Can I separate toluene and CO2 using an MXT capillary column? My detector is an FID with methanizer and everytime i run a mixture of CO2,toluene and dry air, I always get two peaks. The first peak is occurs at 13 minutes while the second one is at 18 mins. But i got confused because when I ran it empty, even for several time, I could still get two peaks. I checked for leaks in the column connections using Snoop and there seemed to be none. Thanks for your guidance
It depends of course on your GC injection and temperature programmes, but CO2 is ways too polar and too volatile to be retained by PDMS capillary columns under typical for BTEX analysis conditions. Very likely none of your peaks comes from CO2. To give any hint it would help to know some more about your GC. Injection? Mobile phase? What blank you ran? Temperature programme?
The carrier gas is helium. The blank I ran is dry air (instrument grade). The oven temperature is maintained at 200 C for 25 minutes. This time, when I try to run a blank, I got one peak only at about 13 minutes. When I try to run CO2 only, the area of that peak at 13 minutes did not change much, So you may be right that none of those peaks I've observed before is for CO2.
Do you have an idea about your dead time? Is 13 min gross or net retention time? I suspect that 13 min might be your injection peak, which would be very late for a typical capillary GC that's optimized to run BTEX. At 200 °C CO2 surely comes with your injection front (as it would at room temperature anyway).
What is your flow rate and your column dimensions? And how do you inject? Some gas volume with a syringe directly on column?