Chromatography Forum

I've been having problems with our Shimadzu UFLC pumps. The flow rate is below the set value ONLY when both pumps are running and ONLY when there is a column attached. If I set it at 100% A with or without a column, the flow is correct. If I set it at 100 % B with or without a column, the flow is correct. Without a column, the flow is fine no matter the A/B composition. I have sonicated the check valves and ran a ton of water, MeOH, and isopropanol through the system. I have tried it with multiple columns and all give the same result. The flow rate is ~80% of what it should be. I don't really understand how the column can decrease the flow rate...unless the increased back pressure is doing something...?

I would appreciate any suggestions!!!

Compressibility compensation not properly set? Bad pressure transducer? Leaking seals or inlet check valve?

If this were my problem I'd verify that the pressure tranducer is working (compare to a physical pressure gauge) then do a leak-down test on the pump.

in additon to Tom's suggestions.

what about the compositional accuracy?
Did you run a gradient step test with one solvent having a UV absorbing compound in it e.g. Aceton 0.1%?
see http://www.lcresources.com/resources/TSWiz/hs450.htm

Test it with a restrictor capillary to get some backpressure.

To eliminate volume contraction conduct your experiment with the same liquid in bottle A and B. Simply insert tubing from bottle B to A.

Simply , plug the outlet of the column and pump the mobile phase at 0.1 ml /min.

Stop the pump around 300 bar and wait for 2-3 minutes.

If the pressure drops more than 10% , it will give you an indication that there is a leak somewhere.