Cryogenic GC TCD

[willnatalie]

Currently doing a -80C run separating out H2, HD, and D2 on a TCD. The separation in He occurs very well and I see all three peaks. Knowing the thermal conductivity of He, I got my H2 to be negative and HD, D2 to be positive.
I know that He is not the best carrier gas so I am currently looking into using Ar. My inital reason for switching the sensitivity toward just the H2.
I run isothermal at -80, I switched the carrier gas a day before and allowed the TCD to stablize a couple of hours before I did any analysis. I made the changes in the Chemstation sofware to allow the flow calculations to occur. I switched the polarity of the TCD and I get a large positive mass representing nothing. I would hope for peaks but alas..
I feeling like am missing something that is basic. Any Ideas?

[willnatalie]

Ran H2 at -80C and got a mess. Ran H2 at 35C and got a beautiful gaussian peak. go figure.

[chromatographer1]

If you are separating the gases using molecular sieve it all makes sense.

I would not use any carrier gas that has a molecular diameter larger than the pores of my sieve which perform the actual separation of the gases involved.

Nor would I use a carrier that competes successfully with a larger molecular gas, blocking the mass transfer of that gas into and out of the pores which again, actually performs the separation of the analyte gases.

But all this is my understanding or mis understanding of the physical chemistry involved. Somebody correct me if I am in error.

Rodney George
consultant

[willnatalie]

ahh.. that did not even occur to me. I am using a mole seive column. Thanks.

I still do find it strange though that I get such a difference in Chromatography at these temperatures (though I know that it is a large difference.). The reason that this did not occur to me is that the RGA instruments that were given to us; when installed, the column (mole sieve) used to separate hydrogen had Argon installed by request of the company.