Chromatography Forum

Hi,

I try in this moment a method for phospholipids analysis but I'have some problems with the eluent (1%THF in 2,2,4-trimethylpentane).

In fact when the eluent flow, the pump pressure is'nt stable (from 0 to 20bar) like if air is in the system. However I have degassing a long time with ultrasound and helium but I've always this problem!

What can be the cause?

With another eluent (water,acetonitril) I d'ont have this problem...

Thx.

I'm not sure why it would only happen with the isooctane, but it could be a stuck check valve. Try removing the check valves and sonicate them is isopropanol and then water. Just make sure you don't turn them upside down so that the inside part doesn't fall out.

I would suspect front end filters ( solvent reservoir, low pressure solvent mixing valve - even though you're not using the mixing feature, the solvent may pass through a filter ).

If they are OK, and the check valves are OK - cleaned as suggested by sassman, then it's probably that your pump plunger seals need replacing. Some instruments ( eg Agilent ) use different seal materials for normal phase and reverse phase. Check tthe pump instructions.

You could also try lifting the solvent reservoir up as high as possible ( about 1m above pump inlets would be nice ) - so the hydraulic head is greater. If you believe it's the solvent, you could try running just with iso-octane to see if pressure is constant ( just inject blanks ) , or replacing the iso-octane with n-Heptane ( similar boiling point ) and running that with the THF ( ensure THF is fairly dry ).

Please keep having fun,

Bruce Hamilton