The adsorption of methanol on reversed phase stationary phases in supercritical fluid chromatography

The surface excess isotherms of methanol from carbon dioxide on reversed phase stationary phases under two different operational conditions - which can be considered subcritical and supercritical conditions depending on the molar fraction of CO₂ in methanol - were determined using the minor disturbance peak method. The shapes of the surface excess isotherms were very similar in subcritical and supercritical conditions for the same column. To verify the influence of the sample solvent on the separation efficiency, two solvents methanol and heptane were used as sample solvents for alkylbenzene samples for the separation on the studied columns with pure carbon dioxide mobile phase. The separation efficiency was determined by calculating the number of theoretical plates. On the embedded amide stationary phase with methanol as a sample solvent the efficiency has increased due to the displacement effect of methanol on the solutes which are retained less than methanol. Then the efficiency for the rest of solutes, which coincide with the elution of the methanol peak tail has decreased as a result of the tag-along effect. The surface adsorbent heterogeneity has been discussed; the bonded ligands on the stationary phase surface demonstrated adsorption a big amount of CO₂, while methanol could adsorb with small amount on the residual silanols on the surface of stationary phase and the embedded (amide) polar group in the bonded phase.