Retention studies on mixed-mode columns in high-performance liquid chromatography

The retention properties of a column prepared by mixing together strong cation exchange (SCX) and reversed-phase (C18) packing materials were investigated using a range of test solutes. The column was found to exhibit chromatographic properties characteristic of both phases. The effects of changes in eluent composition, buffer ion, ionic strength and pH on the capacity factors of different compounds were determined. The dual nature of the retention mechanism allowed the retention of ionisable molecules to be adjusted by altering the composition of the aqueous component of the mobile phase while those of compounds uncharged over the pH range investigated remained unaffected. Results were compared those obtained on a C18 column and it was found that the acidic and weakly basic compounds had higher capacity factors on this column whereas strongly basic compounds had higher capacity factors on the mixed-mode column.