Varian Cp-3800 Flow w/ Mass Spec

[GCguy226]

Hi all,
I have a Varian CP-3800/CP 2200 MS set up and I am having a bit of trouble with the flow rates. I calibrate the EFC and the Purge as directed by the manual. Split vent flow at the recommended 20mL/min, Purge vent flow at 4mL/min with a column head pressure of 10 PSI.
When the GC is on, to achieve the constant flow of 1.2mL/min as desired, the column pressure drops to -7.5 PSI. It will fluctuate from -7.5 PSI to around 0.1 PSI and the Column flow and total flow will also fluctuate.
Does anyone have any optimum setup conditions that I can follow so that this problem can be resolved?

*EDIT*
I changed the output pressure to Vacuum since it is attached to the Mass Spec. When I do this, the total flow fluctuates from around 56 mL/min to 8 mL/min. When the output pressure is at atm, these fluctuations are not present.

[James_Ball]

Hi all,
I have a Varian CP-3800/CP 2200 MS set up and I am having a bit of trouble with the flow rates. I calibrate the EFC and the Purge as directed by the manual. Split vent flow at the recommended 20mL/min, Purge vent flow at 4mL/min with a column head pressure of 10 PSI.
When the GC is on, to achieve the constant flow of 1.2mL/min as desired, the column pressure drops to -7.5 PSI. It will fluctuate from -7.5 PSI to around 0.1 PSI and the Column flow and total flow will also fluctuate.
Does anyone have any optimum setup conditions that I can follow so that this problem can be resolved?

*EDIT*
I changed the output pressure to Vacuum since it is attached to the Mass Spec. When I do this, the total flow fluctuates from around 56 mL/min to 8 mL/min. When the output pressure is at atm, these fluctuations are not present.

I'm not certain on the Varian but I imagine it is like most and calculates column flow using the head pressure and the outlet pressure of the column, it doesn't actually measure it since that would require a flow meter after the injection port. Turn on the vacuum setting so it can calculate it correctly. If you have it set to a constant head pressure then the column flow will drop quite a bit as the over ramps up in temperature, if there is a constant flow setting then it will ramp the head pressure to compensate for oven temperature and keep the flow into the MS constant.

What is the specs on the column? It seems that with such low pressure it must be a large internal diameter column. Is it using helium carrier?