LC retention time/pressure instability

[sepscientologist]

Hi All,
I'm running an Agilent 1100 with a quaternary pump running 100% Dichloromethane through a normal phase silica column
Agilent RX-sil 4.6 x 100 x 1.8 um at 40C. At 1 mL/min I see about 95 bar of pressure drop and retention time of about 3.5 min
and pressure ripple of about 1 bar. Sometimes the pressure will drop lower and I'll see much more pressure fluctuation
along with increased retention times.

It seems like it might be a check valve? Any comments out there? I don't have a lot of normal phase stick time.

[bisnettrj2]

I assume you're running a degasser, and if so aren't seeing a bubble leaving the degasser and making its way to the active inlet valve? Also, if you're only running isocratic, you still need to have all the other channels primed (isopropanol would work), or else bypass the quaternary low-pressure mixing area (can't remember which chromforum user suggested the priming of all channels in the quaternary pump, but they seemed adamant about it, so it can't hurt to do it).

[sepscientologist]

I tried lots of different things but what eventually worked for me was moving away from DCM. For some reason I can't pump it without
having pressure dropouts. I'm running 80:20 hexane/IPA now and performance is solid.

[HPLCaddict]

I've had all sorts of bad experiences in the past with pumping pure organic solvents. Even pure acetonitrile might be a problem with some machines. Due to the low viscosity some check valves don't work properly. Maybe it's been the same reason in your case. I assume the IPA in your eluents raises the viscosity of the hexane considerably, so the check valves now work flawlessly.

[uzman]

If you are using reverse phase piston seals , they are not very effective in normal phase solvents.
When you switch to normal phase solvents , you have to change the piston seals also ( ıf you face problems with normal phase solvents )