Still confused about Agilent 6850 flows and pressures

[Roxanne42]

So after getting input from here, I set one of my GC's to a constant column flow of 1.5 mL/min (constant flow), with the "Purge Flow to split vent" = 0 mL/min and the GasSaver option OFF - thus yielding a total flow of 4.4 mL/min (splitless mode). The oven, electrometer, nitrogen, air and hydrogen are all set to OFF (this is my standby mode).

The problem came when I tried to apply this same method to my second GC (also an Agilent 6850). The inlet pressure does not maintain the setpoint - it keeps going down steadily, eventually causing the GC to shutdown. This GC does have a different column installed and I thought that perhaps the flow was too low for the column in use. But when I raised the column flow to a higher value, the setpoint could still not be maintained. The only ways that I found I could keep a steady inlet pressure was by either turning on the GasSaver feature (minimum of 15 mL/min - thus bringing the total flow to 19.4 mL/min) or by setting the "Purge Flow to split vent 2.00 @ min" to at least 1.0 mL/min (giving a total flow of 5.4 mL/min when column flow = 1.5 mL/min).

Furthermore, I found that after a week, the first GC started the same thing and could not maintain the inlet pressure and I thus set the Purge Flow to split vent 2.00 @ min" to 0.5 mL/min (giving a total flow of 4.8 mL/min when column flow = 1.5 mL/min).


Can anyone explain this to me...?


Thanks!

Roxanne.

[Russ]

Assuming you don't have a leak in the inlet of the second GC, the problem could be with the column you have installed. You did not give column parameters (length, diameter, etc.) which would be helpful but short larger bore columns (15 m x 0.53 mm for example) do not develop enough pressure at low flows for the controller to work properly. At least not on a 6890.

[Peter Apps]

Hi Roxanne

You almost certainly have a leak through the septum, since there is no "spare" flow this easily causes the inlet pressure to drop.

Check with a leak seeker (no GC lab should be without one !)

Peter