Decreasing retention time every shot of a calibrator check standard. Over a run of about 20 samples, 21 minutes each with a gradient of water/ACN, a calibrator check sample at the end of the run will have decreased retention times of the components compared to that same standard at the beginning of the run. This is causing the compounds to not be quantitated properly for one thing. Everything is basically new on the system. What gives?
The column is a Hypersil ODS, 4x125mm, 5 µm, with a guard column in front. Gradient runs from 65/35 water/ACN to 40/60 water/ACN at the end. Flow rate is 1.5 mL/min, column temp 30C.