TCD tailing issue

Discussions about GC and other "gas phase" separation techniques.

3 posts Page 1 of 1
Hi all,

Just wondering if someone can give me some insight as to why my TCD peaks might be tailing (big hump just after peak comes out), why I would get a high area count (order of 1e6) on first injection but subsequent injections always come out much smaller (order of 2e4), why upon using a temp ramp program (40 oC up to 100 or 150 or higher), baseline always just steadily increases and ends high at max temp (signal at 150 or so compared to 5 when temp is 40 oC). Am injecting 500 ul samples but it doesn't seem to matter. Lower volumes don't resolve the issue so don't think I am overloading the column.

I have tried everything I can think of and wonder whether the TCD or the column is compromised or whether a good overnight conditioning might be the solution. I have not found any leaks. If I condition at 210 oC for a few hours, the baseline eventually starts to decrease over time but never gets back to the 5 when it is at 40 oC and then it starts all over again if I do a ramp program. I have managed to finally get all the flows (column flow at 6 mls/min, total flow at 20 mls/min, septum purge at 7 mls/min, column head pressure at 5 PSI) to the point where I am actually getting a relatively nice peak except for the big hump post peak.

Right now, am trying to analyze for formaldehyde but have tried optimizing the TCD response with just analyzing for hydrogen and oxygen on a mol sieve. On the mol sieve column, I get separation for H2, O2 but the humps are there, peaks not that sharp, and I get the issue with the inconsistent area counts. On a Q-plot column (to test for formaldehyde), I get a single peak for air (that's OK) but can't see formaldehyde yet (it might be buried in the hump or it may be an issue re mass and, if the latter, will solve that later) but for now would settle just with knowing about the baseline rise on temp ramp, the inconsistent area counts, the hump on the peak.

As an aside, I did manage to accidently put the oven up to 400 oC (for about 5 min before I caught it) instead of 40 oC and may have done some damage to the Q-plot column (thoughts on that??), however, I was having the same problem before this happened and I don't see much difference in the response post this issue. Did muck up the FID enough to have to replace it, though.

Could upload a pic of the peaks but don't see how to do that.

Thanks for your help.

If you ramped a porous polymer to 400C, you now have a block of plastic in your column. The place to start troubleshooting this problem is with a new column. Then we work from there.

Best regards,

Most peak shape issues are due to the injection port. What are your injection parameters? First thing I'd try is to shorten the time the injector vent is off.
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