Hydrophilic basic compounds have no or very low interaction with the stationary phase by reverse phase mechanism. Instead, the positive charge of the molecule can be used as a means of retention in ion-exchange mode. The problem is that when the molecule has more than one strong basic group, the retention is too strong. For this situation, the weakest Primesep cation-exchange column, such as Primesep C
, should be used. This column has an embedded carboxilic acid with pKa around 3.5. At a pH above 4, the column has a strong cation-exchange properties. At a pH below 3, the cation-exchange properties of the Primesep C
column are significantly suppressed. The typical approach would include a pH gradient from 4.5 to 3.0 by using ammonium acetate – ammonium phosphate buffer systems.