Chromatography Forum

HI,
I am using Agilent 6890A gc with FID. baseline is oscillating continuously from 130 pa to 135 pa with in a time of 1 min 30 sec. If i switch off flame i will get 0- 1 pa stable current

First thought: Do you have at least a 20 psi pressure drop across each regulator from the gas bottle to the instrument? If the pressure drop acorss a regulator is too low, you can get oscialation. Check all gasses.

Thanks for the reply
Although hydrogen and oxygen gas regulator showed output pressure more than 20psi I still increased the pressure but oscillation is continuing but if i keep gc in same condition for more than a day oscillation is decreased from 5 pa to 3 pa

This might be carrier flow, does the inlet pressure fluctuate ?, what happens if you turn the carrier gas off ?

Conceivably a temperature control problem in either inlet, oven or detector, monitor all of these.

Peter

First I want to address the baseline level.

130-150 pA is very high, your baseline should not be higher than 25. You can expect a system in good condition to run between 2-15 pA. Are you using a packed column maybe?

What is the temperature of your inlet, detector, and what oven temp?

If you uninstall the column and block off the detector with a blank nut what is the baseline? Does it increase dramatically when you increase any of the temperatures?

Second, the oscillation. What data rate are you collecting at? The average user runs the FID at 20 Hz, but chemstation (and other CDS) can default to 50 or 100 Hz. Try checking the data rate and make sure it isn't set too high.

Do you have a 2nd inlet? If you have a 2nd inlet with no column installed but the method is set to pressurize the inlet you can get oscillations due to the common carrier gas supply pressure fluctuating as the 2nd inlet tries to pressurize.

Anyway, please provide more details as there are other possibilities.

Yes i am using packed column. I dont have any 2nd inlet.
Temperatures:
Detector: 300 degree C
Inlet: 170
Oven: 150
I will have not tested removing column and using a blank nut. I will test it now and i will inform

Thanks all of you for the reply.
I called agilent technical bench. It turned out to be problem with hydrogen gas. Now baseline is stable

It turned out to be problem with hydrogen gas.

What problem exactly ?

Peter

I told my problem they asked me to check detector gas. Then i checked whole gas line it was ok. Then i changed regulator and cylinder it got rectified. There might be a small leak in regulator .

OK, thanks. So Don was right.

Peter