Still a newb... TCD peak polarity question

[jpd6825]

From the accidental chromatographer. Working in industry... inherited responsibility for a couple of GCs - with really no background in this. GC #1 is an old Varian 3400 with a TCD. The application is quantifying N2 in H2 --- and the carrier gas is H2.

Would someone point me to a reference text or website that would give me a reference list...

what species will give positive peaks and which will give negative peaks in my chromatogram?

Thanks

[GasMan]

You need to look at a table of thermal conductivities for gases, and I believe you can find this in the Hand Book of Chemistry.

If the thermal conductivity of the sample compound is LESS than the thermal conductivity of your carrier gas, you will get positive peaks. So in your case with hydrogen as carrier and nitrogen as the sample, you will get a positive peak for nitrogen. If you were to use nitrogen as the carrier gas and the sample was hydrogen, you will get a negative peak for the hydrogen.

The above statements are generally true, but there may be some cases when you get 'W' shaped peaks due to concentration effects.

Gasman

[chromatographer1]

Everything is Positive, jpd6825.

Everything that is hydrocarbon based and even the fixed gases will give a positive peak (same direction as nitrogen) with hydrogen as the carrier gas.

Only helium has a conductivity close to hydrogen and it will still give a somewhat positive peak. At certain concentrations depending upon the dead volume of your detector, the amount of sample injected, and the volume and flow rate of the gas within the column, non-linear response may occur with helium. But no one uses hydrogen to measure Helium, they use nitrogen or argon, or even CO2.

Hope this helps. The Handbook of Chemistry and Physics is a valuable resource.

Websites for Gas Producers are also helpful, Air Products and Airgas are examples.

best wishes,

Rod

[jpd6825]

Water!

I looked in the CRC handbook and about the only thing that I see with a TC higher than H2 is H2O...

Sounds like I need to talk to the process engineer who is fooling around upstream...

What do you think? Any other reasonable suspects for a large negative peak which has suddenly turned up in our process?

[chromatographer1]

I suspect that you have contaminated carrier gas Ior the wrong gas) and that is the cause of the negative peak.

Is it at the retention time for hydrogen? or helium?

best wishes,

Rod

[AICMM]

JPD6825,

Cannot be water, would come out later than N2 if it came out at all. (Although could be water from much, much earlier runs depending on column but would be a really ugly peak.)

Valve switch, flow upset would be my guess. What does the head pressure do upon inject?

Best regards.

[chromatographer1]

AICMM may be correct.

A leak could be the cause of the negative peak. More info is needed to diagnose this problem. Can you share? We could give scenarios all day long.

Where is the negative peak?

Does it reproduce when only carrier gas is injected?

It is after or before nitrogen peak?

What column are you using? Mole Sieve 5A, or 13X ? A porous polymer? packed column or capillary, or micropacked?

Dear JPD6825,

Forum members do want to help you solve your problem.

best wishes,

Rod