High Noise on GC 7890B

Dear Community

We faced problems with our GC 7890B which is connected to a Leco Pegasus BT. Since this week we have a really high noise, which comes regularly every 5seconds. We already cleaned FID many times, replaced Jet and built the FID out an in.

This is how it should look (orange line):

This is how it looks now:

When zoomed in, the noise looks like this:

Do you have any ideas from where this could come from? we are thinking about an electronic issue (?)

Thanks a lot in advance for your ideas!

KR GCarmen

  • What are your gas supply pressures?  The FID air and hydrogen pressures before the GC should be at least 45psi or higher, up to 100psi.  If one is too low, the electronic pressure control module may have trouble maintaining the proper flows and this sometimes results in very low signal cycling.

    How does the signal look if you turn off the flame?   Does it change if you increase the FID hydrogen flow from the recommended 30ml/min to 40ml/min or change the air from the recommended 400ml/min down to 300ml/min?   How does it look if you remove the column from the FID and cap the bottom of the detector inside the oven?

  • Hello paulsalverda

    Thanks for youre reply and sorry for getting back so late.
    We checked many of the things you mentioned already. We could not find it out at first. But then we build out the FID with the interconnect assembly and we saw, that the spring was reaching too far into the collector (has been built in like this when we got the instrument). It probably caused this disturbance in the signal. Also the high Output which we sometimes had, and sometimes not.
    Problem is now solved. I don't know if this is for help for any other use.
    Question can be marked as solved.

  • Hi  thank you for letting us know the problem is now solved and for sharing the resolution.  Since your question is about the FID, I moved it to the Gas Chromatography forum for better visibility.

    Best Regards,


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