Zwitterionic polymer-terminated porous silica stationary phases for highly selective separation in hydrophilic interaction chromatography

We described two novel zwitterionic polymer-terminated porous silica stationary phases containing the same pyridinium cation and anions of different side chains (carboxylate and phosphonate groups) for use in hydrophilic interaction liquid chromatography (HILIC). These two novel columns were prepared by polymerizing 4-vinylpyridine and grafting it onto a silica surface, followed by quaternization reaction with 3-bromopropionic acid (Sil-VPC24) and (3-bromopropyl) phosphonic acid (Sil-VPP24), which possess positively charged pyridinium groups, and negatively charged carboxylate and phosphonate groups, respectively. The products obtained were verified through relevant characterization techniques such as elemental analysis, Fourier-transform infrared spectroscopy, thermogravimetric analysis, Zeta potential analysis, and Brunauer–Emmett–Teller analysis. The retention properties and mechanisms of different types of compounds (neutral, cationic, and anionic) on the two zwitterionic-modified silica stationary phases were studied by varying the buffer salt concentration and pH of the eluent. The separation of phenol and aromatic acids, disubstituted benzene isomers, sulfonamide drugs, as well as nucleosides/nucleobases were investigated on the two packed novel columns and a commercial zwitterionic column in identical HILIC mode, ensuring a thorough comparison between both novel columns and with a commercial standard. The results illustrated that various compounds could be separated up to various efficiencies based on the mechanism of hydrophilic interaction-based retention between the solutes and the two zwitterionic polymer stationary phases. The Sil-VPP24 column demonstrated the best separation performance out of the three, as well as flexible selectivity and excellent resolution. Both novel columns exhibited excellent stability and chromatographic repeatability for the separation of seven nucleosides and bases.