Chromatography Forum

Our lab has a single HPLC instrument. Up to this point, all of our applications have been developed using reverse phase chemistry. I now have a need to run a normal phase USP method on the same instrument to support production of a new aquisition. The analysis must be performed according to the USP procedure.

The situation is that we will potentially be switching from aqueous mobile phase to 99:1 hexane:IPA as often as twice a week.

I've heard different things about switching back and forth from reverse to normal phase - mostly I've heard that it can be problematic.

Any thoughts?

thanks,

Mike

The crucial step will be flushing out all the water before you start your normal phase methods, since a tiny amount of water can make a huge difference (unlike reverse phase, where water is the weak solvent). Be sure you flush your entire system with IPA before you add any hexane. Likewise, flush well with IPA before going back to water.

Another recommendation I have seen is to use "half-saturated" organic solvents for normal phase so that you at least have some control on water contamination.

If you can, purchase another instrument now and dedicate it to the normal phase analysis.
You will find even a very small quantity of water is not good for normal phase retention.

WK