Chromatography Forum

Hi everyone

I am using a Porapak QS column for detection of CH4 and CO2, with an FID and N2 carrier gas. CH4 RT is 0.5 min, CO2 RT is 1.0 min. Recently we have been getting an extra peak coming off at 0.4 min, it is a large tailing peak that makes CH4 determination impossible. This peak is not present in the calibration gas that we use, but is present when we put a normal air sample through. We have tried a new column and new catalyst in the methaniser, but it did not help.

Anyone got any ideas?

Any advice would be greatly apprecaited.

Thanks.

Wouldn't the first peak off this column be a composite of H2, O2, N2, and CO? Probably one or more of these. (Edit) I missed that you're using methanizer and FID. Only the CO would show in that case. Not much CO in air, so now I'm stumped.

Recently we have been getting an extra peak coming off at 0.4 min, it is a large tailing peak that makes CH4 determination impossible. This peak is not present in the calibration gas that we use, but is present when we put a normal air sample through.

Do you mean you didn't have this peak until recently? what was changed in the system recently? If you run a sample without methaniser, do you still see the peak?

Thanks for the replies.

We haven't always had the problem but I don't know when it started unfortunately, it isn't me who always uses the machine. I think its been a gradual thing though and we haven't changed anything on the machine.

Recently we fitted a new methaniser and the peak didn't disappear. It would be a good idea to try it without the methaniser though, as you say.

I did try lowering the FID temperature today, it's always been used at 350 but lowering to 250 removed the peak. CH4 determination was then possible but the CO2 peak was very broad and had a tailing shape.

This peak could well be oxygen. If you have been passing samples through the methanizer that have a high carbon content, these samples can 'coke' up the methanizer. When oxygen passes through, it reacts with carbon to form CO2, which in turn forms methane which is then detected by the FID.

Gasman

Thanks GasMan. We have replaced the catalyst in the methaniser and it didn't solve the problem.

I don't know if this is important but the peak has a very large tailing back end, ie goes up quickly but comes down very slowly (2nd half of peak is about 20 seconds wide).

The most confusing thing about this is that the peak does not occur for calibration gases!

Try injector port maintenance if it has not been done for a while, maybe detector too.

You probably have carbon deposit on the FID tip and it reacts with O2.

You don't see it with your cal bottle since there is no air(O2) in. It would be great if would have a cylindr with O2 content at ppm level to see if you can reproduce the same peak.

TimJ,

Try turning the heat to the methanizer off, or say, 125C and see what happens then. I would agree with GasMan about coking except you said you replaced the methanizer which should have eliminated that issue. I am also very puzzled by the poor peak shape with the FID at 250 since that should be plenty hot for these components.

By the way, are you using synthetic air for your FID? If so, try shooting some of that to see if the problem still occurs.

Best regards.