Enhancing supercritical fluid chromatographic efficiency: Predicting effects of small aqueous additives

Abstract

Supercritical fluid chromatography is becoming more prevalent, particularly in industry. This is due to the inexpensive, and more importantly, environmentally benign carbon dioxide that is used as the major component of the mobile phase. Water is minimally miscible with carbon dioxide at temperatures and pressures commonly used in SFC. However, the introduction of a polar alcohol modifier component increases the solubility of water in carbon dioxide. Previously, the addition of small amounts of water in the mobile phase was shown to provide significant gains in efficiency in chiral supercritical fluid chromatography, especially with polar stationary phases. In this work, we report the effect of the addition of small amounts of water on efficiency and retention factor with four different SFC stationary phases used for achiral analysis namely FructoShell-N (native cyclofructan-6), SilicaShell (bare silica), PoroShell 120 EC C18 (octadecyl silica) and Xselect C18 SB. This is the first reported use of FructoShell-N, a cyclofructan derivatized phase for SFC applications. We devised a predictive test to determine which analytes show an increase in efficiency using their known chemical constants (logKow, pKa, PSA and Hsum). We also use discriminant analysis to elucidate the most important analyte parameters that contribute to “water enhanced” efficiency gains.