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.


  • Hi

    Actually, AUX EPC, used where we want to give controlled flow direct to column.

    While other EPC is not giving direct flow to column.It's going to inlet like split/splitless/MMI/PP and after that flow going to column.

    Rest valves installed with EPCs are of different type to achieve different purpose/application.

  • Thanks for the answer but I still fail to understand.

    If helium pressure at the inlet of the TD is constant and only controlled manually, why can't it be controlled by the AUX EPC?

    Can't I just connect one of the outlet of the AUX EPC through a tubing block assembly (Agilent ref. G3430-60550) to the inlet of the TD and set the pressure to 6 psi for example?

  • Basically, it comes under design based on application.

    Aux EPC has one oulet and one inlet with each solenoid valve..Three valve are installed in Aux EPC.

    In inlet of Aux EPC gas line and outlet of AUX EPC connected with column under controlled flow n pressure..

    As you are saying to connect outlet of aux to TD, means inlet flow of aux will go to TD through outlet of aux thenafter what meachanism will be for column flow..

  • I might fail to understand how an AUX EPC works because I assume that, if I connect a column directly to an outlet of the AUX EPC and set it for 6 psi, I will get 6 psi of He at the beginning of the column at a flow fluctuating with the oven's temperature.

    When we put a TD in the equation, according to my idea, it could be basically between the outlet of the AUX EPC and the column. My current setup lets me apply manually 6 psi of He, at the inlet of the TD, that will be directed to the column through the TD and its transfer line (outlet of the TD). Then, when there is the need to desorb a tube to the column, the flow is redirected internally with switching valves.

    So, could I use an AUX EPC to set the carrier gas pressure instead of a manual gas regulator (like this one)?

    Thank you.

  • For details, see the 7890B Advanced Operation manual pages 27-31.  A few things to know.  You need 20 psi higher pressure in front of any pressure regulator. For example, if you need to control 50 psi downstream you need 70 psi or more before the controller (up to the 100psi max for EPC modules).  When an Aux EPC channel is used to control pressure for backflush, a bleed restrictor attached to a tee is used in the supply line to the Purged Ultimate Union. That bleed flow is necessary for decent pressure control. If there is nowhere for the excess pressure to go the pressure can build to match the supply. 

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