Hydrogen analysis

[puff]

I started to deal with technical hydrogen analysis. I use packed column with Porapak Q, He as carrier gas 20ml/min, oven temp 110 C, and TCD. I get 2 peaks on chromatogram , first positive tr=1.96min and second negative immadiately after first tr=2.19min. Areas of peaks are comparable (ratio 25/75). Negative peak is H2 but I don't know what can be the first peak from. Can any gas elute before H2 in such conditions?

Thanks for any suggestions.

[chromatographer1]

You are seeing the " W " effect of trying to measure hydrogen using helium carrier. It has to do with the thermal conductivity variation of low percent levels of H2 in He.

Use nitrogen or argon carrier if you want accurate measurement of hydrogen.

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

814-359-5737 voice
814-359-5459 fax
rgeorge@sial.com

[puff]

Thanks for answer. Could you write more about W effect, please? Is it possible to avoid W-efect by change of conditions for example carrier gas flow? I would prefer to stay using He as carrier.

[jhobbs]

I'm doing similar analysis of H2, methane, ethane and so on from jet fuel. Anyway, had to switch to N2 for our carrier gas for H2 analysis. I too would prefer to not have to switch gases, is eliminating a pressure pulse at injection the cause of the "W effect" or to you simply have to use N2 for a carrier gas for H2 analysis?
JEH

[chromatographer1]

Sorry, folks, nitrogen is the best option. The "w" effect is not dependent upon flow but the conductivity of helium when diluted with hydrogen at low levels (<15%). This is well documented issue.

Helium alone is not good. One can use a hydrogen helium mix for the carrier but you need about a 15% 85% ratio for it to work well. Then you have the possible problem of not being able to see small changes in concentration in the sample and of having a stable baseline with a carrier mix.

One can use Argon instead if nitrogen is not available, but the sensitivity is not as good as nitrogen.

Rod

[Rich]

If one is dealing with a hydrogen mixture that is >5% by volume (eg 20-50%), would He carrier then be an option?

Rich

[chromatographer1]

Not in my lab.

Not if my name was attached to the analysis.

Not if the analysis was important.

But that is my opinion.

Rod

[jhobbs]

can anyone provide literature or explain in some detail the "w effect" is. I see a small dip when injecting pure H2 with a TCD detector so it seems I am seeing a "u effect" of sorts.

[chromatographer1]

Chapter 4 in

The Analysis of Gases by Chromatography

by Cowper and DeRose

British Gas Corporation
London Research Station

Pergamom Press 1983

ISBN 0-08-024027-5

[chromatographer1]

If anyone wants to see an example of the "w" effect, send me your email address.

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

814-359-5737 voice
814-359-5459 fax
rgeorge@sial.com