Influence of temperature and mobile phase composition on retention properties of oligomeric bonded phases in reversed-phase liquid chromatography (RPLC)

The effect of temperature and mobile phase composition (methanol-water) on the retention behaviour of an oligomeric series of n-octylsilyl bonded phases in reversed-phase liquid chromatography has been investigated. Plots of lnk against 1/T (van't Hoff plot) and the enthalpy of transfer (ΔHo) yields linear relationships under the conditions studied. The ΔHo values of the aromatic hydrocarbons and n-alkyl benzoates are higher than those of the polar compounds due to their higher level of interaction with the stationary phase. A linear plot of ΔHo vs. ΔSo suggest that the retention process, which is essentially controlled by non-specific (dispersive) interactions between the solutes and the bonded ligands, is identical for all cases evaluated. The existence of similar retention mechanisms is confirmed by the constant value of the enthalpy-entropy compensation temperature of the columns for a given class of componds. As expected, decreasing the methanol content (% v/v) of the mobile phase results in increased eluite retention times. The methylene and phenyl selectivities are found to be independent of the carbon content of the stationary phases and varied only with the eluent composition. In addition to their high stability under aggressive mobile phase conditions as previously reported, the results of this study generally showed that the solute retention process on oligomeric phases are similar to those exhibited by the conventional reversed phases.