Could this situation be an interaction between stationary phase and excipient?
Currently running 6 formulations that contain combinations of HPMC-AS and PVP-VA64 (or Kollidon VA64). Using an Agilent Infinitylab Poroshell 120 Bonus RP (P/N 695968-901) column (4.6 x 100mm; 2.7um particle size). Noticed that the retention time of the NCE in my standards have shifted after I ran 6 formulations. Can the combination of HPMC-AS, and or Kollidon VA64 being deposited on the head of my HPLC column be causing the retention time of my active to shift? Note, I am not using a pre-column.