Low detector response from FID

[knjohnson]

I have a question that hopefully someone on here may have some idea about.

The issue is that the detector response from the FID detector on our Varian CP-3800 is abnormally low, by about a factor of ten. Our solvent peak used to register ~15 V; now, we get ~1.5 V. Peak positions do not appear to be shifted.

The following trouble shooting actions have been done:
- replacing septum twice
- cleaning spring clip in detector
- replacing air cylinder
- replacing injection syringe
- The injection liner has not been replaced, but it does not appear to be dirty or cracked

Our system is GC only and is not attached to MS.

Any suggestions are greatly appreciated! Thank you very much for your time!

[chromatographer1]

How are you collecting the data?

Have you changed the output voltage?

10V and 1V differ by a factor of ............

best wishes,

Rod

[knjohnson]

Thank you very much for the reply.

The GC system is attached to a computer running Varian GC-MS workstation software that automatically saves the data for each run as a MS Workstation data file that opens to a chromatagram and an accompanying .txt file with run data like retention time, peak intensity and area counts.

It is a shared instrument, but everyone that has access keeps from changing anything other than the method file via the computer inferface, so I doubt that output voltage was changed, but we'll check to see.

Do you have any other ideas if it's not the output voltage?

[Peter Apps]

The solvent peak and its signal are miles above the linear range of the detector and the electronics. Are the analyte peaks also 1/10 as big as they used to be ?, if not then you do not have a problem.

If the analyte peaks are 1/10 the size then blow up the vertical axis until you can see the baseline noise. If it is also reduced to 1/10 then the problem is in the electronics, and a setting change is by far the most likely cause for an exact 1:10 ratio. If the noise is the same then the problem is most likely in the detector itself.

Check all the gas flows to the detector with a flow meter, not the onboard pneumatics of the GC. Check the inlet for leaks (with a leak seeker), check the split ratio setting and the actual flow (with a flow meter), check the septum purge actual flow and setting.

Peter

[AICMM]

A quick and easy thing to check; if you were running splitless/split before and someone made you split, it could easily account for the difference.

Best regards,

AICMM

[larkl]

The makeup gas to the FID isn't shutoff is it? This would result in a lower signal.

[walter]

On a Varian the first thing I would check in that situation is the output range of the detector, X10-12 or X10-11. Who changes what on a shared instrument?