HELP

Discussions about GC and other "gas phase" separation techniques.

7 posts Page 1 of 1
My phd project greatly depends on this.
Specific question: Do you normally have a make up box + Reference box in the TCD settings? My GC only has a reference flow rate box in the TCD settings. The TCD is connected to the column for the make up.
The peaks are appearing on the TCD signal graph of Oxygen/Argon (carrier +TCD ref). However, the actual flow rate through the TCD remains 0 (despite the setpoint being 35 ml/min). The comes with PPC (pneumatics) flow controllers for both the FID and TCD. I tried zeroing and calibrating them but still no flow via the TCD detector, even when the required temperature is reached 100 degrees celsius. I set my current at -40 millivolts for a range of 1.

What do I do? I am following the perkin elmer manual. The machine is a revamped Clarus 500 GC (software version 2.22). The only injection method is a Vici valco gas sampling valve, where 100% gaseous sample (from cylinders) is piped in the 6 port valve. There are no liquid samples like GC-MS or HPLC.

Your help is much appreciated!
I don't understand what you're saying here. If you have no flow through the TCD, how are you seeing any peaks at all. No flow through the carrier side means no flow through the column. Are you measuring the flow rate with a meter designed for that purpose (truly what's coming out of the ports on top of the detector when it's mounted on the GC)?
Thanks for your reply.
To clarify, for the gases entering the back of the GC, I dont have any external flowmeter devices. If you look in the operating manual, GCs can come automatically fitted with PPC pneumatics, so I can set the pressure/flow entering the detectors and packed column.

As for the TCD (thermal conductivity) issue, I was surprised about it just like you. However, I realized I might be out of the range of detection of the PPC pneumatic controller. In the operating manual, it says 60-90 psig (which is approximately 4 bar or 400 kPa). I had it at 181 kPa. The flow set point for the TCD reference (argon) was 30 ml/min (that is the equivalent of around 181 kPa approximately). So I need higher Argon outlet pressure from the cylinder and a higher set point.

Would this work ?

Thanks.
Rule #1, never completely trust pneumatic flow controls. You should always have a way to check that they're working correctly. A digital flow meter is very important. I use an older version of this:

https://www.agilent.com/en/products/lab ... flow-meter

There are others on the market as well. The only way I know that will tell you if you're getting 30 cc/min through your TCD is to put a flow meter on the outlet of the column and reference sides.

I'm not talking about the gases going into the GC. It's what's coming out of the TCD (through the column) that's important. The electronic pressure control is merely a regulator you can set very accurately without turning a knob. They're calibrated to do this. They step the pressure down from some other point. 60 psig from your tank means that the PPC can only deliver less than that.
Thanks for your reply.
I was in the lab and discovered what you said. The main reason for my surprising observation is a discrepancy between the flow rate measured by the GC and the flow rate measured by the bubble flow meter (I
    do not have an external digital flowmeter, not this project on a tight budget, so just using a simple glass bubble flow meter).

    The detector PPC flow modules are automatic pneumatics, however, the carrier gas flow module (carrier to packed column) is manual pneumatics. I disconnected my column from the injection sampling valve and attached my bubble flow meter hose. Then measured the actual flow rate measured by meter and flow rate shown on GC display. There was a large discrepancy.
    The main reason is:

    1- I selected ArCH4 from the drop-down menu rather than Helium. I am using pure Argon only. ArCH4 and Ar are obviously different size molecules so will give different flow rate/pressure differences on the PPC module. I know that for carrier gas. For the detector reference gas, it must be the same reason because I am using Argon for that as well.

    2- The options on my GC are only He, H2, N2, ArCH4. I am using CH4 for my experiments but I don't want to use it as a carrier or detector reference. It just goes into the reactor.

    I think the solution is to try and use Helium if possible for detector reference. Share with someone nearby. Or I can stick with ArCH4 and keep using the calibration curve to check the right flow rate. I will keep using the calibration curve. Helium cylinders are expensive and there is a shortage of helium sadly.

    Which brings us back to rule number 1 which you mentioned: never trust GC pneumatics on their own, always use an external device, in my case, it is my lovely bubble flow meter.

    As you said, pressure setpoints on the GC simply act as a regulator guage, just show the outlet pressure like a regulator. Thank God for that!, my cheap single stage argon regulator only shows outlet flowrate in L/min and cylinder contents in bar/psi.

    Thank you for sharing your experience, greatly appreciated!

    All the best!
    Chemengnemesis wrote:
    Thanks for your reply.
    I was in the lab and discovered what you said. The main reason for my surprising observation is a discrepancy between the flow rate measured by the GC and the flow rate measured by the bubble flow meter (I
      do not have an external digital flowmeter, not this project on a tight budget, so just using a simple glass bubble flow meter).

      The detector PPC flow modules are automatic pneumatics, however, the carrier gas flow module (carrier to packed column) is manual pneumatics. I disconnected my column from the injection sampling valve and attached my bubble flow meter hose. Then measured the actual flow rate measured by meter and flow rate shown on GC display. There was a large discrepancy.
      The main reason is:

      1- I selected ArCH4 from the drop-down menu rather than Helium. I am using pure Argon only. ArCH4 and Ar are obviously different size molecules so will give different flow rate/pressure differences on the PPC module. I know that for carrier gas. For the detector reference gas, it must be the same reason because I am using Argon for that as well.

      2- The options on my GC are only He, H2, N2, ArCH4. I am using CH4 for my experiments but I don't want to use it as a carrier or detector reference. It just goes into the reactor.

      I think the solution is to try and use Helium if possible for detector reference. Share with someone nearby. Or I can stick with ArCH4 and keep using the calibration curve to check the right flow rate. I will keep using the calibration curve. Helium cylinders are expensive and there is a shortage of helium sadly.

      Which brings us back to rule number 1 which you mentioned: never trust GC pneumatics on their own, always use an external device, in my case, it is my lovely bubble flow meter.

      As you said, pressure setpoints on the GC simply act as a regulator guage, just show the outlet pressure like a regulator. Thank God for that!, my cheap single stage argon regulator only shows outlet flowrate in L/min and cylinder contents in bar/psi.

      Thank you for sharing your experience, greatly appreciated!

      All the best!


      Has been a long time since I use a TCD, but the one thing I remember was needing to keep the same gas on both sides and try to keep the flows on each side the same also. As long as those are the same then it shouldn't matter what the instrument is set to as that just calculates a flow from the head pressure reading.
      The past is there to guide us into the future, not to dwell in.
      James, I know what you mean, in theory, you have to set make up and reference gas flows for the TCD. This balances the bridge and sets the current.

      However, in my GC, the TCD only has one connection TCD reference (FID has 2). There is no make up. I am assuming the sample + carrier is injected via the valve, passes through the column and enters the TCD via the oven (this can be considered as 'make up', so we equalise both of these flowrates ?


      James_Ball wrote:
      Chemengnemesis wrote:
      Thanks for your reply.
      I was in the lab and discovered what you said. The main reason for my surprising observation is a discrepancy between the flow rate measured by the GC and the flow rate measured by the bubble flow meter (I
        do not have an external digital flowmeter, not this project on a tight budget, so just using a simple glass bubble flow meter).

        The detector PPC flow modules are automatic pneumatics, however, the carrier gas flow module (carrier to packed column) is manual pneumatics. I disconnected my column from the injection sampling valve and attached my bubble flow meter hose. Then measured the actual flow rate measured by meter and flow rate shown on GC display. There was a large discrepancy.
        The main reason is:

        1- I selected ArCH4 from the drop-down menu rather than Helium. I am using pure Argon only. ArCH4 and Ar are obviously different size molecules so will give different flow rate/pressure differences on the PPC module. I know that for carrier gas. For the detector reference gas, it must be the same reason because I am using Argon for that as well.

        2- The options on my GC are only He, H2, N2, ArCH4. I am using CH4 for my experiments but I don't want to use it as a carrier or detector reference. It just goes into the reactor.

        I think the solution is to try and use Helium if possible for detector reference. Share with someone nearby. Or I can stick with ArCH4 and keep using the calibration curve to check the right flow rate. I will keep using the calibration curve. Helium cylinders are expensive and there is a shortage of helium sadly.

        Which brings us back to rule number 1 which you mentioned: never trust GC pneumatics on their own, always use an external device, in my case, it is my lovely bubble flow meter.

        As you said, pressure setpoints on the GC simply act as a regulator guage, just show the outlet pressure like a regulator. Thank God for that!, my cheap single stage argon regulator only shows outlet flowrate in L/min and cylinder contents in bar/psi.

        Thank you for sharing your experience, greatly appreciated!

        All the best!


        Has been a long time since I use a TCD, but the one thing I remember was needing to keep the same gas on both sides and try to keep the flows on each side the same also. As long as those are the same then it shouldn't matter what the instrument is set to as that just calculates a flow from the head pressure reading.
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