Chromatography Forum

Hi all,

I have an issue with an old 6890N with two FIDs installed. We've always run 400 mL/min air to 40 mL/min hydrogen with no issues. Recently the actual reading on the back detector continually fluctuates between 398 and 402, the GC saying "not ready" due to back detector air flow. Baseline/signal look good while it remains lit. However, every so often, it will just turn itself off and shut down air and hydrogen flow. Not sure what would cause this fluctuation. Software doesn't allow to ignore the setpoint of 400 mL/min. I have verified there is more than adequate air supply in to the system, as I can turn the air up to 500 mL/min and it'll reach the setpoint. Any ideas, thoughts, tricks would be greatly appreciated.

Try a larger number in the "Lit offset" box, like 2.0 instead of 0.2.

I thought the lit offset had to do with the instrument sensing the difference between having a flame and not having a flame. When the instrument shuts downs, the flame is on, and the baseline is relatively constant and usable. I will try this however and see if it works. Thank you.

I thought the lit offset had to do with the instrument sensing the difference between having a flame and not having a flame. When the instrument shuts downs, the flame is on, and the baseline is relatively constant and usable. I will try this however and see if it works. Thank you.

Yes. That is the purpose of "lit offset". However depending on the level of baseline signal and high lit offset (2 or more) FID flame may shut off.
I would rather set lit offset to 0.2 not 2.
Fluctuations of air flow may be caused by faulty cylinder regulator or EPC.
What is the air pressure on the outlet of cylinder regulator ? (5 bar is O.K.).

Pressure off system going to the FIDs is approximately 80 psi. It is travelling through a zero air generator from our house air and has a pressure gauge just prior to the zero air. I did test the FIDs at much higher flow rates and the system seemed to handle it ok. I suspect the EPC as well. I will change the lit offset and see what comes of it. Thank you.

Check with an independent flow meter whether the air flow really is fluctuating, if not then it is a fauly readout. If it is fluctuating then the EPC is bad. A possible workaround for both is to set a slightly different flow rate - say 405 ml/min.

Peter