GC analysis of greenhouse gases, CO2, CH4 and N2O

[soak]

Hi guys and gals,
We have a PE Autosystem GC running with an ECD and FID (with methaniser) and are currently trying to analyse all three gases. The problem is with the methane as it is co-eluting with a peak that comes out just before it (oxygen peak??). Any suggestions to solve this would be greatly received. Have tried lowering carrier pressure, decreasing temperautre.
Cheers
Si

[Don_Hilton]

Column selection would be a possibility - what are you using?

[soak]

A Porapak Q column. I've never had this problem when just looking at CO2 and CH4 on a FID, but have recently put a split in after the column to go to both detectors. I can get N2O and CO2 fine but the methane is proving elusive!
Cheers

[chromatographer1]

Could the peak be CO ?

If you have a short Q column (less than 4 meters) the separation of fixed gases and methane will be minimal, especially if you have dead volume (and you do with the methanizer).

Try to minimize the dead volume in your connections or get a longer column (more plates).

best wishes,

Rod

[soak]

We have a column that is about 4m and have used it in the past to look at just CH4 and CO2 and had little problem with the methane peak co eluting with the CO peak. The problem has only arisen since the split has occured. Does this sound plausible? Would just buying a longer column solve this? Should I try and reduce the lengths of the capillary tubing (used for the spilts) to the shortest they possibly can be? All your help is appreciated

[chromatographer1]

OK, now we are getting somewhere.

The issue began after the spit was installed.

There it is.

You may be having a restriction causing the dead volume which when the gases flowed briskly was no problem to .... it is a problem.

Or it may be that slowing the flow by splitting it is causing the problem.

I would try increasing the flow to see if that might help. I would also make certain I was using a methanizer that was packed FULL and tight. Minimize your dead space in it and your problem may go away.

best wishes,

Rod

[soak]

Thanks for this, it's really helping. However, if I increase the flow will this not just push the CO peak into the CH4 peak?

[chromatographer1]

Not unless your methane has a broken leg

It may keep the plugs from mixing even more in the dead space if the flow is increased.

Rod

[soak]

I have had the carrier as high as 35psi, any suggestions how much higher I should go?

[chromatographer1]

What was your flow rate through the methanizer before you split the column output?

Raise your pressure until the same flow is passing through the methanizer.

I hope you have balanced the flow through each detector. Did you measure the flow through each detector?

If the methanizer has a LOW flow then increase the restriction on the ECD.

Rod