Response of a TCD detector to different gases

[sagipas]

Hello,
I have recently purchased and installed a Mol sieve 5A column in my 5890 GC. My goal is to measure H2 with N2 carrier.
I am trying to check that my system is working properly but I do not yet have H2 to examine...

I am running the column at 5 psi, 60C and the TCD temperature is 200C.

Is it normal that I can not see an oxygen peak with a TCD and an N2 carrier?

If so, what other gas can I use (besides H2) to just check if my system is on the right track...

Thank you very much,

Sagi

[chromatographer1]

nitrogen has a similar thermal conductivity to oxygen. A TCD responds to gases with a DIFFERENT conductivity from the carrier gas. Look up the conductivities of the gases you have on hand and compare them to the conductivity of nitrogen. When you find one that is more like hydrogen than nitrogen then you have the gas you need.

You should be able to look these values up on the internet. Try wikipedia.com for example.

best wishes,

Rod

[sagipas]

Thanks a lot chromatographer1
I did find gases which have different thermal conductivities, however, my problem is I am getting a nice baseline
signal from te detector (no drift) but no peaks whatsoever. There is flow through the column and the pressure setpoint is maintained.

Could this be due to bad connection between the column and the TCD? or am I missing something else?

Sagi

[Johnny Rod]

Have you got any helium to try injecting? They'll both come out pretty quickly (He and H2). If you do an injection of air you should see peaks as the concentrations are high, even in N2 carrier.

[chromatographer1]

I repeat:

Nitrogen has a similar thermal conductivity to oxygen. A TCD responds to gases with a DIFFERENT conductivity from the carrier gas. Even at high condentration of air it is likely you won't see much, if anything.

The air peak you wish to see has a similar conductivity to nitrogen. Do you understand when I say a TCD measures the difference between conductivities? If your gas has a similar conductivity to nitrogen you will see NO peak.

Johnny Rod gave you good advice. Helium will elute slightly AHEAD of hydrogen on a MS5A column. It is an excellent choice to trouble shoot your detector and to confirm it is working properly. I hope you have that available. If you examine the conductivity of helium you will see it differs greatly from nitrogen and oxygen and is similar to hydrogen.

best wishes,

Rod