Evaluation of an amide-based stationary phase for supercritical fluid chromatography

A relatively new stationary phase containing a polar group embedded in a hydrophobic backbone (i.e., ACE®C18-amide) was evaluated for use in supercritical fluid chromatography. The amide-based column was compared with columns packed with bare silica, C₁₈ silica, and a terminal-amide silica phase. The system was held at supercritical pressure and temperature with a mobile phase composition of CO₂ and methanol as cosolvent. The linear solvation energy relationship model was used to evaluate the behavior of these stationary phases, relating the retention factor of selected probes to specific chromatographic interactions. A five-component test mixture, consisting of a group of drug-like molecules was separated isocratically. The results show that the C₁₈-amide stationary phase provided a combination of interactions contributing to the retention of the probe compounds. The hydrophobic interactions are favorable; however, the electron donating ability of the embedded amide group shows a large positive interaction. Under the chromatographic conditions used, the C₁₈-amide column was able to provide baseline resolution of all the drug-like probe compounds in a text mixture, while the other columns tested did not.