Novel Data on the Effect of Tetrahydrofuran as an Organic Co-Modifier in RP-HPLC

The aim of the study was to characterise the unique chromatographic properties of tetrahydrofuran (THF) based on hydrogen-bonding interactions with weakly acidic compounds (p K a = 7.4–12.48) including steroids with phenolic hydroxyl groups, their substituted derivatives and heterocyclic amides having different polar functional groups (log P = 1.15–4.78). The results suggested that the organic modifier does not simply affect retention by changing the hydrophobicity of eluent, but rather specifically modifies the nature of the analyte–stationary phase interaction. In the water/isopropanol (IPA)/THF eluent mixture THF forms a hydrogen-bonded complex with the phenolic steroid compounds. The apparent formation of the THF–analyte complex depended on the proportion of components in the ternary mobile phase employed (from 70:30:0 to 70:0:30 v/v/v). The weakly acidic model compounds showed an increasing retention time with increasing THF concentration. This effect of THF was found to be a solvent-specific interaction, which was only observed in the presence of IPA. The systematic modification of the phase ratio of organic modifiers exerts a great influence on retention time and changes the separation processes over a wide range. In the case of other protic solvents (methanol, ethanol) we could not observe this selective chromatographic behaviour. From the point of view of chromatographic practice, the use of THF–IPA co-modifiers may increase the selectivity and provide excellent possibilities for separation of weakly acidic compounds including the large family of phenolic compounds.