Benzalkonium Chloride and cyano columns

[mikea]

I'm wondering if anyone else has analyzed BAC using a cyano column, and if so, have you experienced small changes in retention times of the 4 analogs? I seem to recall something in the literature that -CN columns are notorius for rt drift. We are using a Supelcosil LC-CN 150x4.6, mobile phase 40:60v/v 30mM sodium phosphate, pH 7:ACN. The method is isocratic and we get great peak shape and resolution of all 4 analogs, but over the course of 70-100 injections of the same solution, all 4 analogs seem to increase in retention time by ~1.5 minutes.

Any insight would be greatly appreciated.

[Uwe Neue]

The surface bonding of CN columns is generally rather unstable - orders of magnitudes worse than a good C18. There are ways to improve this, but this fundamental instability has caused sufficient concern for me to not recommend CN columns at all.

The increase in retention is likely to be due to an increase in the silanol population. It is possible that you could do the same assay with a silica column - without retention drift.