Zwitterionic polymeric monoliths for HILIC/RP mixed mode for CEC separation applications

Polymer-based monoliths with zwitterionic surface character were synthesized in capillary columns following a two-step approach to provide versatile electrochromatographic stationary phases exhibiting potentiality of both hydrophilic interaction and RP separation modes. UV-initiated free radical copolymerization of N-acryloxysuccinimide and ethylene dimethacrylate was performed using azobisisobutyronitrile as initiator and toluene as porogen. One of the originalities of this approach relies on the dual role of the N-acryloxysuccinimide monomer that is successively used during the preparation protocol to first covalently graft chromatographic selectors on the monolith surface via simple nucleophilic substitution reaction and then to generate negative charges through hydrolysis of remaining N-hydroxysuccinimide units. In this respect, the grafting of hexyldiamine affords potential cationic surface charges. It is shown that it is possible to tune, controlling the pH of the mobile phase, the intensity and direction of the generated EOF. Moreover, the nature of the interfacial interaction process responsible for the observed separations is well governed by the composition of the mobile phase. Polymer backbone hydrophilization is proposed as an efficient way to improve the HILIC behavior of poly(N-acryloxysuccinimide-co-ethylene dimethacrylate) based monolithic CEC columns together with the grafting of an alkyldiamine incorporating a shorter aliphatic segment.