Chromatography Forum

Hi all,
I am working on a GC from SRI with TCD and FID to analyze hydrogen and carbon monoxide. Recently, the FID baseline has become extremely bumpy after a couple runs. My runs are about 5 minutes long, and the first three to four will have a nice flat baseline but after that bumps show up and start substantially affecting my measurements. The TCD trace remains normal.
I have baked it out multiple times, run a backflush through the HayeSep, and have replaced the wire connecting the FID electrode to the amplifier/collector. The columns look fine as far as I can tell (I haven't replaced or opened them up but can if that seems like the likely culprit). I have taken off and re-tightened the nuts on the FID/methanizer, and didn't see any obvious debris or breaks.
We have other identical GCs running from the same carrier gas and hydrogen generator that are behaving themselves.
Any advice on next steps would be greatly appreciated. Thanks!

Some types of FID have 2 electrodes (not sure about SRI), try to check both.
1. Collector - from big cylinder collecting ions.
2. Polarizing electrode touching head of FID jet (+200V polarizing voltage).

Not a "GC-guy" but I asume it's isothermal?

Then what about late eluters?
Will the baseline become bumby too if you extend your runtime to about 10x or more? Will it get stable again after some time? Then it may come from your sample and/or injector, introducing some late eluters?

I have baked it out multiple times, run a backflush through the HayeSep

I would install a brand-new column and see what happens.