Packed column supercritical fluid chromatography with carbon dioxide-polar modifiers : Influence of carbon dioxide density on retention

Abstract

The influence of the CO 2 density in mixed mobile phases containing methanol, acetonitrile or carbon tetrachloride was investigated by packed column supercritical fluid chromatography. With bare silica and polystyrene-divinylbenzene (PRP1), the retention of model solutes was found to decrease with increase in the CO 2, density by the same order of magnitude as with neat CO 2 on bare silica. This is thought to be the result of variation in the solvation of the solutes in the mobile phase whereas the solvation of the stationary phase is governed by the amount of modifier in the mobile phase. The same relationship as with pure CO 2 was also found between retention and temperature on bare silica but the maximum retention depended on the boiling point of the solute. Liquid and gas chromatographic like behaviour occurred with silica and PRP1, respectively. Examples of density gradients for improving the separation time and selectivity are given.