Chromatography Forum

Hello fellow members,

I am using a 7890 GC in my analyses. Recently the instrument would not give a chromatogram on injection of a test solution. There is carrier flow through the column, the inlet liner is new, the detector signal makes a large movement when the detector is lit, yet on injection of a solution there is no signal generated. There is just the baseline with its variations.
I noticed while checking the inlet flow that on injection the flow increases from the set point value for some minutes and only returns when the GC run is stopped.

Can some one help me with this problem?


Thanks
foodchembb

It sounds like you are running in constant flow mode (inlet pressure rises as the oven temperature rises to keep the flow constant). In the inlet parameters of your method, you can change that if you want to run at constant pressure.

You should still see peaks if you inject a sample. I assume your detector is a flame? If yes, make sure that your method is set to monitor the signal from that detector (properly located in your system - either front or back). Associating the injector with the detector is important in ChemStation methods.

Sounds like you might have zero flow through your column. Disconnect your column from the FID. Stick the end of the column into a vial of solvent and confirm that you have bubbles (your carrier gas flowing). I do this every time before installing a column into the detector to ensure I have column flow.

There are a large number of possible causes.

How do you know that you have flow through the column ?

When you say that carrier flow increases during the run, how do you measure flow ?

What happens to the inlet pressure during a run ?

If you inject some gas from a cigarette lighter (using an ordinary syringe if you do not have a gas-tight syringe) do you see a peak and how long does it take to elute ?.

The syringe might be blocked - have you tried another one ?

Peter

Dear Foofchamb,
please check your detector Jet. many times it seems if your Jet is chocked then the signals will reduce and after sometimes they will not appear even you inject higher injection.
clean your Detector jet by methanol. also check your GC syringe which you are using.