GC-MS pressure problems,

We have a GC7890A and an MS 5975C in our lab and the column installed in the system is ASTMD6584.
after the installation of this column we encounter pressure problems, for example we put 3 ml/min but the system only gives 2.8 ml/min. and the system still gives a low pressure signal. we changed the flow to 2 ml/min the system gave 1.8 ml/min
the system always gives a low flow compared to that programmed.

  • What are the column dimensions?   Is it a Split/Splitless or MultiMode Inlet?  Is the column configured properly, from inlet to Mass Spec (vacuum) ?   What is the total flow setting? The total flow must be high enough to supply the column, septum purge, and split vent flow. It should be set at 20 ml/min or higher as control is difficult below that.  What is the inlet mode? Split or Splitless or Pulsed Split or Pulsed Splitless?  What splitless or purge timing? What liner is installed?

    What is the gas supply pressure?  It must be greater than 20 psi HIGHER than the maximum inlet pressure that will be needed. Typically 60-80 psi.

    What is the inlet pressure setpoint?  Effective control of low pressures is very difficult.  It is recommended to install columns and use flows that need 5psi or higher inlet pressure.

    3 ml/min is too high a flow into a 5975C GCMS.  The ion source functions best between 0.7 and 1.5 ml/min of helium and 0.4 and 1.1 ml/min if using hydrogen carrier.  

    If the MS uses a diffusion high vacuum pump, 3 ml/min can cause backstreaming of the diffusion pump fluid.

  • -the column is configured properly and the column dimensions are: 15-0,320-0,10.
    -it's a split mode and the total flow is 14 mL/min
    -septum purge flow: 3mL/min.
    -the gas supply pressure : 8 bar

  • Using the Agilent Pressure-Flow calculator, it would take 122°C initial oven temperature required to get above 5psi inlet, the recommended minimum for good column flow control.  That is too wide a bore and too short a column to use effectively with the vacuum of the MS pulling on the end of the column.

    The total flow is too low at 14 ml/min. 20ml/min minimum to get good control. If the inlet is 3, the column is 3, there's only 8 ml/min out the split vent for the system to use for inlet pressure control. It's just not enough gas flow. Turn the split vent flow up to 30 or 40 ml/min and your column flow issue will go away.  The MS source still won't be working properly at that high a flow rate, though.

    If you need that low a split ratio 3:8 -- 3 ml/min column and 8 ml/min split -- you should run splitless as the split ratio at those flows and very low inlet pressure is really variable. 

    You should change to a 15m, 0.18mmid column or a 30m x 0.21mmid or 0.25mmid for use with a Mass Spectrometer.

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