u/krisphers

Hi all,

I’m troubleshooting an Agilent 7890B GC setup for CO2 reduction product analysis and I’m unsure how tight the microneedle/resistor valve that bypasses my molsieve column should be.

For reference, the current configuration is:

injector to PLOTQ to molsieve to TCD/FID

when the valve actuates, the flow is restricted to the PLOTQ and the molsieve is effectively cut off

I am using the PLOT/Q for CO2/large hydrocarbons and the molsieve for permanent gases. I understand that the idea is to balance the pressure and the flow when the molsieve is cut off by tightening the needle valve, but I don’t really know what “correct” looks like in practice. When I've actuated the valve that cuts off the molsieve, there seems to be no difference in the column pressures, flow rates, and velocities no matter how tight or loose the needle valve is (I have seen a slight drift in the baseline).

I was wondering if people had any experience with this. How tight do you normally tighten these resistor needle valves and are there any signs in the TCD/FID signal that I can use to tell if the valve is too tight vs too open?

Thanks!