, CH4 and CO (not horrible but low.) In the meantime, you will get good sensitivity to hydrogen. Then, on the second channel you run either hydrogen or helium and get excellent sensitivity to CO2 (and C2's as well.)

If you have a single channel micro configured in the normal manner, you are in a bind. The Q acts as a pre-column to the sieve to keep CO2 off the sieve so you won't see CO2. You then have to decide if you want to see H2 or the others due to the problems already noted with hydrogen.

Best regards,

AICMM

[buxbaum]

What about using a mixture, lets say 50%Ar/50%He?
With a thermal conductivity inbetween the two, shouldn't it be possible to detect all the components with a somewhat lower, but sufficient sensitivity @ the TCD in one channel?
Does anyone have experience with gasmixtures for carrier gases?

[chromatographer1]

It would be an interesting experiment. The issue is the unusual thermal physical characteristics of hydrogen in the presence of helium.

While I have heard people trying this back in the 1960s I did not hear of any successes with it. But if someone were to discover a way to make this work there is probably a commercial value to it.

If I had the gases and the equipment I would give it a try.

best wishes,

Rod