oscillation on splitter plate (CFT)

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?)

  • Hi, is this the purged 2 way splitter? If so, based on the image below the analytical column coming from the purged union would be in the Column position. Is Detector 1 going to the FID or the QTOF? What are the lengths and dimensions of the restrictors?



    Alex Graettinger

  • Oscillations - like what?  Pressure reading?  Retention times?  Vacuum readings?   What units and how much?    Is it a purged splitter plate?  If so - - is the purge supplied by an Auxiliary EPC module with the required bleed restrictor mounted on top of the oven ?

    Tailing can be cold spots, too, especially for higher boiling point compounds. Is the GC pushed up tight to the MS transferline so that there is little to no gap ?   What is your transferline temperature?  Removable Ion Source temperature?

  • yes, it's the purged 2-way splitter. Bottom is column entry, then detector 1 is FID, detector 2 is qtof. (switching FID and qtof position doesn't change anything)

    We had 3.7mx0.15mm to qtof and 1mx0.15mm to FID with constant flow 0.8 ml/min for a 1:1 last year. Changed for more like 2:1 to get more on the QTOF (1.9mx0.15mm to qtof and 1mx0.15mm to FID with constant flow 1.1 ml/min). Split ratio was fine, but with the problems as described above, we have switched back to the configuration from last year, with no improvement (oscillations and tailing). 

  • Oscillations in baseline (TIC) and in pressure reading.

    Blue is FID, red is MS. I recorded the pressure actual from inlet, backflush (EPC6) and splitter plate (EPC4). The pressure starts oscillating after 1 minutes after temperature increase from starting temp. (60 °C). The oscillations in pressure are 0.15 10^-2 of whatever unit it is recorded in. (my reading starts at 1 10^-5, but on the GC it says 4.4 psi. my reading at the end is 4.18 10^-2)

    The splitter plate was mounted by the service technician. I don't see a restriction on top of the oven. There is a connector piece (union) and the tubing disappears towards the EPC. (I cannot check as the Gerstel autosampler is mounted on top.)

    The GC is pushed as tight as possible to the qtof. There are two spacer bars mounted at the qtof and the GC is tight with the spacer bars. This leaves the GC with 1 cm space from the QTOF. 

    Transferline is 300 °C, source temperature is 230 °C. Both temperatures are like this since ever. 

  • Vacuum reading of quad vac is stable at 3.26e-05 Torr, TOF Vac is not super stable, probably since the new turbo pumps (replace 2021 and 2022), 2.20e-07 to 2.4e-07. (not sure about the stability of the TOF vac, might have been the same before, but we don't check when everything is fine...)

  • Vacuum gauges and pressure readings are not always stable even when the vacuum or pressure is stable.  The vacuum gauge is reading very, very few ions of random gas molecules that bounce into the gauge, get ionized by electrons from a hot filament, and are then captured.  The TOF vacuum reading tends to be slightly less stable appearing than the Quad vacuum reading as there are so few molecules in there that the gauge has a more difficult time.

    Are the backflush Purged Ultimate Union and the Splitter plate supplied by EPC channels -- a channel off of a three channel Aux EPC module or the channel off of the collision cell EPC module -- or are they supplied by a PCM - a pressure control module?  If it is supplied by an EPC channel, there should be a Tee and a bleed restrictor mounted in place of that 1/16" union for both the PUU and the Splitter.   The EPC module must have enough flow going through it to control the pressure properly and bleeding off some is the only way.  A PCM supplied channel does not need that bleed restrictor.   Please contact Agilent about that.  

    I looked again. That coil of tubing top right is a bleed restrictor, so you have the right setup on at least that channel.  What is that at the red arrow? Make sure it's not capped off. It looks like a piece of tubing to make it easier to connect a flow meter. It must always be open.

    I just read from the top again. What are your column 1 and column 2 flows?  before and after the PUU backflush device?    Column 2 must be at least 0.2 or more ml/min higher than column 1.   How are you calculating the split ratio between the two devices?  How did you choose the lengths of the restrictors?

    The oscillations in your chromatograms above are not pressure, but signal. Please uncheck the FID signal so that only the MS TIC Scan signal is shown, then zoom in on the area of oscillation and share that chromatogram.  --- or to speed this up we need to figure out how I can look at your actual data file.

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