Using an AUX EPC for helium pressure control at the inlet of a Markes Thermal Desorption unit

Hello everybody,

Our GC-MS has two setups:

- classic: GC injector - column - MSD,

- less classic: thermal desorption (TD) unit - transfer line until GC oven - ultimate union - column - MSD.

The less classic setup operates at constant helium pressure controlled though a manual valve and a gauge installed before the TD unit and after the gas filters. There is the need to swap columns between setups as they are different.

We have an unused AUX EPC installed (I think it was meant for a purged union for backflushing purpose) and I was wondering if I could connect one of the outlets of that EPC to the inlet of the TD unit to start controlling the pressure though the GC-MS software. This way I would have a better control of the method pressure and, most importantly, I could leave both columns in the oven no matter what setup I'm using (I would always control a small pressure of He in the unused column).

However, I have been told that it could not be that kind of EPC. It had to be a Split/Splitless or Multimode EPC.

Can someone explain what is the difference between those EPC's?

Thank you.


  • Hello - sorry for the late reply on this - am guessing you've probably gone ahead and ordered whichever EPC you chose. In short you could use an AUX EPC, or PCM but the SSL EPC (or MMI) is preferable as it:

    1) allows you to run different split flows whilst having the same retention times (the pressure supply adjusts to compensate for the increased flow)

    2) gives you a readout of "total flow actual" which is not only great for diagnostic purposes when looking for inevitable leaks, but can also be recorded as a signal so that you have a record of flow during injection - which is great for QA/QC when you run a batch of samples and wonder why you have an outlier - you can go in and check that the flow was stable and reproducible during injection.

    Best regards, Matt.

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