Why can my GC can detect O2, N2 and CO but not CO2.

Discussions about GC and other "gas phase" separation techniques.

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Hi all,

I am using GC to analyse gas samples containing N2, O2, CO and CO2.
My detector shows a peak for N2, O2 and CO but NOT for CO2.
This is fine - it is the CO peak that is interesting to me and I do not need to observe the CO2 peak.

My question is: WHY can I not observe CO2?
I am writing my thesis and trying to explain why CO2 is not observed and I can't seem to find any information online. Does it adsorb to the surface of the column? Is it to do with particle size?

I am using a Perkin Elmer Arnel autosystem XL GC using a molecular sieve packed column fitted with a TCD, H2 was used as the carrier gas.

Any help would be much appreciated.

Kind regards,
Are you sure that CO2 is eluting from your system? A quick search of the Restek literature does not reveal an app note where those analytes are separated from each other on a molecular sieve.

I don't see CO2 on my mol sieve because my oven temperature is at 40 °C isothermal (all I want is H2, O2+Ar, N2, and methane). If you don't go pretty hot for an oven temperature, it could be stuck on your column.

I am definitely sure there is CO2 in my sample as I am using CO2 saturated solution.
(My experiment involves chronoamperometry and manual injection of samples from the vessels headspace into GC to determine how much CO is formed from CO2 reduction over time)

My oven temperature is 160 degrees.
Occasionally I see a very broad peak that messes up my baseline which I think may be due to CO2 adsorbing onto the column? But I don't know this for sure.

CO2 is absorbed onto a Molecular Sieve column, and it will take ages for it to come off at 40 °C. I would suggest holding the oven temp at 40°C until the CO has eluted, and then ramp up at 20°C/min to about 250°C.

Hi gasman,

My oven is at 160 degrees not 40 degrees. Do you expect CO2 to elute at this temperature?

No. He meant that you have to go to 250 °C or so to see CO2 come out. Since you only go to 160 °C, the CO2 is going to take a long time to come out.

The 40 °C was in reference to my comment. I don't care about CO2 on my system. I only care about the O2 and N2 really AND I'm ok with O2+Ar coeluting.
Your system has a backflush valve?
Are you using only one MolSieve (13X?) column or another columns too?

In MolSieve column with foreflush only, the CO2 takes a long time to be desorbed from the column.
Probably the CO2 in you sample remains trapped inside the column. As our coleagues said, it's necessary to increase the temperature above 250°C to allow CO2 ellution.

CO2 is strongly retained on a molesieve 5A column. It comes off more quickly on a molesieve 13x but still is highly retained.
Using a 3m SS 2mm ID 60/80 mesh MS5A column using He carrier at 30cc/min. at 150°C will elute CO2 at 4 times the retention time of CO. Lowering the oven temperature will increase the retention time dramatically.
Any imperfections in the packing of the MS5A will distort the peak shape of the CO2 also.

best wishes,

retired Rod
As mentioned in a responses to your question, the Mole Sieve is "eating" the CO2. This is a simple fix...assuming this is an isothermal applicaiton and you have the ability (or control) to switch a few column valves. You just need the correct plumbing configuration. Reverse Column Step or Back-Flush to Detector. Front column is a short Hayesep N 100/120 for setting the reverse, second column is a 5A Mole Sieve. You'll need two 6 port valves or one 10 port valve. All depends on the plumbing config. The CO2 is reversed back to the detector BEFORE the mole sieve column, and is output as CO2+.

NOTE - everything CO2 & heavier will be included in the CO2+ peak. e.g. water, methane, ethane, carbon tetra-hexamethyl-chickenwire...all will be in the CO2+.

Second option - PTGC heat the living snot out of it after the CO peak elutes.

If this is still an issue and you're interested in our solution, respond back with you email address & I'll send you an application plumbing diagram with all the details. OR call (918) 662-7000 & ask for GreGG
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