Selectivity vs. polarity: The fundamentals of chromatographic separation

Abstract

Stationary phases for capillary gas chromatography are most often classified as non-polar, low, medium, or high polarity. Some stationary phase have been described as appearing “more polar” than another phase by virtue of increased retention of a particular solute type (e.g., aromatic, olefin, alcohol), and less polar with respect to another. This concept, however, may be too simplistic to explain some observed chromatographic behavior. The concept of selectivity, which also incorporates specific interactions between the analytes and the stationary phase, offers a better way of understanding these retention phenomena. Examples presented here highlight a stationary phase that is “less polar” than 100% dimethylpolysiloxane with respect to polar analytes, such as alcohols, but “more polar” with respect to aromatic analytes, and two stationary phases with the same apparent “polarity”, but different chromatographic behavior. These phenomena are discussed in terms of stationary phase composition, and the specific solute-stationary phase interactions that result in their unique selectivity.