post column removal of CO2 for N2O and COS analysis

[Jochen]

Dear all,

I have a tricky question regarding a specific removal of CO2 from air samples in order to analyse N2O isotopes and carbonylsulfide (COS).

My current set-up is as follows:
I extract old atmospheric air from ice core samples and remove CO2 chemically with an ascarite trap (basically an alkaline absorber für acidic gases). On this CO2 free air sample we then analyse CH4 and N2O for its stable isotopes in a GC-IRMS system. Although CO2 elute before N2O on our GS-carbonPlot column (Agilent), the bulk of CO2 has to be removed with the ascartie trap otherwise the small N2O peak sits on the flank of the 1000 fold larger CO2 peak and then isotope analysis is not possible. So I have to get rid of the CO2 some how to get the N2O isotopes.
On the other hand I would also like to analyse the concentration of COS, carbonlysulfide, within the same chromatogram (for us the sample material is a limiting factor since we only get a small sample from the ice core and therefore want to measure many parameters on the same sample) . However, COS is absorbed in the ascarite trap together with CO2. So it seems that there is no way out.
My questions are the following.
Do you know of specific absortion material as an alternative to ascarite which only absorbs CO2 but not COS?
Or the alternative way, is it possible to remove CO2 after the GC separation in a special capillary or membrane without peak broadening? (cryogenic separation with liquid nitrogen is not an option as CO2 and N2O are too similar). For this idea I could switch an extra valve to let the CO2 peak pass this 'post-column trap' and get a nice baseline for N2O analysis. When the COS peak elutes ca. 10 minutes later, I could bypass this trap and analyse it in the mass spec.
If you have any idea please let me know.

cheers

Jochen

[MMcI]

Hello Jochen.
What volume of sample is available?
What is the expected concentration of CO2, N2O and COS?
I have used a 6Mx1/8" Hayesep Q column to see
neon, hydrogen, CO2 and N2O in room air using DID GC.
The 0.5 ppm N2O was separated from the 330 ppm CO2.
I did not look for COS but a sulfinert column path would be necessary
to prevent losses.

[pi3832]

Mole sieve? It'll getter CO2 like mad, but I've no idea if it's going to also grab the N2O or COS.

[chromatographer1]

It sure will. Not recommended, pi3832.

Rod

[Jochen]

Hi,
my sample size is ca. 15 mL of air with mean concentrations of around 250 ppm for CO2, 200 ppb for N2O and 300 ppt for COS. The bulk air components N2, O2 and Ar are removed in a temperature controlled activated carbon trap, so only the trace gases are "injected" onto the carbonPlot column. This column does a nice job in separating many trace gases but it will certainly overload the column if CO2 is not removed with the chemical trap. I still think the only solution is to find a specific absorbtion material for CO2 which does not absorb COS.

cheers and thanks for your held

Jochen

Hello Jochen.
What volume of sample is available?
What is the expected concentration of CO2, N2O and COS?
I have used a 6Mx1/8" Hayesep Q column to see
neon, hydrogen, CO2 and N2O in room air using DID GC.
The 0.5 ppm N2O was separated from the 330 ppm CO2.
I did not look for COS but a sulfinert column path would be necessary
to prevent losses.