kindly help me to analyze CO2 with GC/TCD

[jagan_iit]

Dear friend

Thank you for coming forward to help me. I need to analyse CO2 in low range (10-300 ppmv). I have GC/TCD (Nucon-5765) with Poropak Q packed column(1/8", 6 ft), and He as carriere gas.
My sample containes residual benzene,touene xylene and CO2. My aim is to analyse all these compounds. My GC system is a dual channelled and able to use both FID and TCD simultaneously for two different columns. So I'm using an isothermal conditions of
Inj @ 200 deg C, Oven @ 55 deg C and Det @ 210 deg C
to get peaks for aromatics in channel 1 with FID and peaks for CO2, N2,O2 in channel 2 with TCD. But I'm able to analyze co2 as low as 1% (approx 10000 ppmv) only.

Is it possible to analyze co2 in low range,
Can you please suggest me operating condition for the analysis of co2 in low concentration range?
Thank you.

[chromatographer1]

You should not have a problem measuring CO2 at the levels you describe. Howver, since Porapak Q keeps the CO2 relatively close to nitrogen you might wish to change to a Porapak R or a Hayesep A column.

If you can use a smaller mesh size of porous polymer for a sharper and more easily detected peak for CO2 (100/120 is good)

If you have a thermistor TCD detector, lower the temperature of the detector to improve sensitivity and slow your column flow rate.

If you had a pulsed helium ionization detector from Valco you would have the best of both worlds. It is very sensitive for CO2 and the aromatics.

Increase your injection size to 2mL and you should be able to see much less than 100 ppm of CO2. Increase it even further and decrease your flow to 20cc/min or less and you will increase your sensitivity to ppm levels.

If you have a large dead volume in your TCD that will be a limitation for your sensitivity.

The aromatics can be done on almost any column, methyl silicone or Carbowax 20 is preferred. 5% loading on Chromosorb P or G or 10% loading on Supelcoport type support.