Investigations on the chromatographic behaviour of zwitterionic stationary phases used in hydrophilic interaction chromatography

Two commercial stationary phases possessing a sulfobetaine zwitterionic bonded ligand (ZIC-HILIC and Nucleodur HILIC) were compared under hydrophilic interaction chromatographic (HILIC) conditions. First of all, the separation of 12 model compounds chosen among neurotransmitters and presenting a diversity of ionization states (anionic, cationic and zwitterionic) was studied under varied operating conditions. The effects of the percentage of acetonitrile, ammonium acetate concentration and temperature of the mobile phase were compared on the two columns. Secondly, a generally applicable retention model was established, based on chromatographic retention data (log k) acquired for 76 model compounds. The chosen compounds are small molecules presenting a wide diversity of molecular structures and are relevant to biomedical and pharmaceutical studies. To account for their retention behaviour, a modified version of the solvation parameter model was designed: two additional molecular descriptors were introduced, to account for ionic interactions with anionic and cationic species. The retention equations obtained allow a rationalization of the interactions contributing to retention and separation in the HILIC systems considered.