TCD-FID in Series

I have the TCD and FID setup in series and I am noticing the top of the peaks in the FID are very noisy? Do you know how I can correct this? Detector parameters are listed below.

 

TCD

Temp: 300°C

Reference: 20 ml/min

Makeup (He): 2 ml/min

 

FID

Temp: 300°C

Air: 400 ml/min

Hydrogen: 30 ml/min

Makeup (He): 25 ml/min

  • Since the FID and TCD are in series, I would guess that the extra noise on the peak through the FID is due to the TCD valve switching between carrier and reference.  Try setting the FID signal rate to 5 Hz.  Also, the FID makeup flow can be reduced or turned off completely.  The TCD reference and makeup flows provide enough extra flow so the FID makeup can be 5 mL/min or less.

  • Thank you Lance.

    Yes, the noise is definitely from the switching valve on the TCD. Ill try your suggestions to see if it helps as the FID was set to 20 Hz

  • Hi Lance.

    Setting the FID to 5 Hz fixed the peak noise issue. I did notice however that when lowering the makeup flow on the FID < 5 ml/min (set it at 3 ml/min) there was loss of sensitivity in the FID. I set it back to 25 ml/min and the sensitivity was restored. The current set up is below but sometimes the FID has trouble lighting which I'm assuming is because there is too much flow and its blowing out the flame. When I turn he flows off for the TCD, the FID lights fine. Any suggestions to remedy this?

    TCD

    Temp: 300°C

    Reference: 15 ml/min

    Makeup (He): 4 ml/min

     

    FID

    Temp: 300°C

    Air: 400 ml/min

    Hydrogen: 30 ml/min

    Makeup (He): 25 ml/min

  • You're correct in that the FID is having trouble lighting because the helium flow is too high.  One workaround is to turn on the FID before the TCD so that the TCD reference flow is off while the FID is lighting.  Another idea is to use a higher H2 flow at the FID - that should make lighting easier with the higher helium flow.

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