CO2 Detection by FID

[DocHollywood]

I'm using a 5890 series 2 GC with an HP Plot U column leading to an FID detector. I have a 1 ml sample loop. I am trying to detect methane contaminants in 99.99% pure CO2. I see methane with out a problem. What is odd is that i see a small peak after methane which i think is CO2. The peak retention time is before ethylene and ethane. Since the column is polar it can't be any other 1-2 carbon species. Could some of the CO2 be reduced in the Flame of the FID? Once again the peak is very small, similar to 12 ppm methane.
Thanks

[chromatographer1]

There could be a peak from another non-carbon molecule to which the FID will respond, but it is very likely the 'peak' is actually the flame or conductivity 'upset' caused by the CO2 balance entering into the flame.

Even water can cause an upset in the FID response. I remember seeing a peak greater than 100 ppm by weight in an injection of water standards when the oven temperature was high enough to sharpen the water peak eluting through a Porapak Q column. When you change the gas composition or the conductivity across the electrodes on a FID, the current change can be seen as a 'peak' in the chromatogram.

It is quite likely that you are not seeing NO or NO2 but CO2.

best wishes,

Rodney George
consultant
chromatographer1 at aol dot com