Dual hydrophilic interaction-RP retention mechanism on polar columns: structural correlations and implementation for 2-D separations on a single column.

We investigated the retention of phenolic acid and flavone antioxidants on five polar columns in buffered aqueous ACN mobile phases. All columns show mixed retention mechanism: RP in highly aqueous mobile phases and normal phase (hydrophilic interaction LC, HILIC) in mobile phases with high concentration of ACN. The Silica Hydride and the ZIC HILIC sulfobetaine zwitterionic columns show rather limited retention in the RP mode. The Luna HILIC column shows higher retention in both the HILIC and the RP modes in comparison to the PEG and DIOL columns. We characterized the selectivity of various HILIC systems using linear solvation energy relationship model with molecular structure descriptors characterizing selective molecular size, dipole-dipole and proton-donor/proton-acceptor interactions and we investigated the effects of the mobile phase composition on the linear solvation energy relationship characteristics of the separation phase systems to select suitable conditions for orthogonal HILIC separations in combination with RP systems. Dual retention mechanism offers possibilities for using complementary selectivity in the HILIC and the RP modes for sequential 2-D separations of natural antioxidants on a single Luna HILIC column. Column equilibration time of 15 min between alternating RP and HILIC gradient runs is sufficient for reproducible results.