Chromatography Forum

I have a Dionex/Thermo U3000 HPLC with a syringe autosampler. When the autosampler valve is in the Load position the autosampler is bypassed and the mobile phase goes directly to the column. In this mode, my pressure is about 190 bar. When the valve switches to the inject position the mobile phase passes through the sample loop and needle before going to the column. In this mode, the pressure drops to about 70 bar. This large pressure fluctuation is causing retention time drift.

Does anyone know what causes this and how I can fix it?

Thanks for looking.

Had similar problem, unfortunately propably 6-positon stator - rotor in your autosampler need exchanging. You should be prepard for some costs.

Kreall

And what is the back pressure before injection? Did you check if the injection system is leaking?

Thanks for the replies.

I am also suspicious of the autosampler valve rotor since that is the only component that the mobile phase is flowing through when the valve is in the load position.

The pressure reading is about 170 bar before injection (valve is in load position) but as soon as the valve switches to the inject position the pressure drops to about 70 bar. I have looked for leaks in the autosampler and couldn't find anything. I tightened all of the fittings anyway and that didn't change anything.

was wondering whether u found out what was wrong?

Inject position includes loop, needle and seat (3 areas that can can be plugged up) plus column. Bypass is column only (but with one groove on the rotor seal)
Inject position should be slightly higher with the added 3 components of the flow path.
Insert a viper fitting plug on the injector valve port OUT to TCC. set pump at 0.2 ml/min and manually toggle the valve and wait.
it should pressure out (max out your pressure limit parameter), if not there is a leak in the rotor seal or stator. The 3 grooves on the rotor seal can wear out and the damage is visible only with a microscope.