low response in FID

[rochelle]

Hello guys! Its me again... We always had a problem in our GC... maybe we need a new one... kidding aside or maybe we really need someone who can answer our question... Hope u can help us.

We are analyzing TPH Gas/ BTEX using gas chromatograph HP 5890 with a tandem of FID/PID. The method that we are using is USEPA 8021. What we observed before is that the response of the two detector was not the same. FID is almost half lower than the PID. We don't mind the gap of the two detector until our surrogate recovery exceeds our limits. Lets say the limits for FID is 50-150%, we are getting 40% but not with PID. PID was always consistent in getting a good recovery...

What shall we do? We already tried another set of calibration standards. Here's the tricky one when we run QC samples in the GC the response is in normal range ( i mean near the Lower Limit )but when we run soil samples the response is very low...

Thanks!

[AICMM]

Rochelle,

The lower response of the FID relative to the PID should be expected. The PID sees fewer compounds but the ones it is intended to see (BTEX) it should see much better than the FID. Therefore, I hope you are supporting two calibration curves, one for each detector, in your method file and you are supporting two sets of surrogate recovery data.

If the FID is not sensitive enough, the place I would go look is the flow rates for the hydrogen and air. Make sure these are where they should be.

Regarding low recoveries for soil samples, this is not that uncommon especially if you have organic material floating around in the sample after you add water. If you are saying that the surrogates fail on the FID side but not on the PID side for the same sample, that is a bit more puzzling, especially if the sample is basically clean.

Best regards.

[JI2002]

If PID works fine and FID fails, it's FID detector problem. Check FID flow rates and/or clean it.

[Chandresh K. Soni]

Dear Rochelle
If your FID Gives the Lower Response then Pls Check Your FID Assembly or Clean it by Various Solvents( Because Many times Comp. Deposite on the Jet of FID and it wil Reduce the Sensitivity of the FID . Also if the Ratio of Air and Hydrogen is Vary ( If the flow of Hydrogen is less then required then this type of Problem is Faced out.)

Best Regards