Most reversed-phase HPLC columns, such as C18, are incompatible with 100% aqueous mobile phases due to dewetting (collapse) of the stationary phase. Primesep columns combine reversed-phase with polar functional groups which eliminate the problems of dewetting. The Primesep 100 column in this application with an acetonitrile (MeCN, ACN) shows a retention time of 2.5 minutes. In 100% water, the retention time is 3 minutes even after sitting 24 hours in the water mobile phase. On a C18 phase, the analyte retention would have been lost and elution at the void (1.5 minutes) would have resulted.