Chromatography Forum

hi ,
please help i have a proplem with pump (Knauer , Germeny )
the pump don't create stable pressure (i.e within 5 min the pressure vary from zero then 3 then 22 then 7 then 2 then zero and so on )

The simplest cause would be air in the pump. Are you working with well degassed solutions and have purged the pump well? Sometimes to remove air you have to loosen the fitting from the outlet check valve and allow mobile phase to leak out.

My next guess would be a troublesome check valve, but at this point I'll hand over to the more experienced troubleshooters.

DoryFish, you nailed it. Air bubbles are *always* the prime suspect, followed by sticky check valves. If it's not air, removing the check valves and sonicating in methanol can clean up the sticky "film" that seems to develop on sapphire ball check valves used with acetonitrile. If that doesn't help, replacing the check valves is the easiest/best solution.

thanks all

If the recommendations don't fix the problem , please also consider the piston and the piston seal.
In some cases , those are also create pump pressure instability problems .

You may also see solvent leak under the pump heads if this is the case , but it may take some time since you have not a stable flow.

thanks alot i have remove safety valve and clean it my problem solved