Unusual effect of flow rate on retention in analytical supercritical fluid chromatography exemplified by polyethylene glycol separation.

We report a case of a peculiar effect of flow rate on retention in a separation of polyethylene glycol oligomers via supercritical fluid chromatography. During method development, we tested flow rate gradients and notices that for some PEG oligomers retention times at flow rate gradient were lower than at constant flow with the largest flow rate value used in a gradient. For instance, at BEH stationary phase and CO-MeOH gradient from 10 to 35% at 20 min a PEG oligomer having mass of 1225 Da has a retention time 14 min at 1 mL/min flow rate, 10.3 at 2 mL/min and 9.5 min at 1-2 mL/min flow rate gradient. The effect is not unified for all PEG oligomers, it occurs only starting from a particular PEG molecular weight which depends on the stationary phase type and/or mobile phase conditions. We believe that such an unusual flow rate effects can happen in SFC on various occasions, not exclusively for flow rate gradients, and thus should be taken into account during method development or method transfer.