New Insights into the Effects of Tetrahydrofuran and Dioxane on the Selectivity of Ternary Eluent Compositions in RP-HPLC Systems

The present paper discusses the tendencies in the retention changes of a diverse set of organic model compounds on HPLC C18 stationary phases in aqueous ternary eluent mixtures containing tetrahydrofuran or 1,4-dioxane, and one short chain aliphatic alcohol (ethanol, n -propanol or isopropanol). The set of compounds consisted of steroid and non-steroid molecules with different hydrogen bond donor and/or acceptor abilities. Every eluent mixture contained 75 V/V% water and 25 V/V% organic solvent(s). The composition of the mixture was changed in 5 V/V% steps, starting with the binary alcohol–water mixtures and finishing with the binary ether–water mixtures. The results show clearly the dependency of retention times on the eluent composition, the size of the molecules, and the occurence of the hydrogen bond donor/acceptor groups. The isopropanol–tetrahydrofurane–water mixtures resulted in selective changes in the retention times of the compounds with acidic groups or with other non-acidic OH or NH protons in the neighborhood of electron-withdrawing groups. Every compound has shown elevated retention times in the isopropanol–dioxane–water, n -propanol–tetrahydrofuran–water, or n -propanol–dioxane–water mixtures. Clear trends could not be observed in the eluents with ethanol. The probable reason for the retention enhancement is the adsorption of the organic components of the mobile phase on the surface of the stationary phase. The different effects of the alcohols may originate from the interaction of their varying aliphatic alkyl chains with the C18 chains. This phenomenon may result in different availability of the C18 chains for dioxane, tetrahydrofuran and the model compounds.