periodic baseline up and down

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TG's picture
periodic baseline up and down

we have a HP 5890 Series II gaschromatograph.
We changed lately the capillary column und run now with a varian column for fatty acids.
Carrier gas: Helium    FID supporting gases: Hydrogen and synth. Air
Now we have a constantly periodic up and down of the baseline. Looks like a saw tooth pattern.
I tried a run with a standart component and also a blank run (without any injection), always pretty the same.
Have you any idea what kind of problem this could be ?
How would you solve it ?
Thanks for any help !!!

Dr. Analytical
Dr. Analytical's picture
This sounds like a detector

This sounds like a detector problem.
First, let's eliminate some other possible problems.  Change the pressure or flow and watch the baseline.  Then, change the temperature.  If you do not see any change in the response after these changes, then you probably have a problem in your detector.  If the pattern is very regular, with very sharp corners (the "sawtooth" that you describe), it is probably an electronic issue, perhaps the power supply.  Try turning the detector off, letting it cool, and then turning the power on again.
I have another 5890 expert that I can consult with, but won't see him until tomorrow.  I will write back later in the week if I learn any more.

TG's picture

I checked my pressures and flows again. Here are my conditions:
Helium: pressure: 2,8 bar   column head pressure: 2,0 bar    flow through the column: 0,5 ml/min     split ratio: 9,5
Air:  pressure: 5 bar    flow: 300ml/min
Hydrogen:  pressure at the generator: 2 bar     flow: 40 ml/min
I also checked the temperature program again, but since it starts with 140°C and ends at 240°C I don´t think the temperature is the problem.
I did a run with just a solvent (toluol) and two more runs with diffrent standart mixures, but it was always the same.
I get the right number of peaks, but the baseline is still in that kind of pattern.
I also did a zero adjustment of the detector signal without a change. Shuting off and cooling down the dectector also doesn´t help.
If it´s really the detector, what can I do on my own without further experiences in repairing a FID ?
Is there anything you what do before calling a technician? How can I proof it´s the detector?
Thanks for your help !

Dr. Analytical
Dr. Analytical's picture
Thanks for the extra

Thanks for the extra information.  This is a good start at learning troubleshooting.
First, I usually want to see the air:hydrogen ratio at 10:1.  Try setting the hydrogen to 30.
Now, make an injection of solvent at one temperature.  Then change the oven temperature by at least 25 degrees and make another injection of the same solvent.  Compare the baseline pattern.  If there is a change in the pattern at a different temperature, then you may have a problem with the makeup gas.  This might be caused by a leak or bad ferrule at column end on the detector side.  Check all fittings for leaks.  Replacing a ferrule may be necessary.
If there are no changes, you may have an FID gas controller problem.  Most problems are with the hydrogen controller.  Try adjusting the controller, or installing a different one if you have one available.