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oscillation on splitter plate (CFT)

nbohni

We were having many issues the last months with our GC-7200 qtof system. Many have been resolved, but we are left with tailing on later eluting peaks (phthalates, starting at about 250 °C) and oscillations coming from the splitter plate. 

Our system is as follows:

KAS-inlet (Gerstel), 15m column - backflush - 15m column - splitter plate - FID/qtof

The tailing is only on the MS side, and not on the FID. Oscillations are on both channels (and on the actual pressure reading of the corresponding EPC).

We extensively checked for leaks (with a duster), so splitter plate is good. I checked the inertness of the inlet side (including backflush) with the GC degradation mix from Restek (I know that it shows me leaks up to the backflush), so inlet/backflush is fine too. 

Also, analytical columns are in few months only and the system has hardly seen any samples. Transfer line has been exchanged again last week and tailing stayed. (Again, resolution is perfect on the FID!) MS source has been cleaned several times and hardly any samples have been on it since. 

Any ideas what the oscillations on the splitter plate are coming from and how to resolve it?

Any ideas on the tailing of later eluting compounds? (Could it possibly be the transfer line that's not heating evenly? How to check for this?)

  • 0 in reply to paulsalverda

    The oscillations are also in pressure. 

    Oscillations are on FID, TIC and Pressure Actual of EPC4. 

    I have removed the plastic cap from the bleed restrictor. (It has been on since initial installation, 8 years ago.) It's the backflush (EPC6). 

    Both backflush and splitter plate are supplied by EPC channels. (according to that the control panel calls the channels EPC4 and EPC6, they would be called differently if they were PCMs, no?)

    Column 1 is 1.2 mL/min and column 2 1.4 mL/min. I have calculated the split ratio with the provided Excel form: splitter3calc-rev1 (initially for another splitter plate installed on the instrument without makeup flow, we then added the splitter plate with makeup flow but kept the dimensions and it has worked 'til last autumn)

  • 0 in reply to nbohni

    The scales are so small - 10^-2 and 10^-3. Are you sure that this is an issue and not that the signals are all so small that you're seeing it zoomed in?  I've never plotted EPC control down at that level to check.  The 7200 data files I have show wavy baselines at that scale.    Can you compare it with data files from earlier?

    Here are two FID signals with the only baseline sections I could get onscale at x10^-2...and you are showing FID at 10^-3.   This is using MassHunter Qualitative analysis Version 10.0. What software version are you using?

     

    All three signals show the oscillations which is a great clue.  What is the supply pressure of helium?  It has to be >20psi higher than any possible setpoint at the back of the instrument after any traps.  If the gas traps are older then the supply might need to be higher.  That's at least easy and quick to check. I set the helium to 80psi on the tank gauge as there's usually a mark there.

    After that I would guess that there's a tiny possibility that there's an EPC issue. Doubtful, but possible. We're getting close to where a visit by a Field Service Engineer might be faster.

  • 0 in reply to paulsalverda

    We could resolve the oscillations! The gas was wrongly configured and setting it back, without changing anything other than that, did result in a much smoother pressure line and oscillations are no longer seen on the FID and qtof trace. (I don't know how this misconfiguration had happened, but I'm happy that the issue has been resolved.)

    pressure actual from EPC4 (purged 2way splitter), above: N2 set as gas, below: He set as gas (we use Helium)

    here are the FID and MS traces of these same samples. 

    The tailing problem on the MS is currently taken care of from our Swiss service technicians. 

  • 0 in reply to nbohni

    The scale of your chromatograms is incredibly low, surprisingly low.  Most would never have seen these tiny perturbations.  Glad it's resolved.

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