Low FID signal

[biocat]

I have an Agilent 6890 with dual injection ports and dual FIDs. When I inject from the front injection port to the front FID, I get a solvent peak of about 3500 pA. When I inject from the front injection port to the back FID I get about 300,000 pA for the solvent peak (i.e. much higher). I swapped the jet and collector assembly from the front FID to the back FID and I get 300,000 pA. From this, I gather the injection port is ok and the jet/collector assembly is ok. Any idea on what could be wrong? Something with the electronics? Measurements are being done through ChemStation through an ethernet connection to the GC. Thanks!

[dlbenach]

After you swapped the jets and collector and got 300,000 pA, was that injecting from the front inlet to the front detector (which now has the parts that were on the back detector? If so, that says there is a problem with the original front detector parts. It's possible your original front detector jet is slightly clogged. I've seen this before but not with such a large loss of signal. How are your retention times? If the retention time is longer for the injection in which you got 3500 pA, this also suggests a clogged jet.

[biocat]

Argh, nevermind. The air flow rate to the front detector wasn't set right (way too low) and I didn't notice it. We never change those settings, so it didn't come to mind to check it. All good now.

[Meerkat]

Which begs the question: what is the optimal air flow and H2 flow on a FID
or does it really matter as long as the Flame is on and set at the correct Temperature?

[kubowicz.tomasz]

Hello

Which begs the question: what is the optimal air flow and H2 flow on a FID
or does it really matter as long as the Flame is on and set at the correct Temperature?

Normally Hydrogen/Air - 10-12% which in practise gives typical values:
30/300
35/350
40/400
45/450
The best way is to check analyte(s) response vs. different H2/Air ratio and pick the best one

Regards

Tomasz Kubowicz

[biocat]

We have ours set to 40, 400 and 45 mL/min for H2, air and N2 (makeup).

[kubowicz.tomasz]

Hi

We have ours set to 40, 400 and 45 mL/min for H2, air and N2 (makeup).

It is all about method development...
If you're using gas generators it doesn't matter but if you have gas cylinders you can calculate H2 and Air consuption.

Regards

Tomasz Kubowicz

[Peter Apps]

You can rig up one of these:

THOMASON, M.M., BERTSCH, W., APPS, P.J. and PRETORIUS, V. 1982. A simple device for the optimisation of flame ionisation detectors and determination of absolute sensitivity. Journal of High Resolution Chromatography and Chromatotgraphy Communications 5: 690-692.

But you will see only marginal improvements over what the manufacturers recommend for their instruments.

Peter