Adding a 2nd column to GC-MS

We have installed a second column on the back inlet and need help configuring the flows for both analytical columns and the transfer line. We prefer flow of 1 mL/min for each column, but only the back inlet will be used for now. Should we operate in constant flow or constant pressure? How should we adjust the PSD purge flow?

  • Operating like this puts sub-optimal flow into the GCMS ion source of more than 2 ml/min.  Please make sure that you have a system with a turbomolecular high vacuum pump and not a diffusion pump as 2 ml/min exceeds the maximum allowable flow into a diffusion pumped system.   The ion source will not function well at these flow rates and this is not a recommended way to do what you want to accomplish.

    But if you absolutely must give it a try, to combine flows there has to be some pressure/flow from the PSD so that the flow always goes forwards to the GCMS and can never back up into the PSD supply line. 

    As an example that won't work -- both columns could be set to constant flow and the PSD to constant pressure calculating the total flow into the MS through column 3 that is at least 10% or more higher than the total of both columns so that the PSD is providing flow.  So let's see what the Pressure Flow Calculator built into the software shows.  At 35° C oven to get 2.2 ml/min into the MS would mean the PSD would be set to 5.5 psi. 5.5 psi constant pressure at the PSD would give the two analytical columns from the injection ports a fixed exit pressure, so easier for their EPC modules to control against.

    As the oven temperature increases to elute your peaks this goes wrong, though. At 250° C oven and 5.5 psi PSD, the flow in column 3 would only be 0.904 ml/min - so the excess from the two columns going into the splitter would be going up the PSD supply line and out the vent. Part of the PSD supply line is outside of the oven and so there would be a great possibility that peaks would condense there and you would have ghost peaks in your runs forever after.


    You could try both columns from the inlet to the splitter at 1.0 ml/min constant flow and from the splitter to the MSD constant flow 2.2 ml/min.  Let's look at the splitter PSD pressure again.  The 2.2 ml/min into the MS at 35° C oven would still be the same as above, but at 250° C........

    The PSD supply would have to be controlled up to 16.867 PSI.  That would mean that both injection ports would have to be something like 22.809 psi at 250° C oven supplying towards that high pressure at the PSD.

    Again, it gets weird.  At 250° C oven, the inlets are both at 22.809 psi trying to drive 1.00 ml/min from both towards 16.867 psi at the splitter driving 2.2 ml/min into the mass spectrometer which is unhappy at that high a flow rate.

    So folks want to try to lower the flow into the MS by running one column at lower flow than the other column. Again, strange things can happen if the column dimensions are not calibrated and entered exactly and forward flow control at very low flows gets a bit strange, too.  So 1.00 ml/min on column 1, 0.50 ml/min on column 2 and Column 3 would have to be run at 10% more than the total of both Column 1 and Column 2, so at least 1.65 ml/min which is still higher than the 1.2 ml/min optimum into the ion source.  You would have to remember which one is low and which high, injection port dependent. You would either have to never cut the columns or remember to recalibrate the lengths every time you do any maintenance.

    It’s complicated. So much so that this is not recommended except for advanced users.


    Now the PSD purge flow is so that there is always forward flow, so the default of 3 ml/min works most of the time. If the PSD EPC does not come "ready" use more flow.  If the PSD pressure is low because of not enough restriction to the MS and the vacuum is really efficient it might require a higher purge flow so that the EPC pressure will stabilize and come ready.

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