Preparation of an imidazole-bonded silica gel stationary phase for supercritical fluid chromatography

The development of stationary phases capable of providing diverse interaction mechanisms is of critical importance for expanding the applicative scope of supercritical fluid chromatography (SFC). Although imidazole-functionalized materials have demonstrated considerable promise in high-performance liquid chromatography by virtue of their hydrogen bonding, π-π, and electrostatic interactions, their application and systematic evaluation in SFC remain largely unexplored. To address this gap, we herein report the synthesis, characterization, and chromatographic evaluation of a novel imidazole-bonded silica stationary phase (SiO₂-IM-2) for SFC. The successful immobilization of 2-methylimidazole onto silica was confirmed by Fourier transform infrared spectroscopy, nitrogen adsorption-desorption isotherms, and elemental mapping. The separation performance of the SiO₂-IM-2 packed column was systematically assessed using polycyclic aromatic hydrocarbons (PAHs), non-steroidal anti-inflammatory drugs (NSAIDs), anilines, and sulfonamides (SAs) as the probes. Under optimized conditions (10 % methanol modifier for NSAIDs, SAs, and anilines; 6 % methanol for PAHs; 308 K; 130 bar), the SiO₂-IM-2 column afforded rapid and baseline resolution for all four classes of compounds, achieving high column efficiency (up to 33,386 plates m^-1 for ketoprofen) and excellent resolution. Notably, the stationary phase exhibited superior performance relative to conventional C18 and unmodified silica columns. Mechanistic investigations indicated a synergistic multi-modal retention mechanism involving the imidazole ring as the principal interactive moiety during SFC separation. Overall, this study introduces the SiO₂-IM-2 phase as a robust and selective stationary phase for SFC, thereby expanding the portfolio of available separation materials for this technique.