decreasing retention time

[Kiesels]

I am trying to develope an HPLC end- analysis method for the dissolution of solifenacin succinate. I have a decent peak response resolved from baseline intereferences, however over the course of the analysis the retention time of the component of interest decreases steadily (0.3 min over 25 injections). I am using a phenyl-hexyl column, temperature controlled at 25C. My injection volume is currently 50 uL. My mobile phase is 70/30 Water:ACN with 0.1 % phosphoric acid. Am I saturating my column with the compound or some component that does not elute and is competing for active sites? Any suggestions would be much appreciated.

[HW Mueller]

Equilibration problem? Too acidic?

[Chris Pohl]

Kiesels,

While there can be several possible causes of this problem, the most common would be a sample component not eluted with your mobile phase. A possible solution would be to add a cleanup at the end of each sample run where you step to a higher solvent concentration to elute components which are strongly retained. One way to test for this is to determine whether or not the change in retention time is correlated to the injection frequency. Make 4-5 injections in close time proximity and calculate the rate of retention time loss per minute, then wait a couple of hours and make another injection. If the rate of change is unaffected by the gap in the injection sequence, the problem is probably an eluent component. If the rate of the retention time loss became less when you waited a couple of hours between injections, the problem is most likely sample related.

[Kiesels]

I recently ran an analysis which had a total of 71 injections. Over time the magnitude of the retention shift decreases, which leads me to believe that there is a point of equillibration. Up until now, I have been flushing the column with acetonitrile after every run, but I think now I will try not flushing it and see if my retention shift eventually dissipates.

Thanks for your suggestion Chris, I will also try your idea if time permits me.