Temperature effects on selectivity using carbon dioxide-based mobile phases on silica-based packed columns near the mixture critical point

The temperature dependence on selectivity using modified carbon dioxide mobile phases with silica-based packed columns is found to be primarily related to the hydrogen bonding character of the analytes. Selectivity between hydrogen bond acceptor compounds is enhanced at temperatures above the mixture critical temperature. Selectivity for these compounds is further enhanced using hydrogen bond donating modifiers, such as alcohols. Selectivity between hydrogen bond donor/acceptor compounds is enhanced at lower temperatures, below the mixture critical temperature, regardless of the modifier used. Analysis of the enthalpic contributions to retention indicate that the main driving force behind these variations in temperature and modifier effects are the differences in solute heat of adsorption on the silica stationary phase.