Preparation of ionic liquid stationary phase copolymerized with anion and cation for mixed-mode liquid chromatography

Ionic liquids (ILs) are a promising material to develop stationary phases for mixed-mode chromatography (MMC). In order to investigate the effect of anion type and polymerization method of ionic liquid, this work prepared and compared two stationary phases (Sil-IM-Br and Sil-IM-SAG) based on the polymeric ionic liquid, one of which copolymerized cation and anion. Two stationary phases were characterized and provided satisfactory retention repeatability (RSD < 0.3 %) and column efficiency (84600 plates/m). Chromatographic conditions (ACN content, salt concentration, and pH) were varied to reveal the retention characters and interactions between stationary phases and solutes. The results showed that both two developed stationary phases can operate under the mixed mode of RPLC/HILIC/IEC, but diverse selectivity was provided. So, the retention mechanisms were studied by linear solvation energy relationship equation (LSER) and thermodynamic research. Two stationary phases showed totally different retention towards carboxylic acids, which resulted from the effect of anion bonded. Lastly, the developed stationary phases were applied to separate alkylbenzenes and polycyclic aromatic hydrocarbons (PAHs) under RPLC. Furthermore, 7 carboxylic acids and 7 nucleosides and bases and complex samples were successfully separated under HILIC mode, Sil-IM-Br and Sil-IM-SAG provide better performances and different selectivity when compared with commercial mixed-mode columns. This work proves that cationic and anionic copolymerization provides a new method for stationary phase design, and better selectivity can be obtained by changing the anions.