Deviation from van't Hoff dependence in RP-LC induced by tautomeric interconversion observed for four compounds.

The most encountered situations in reversed-phase liquid chromatography for temperature dependence of retention are those obeying the linear equation known as the van't Hoff plot. When studying compounds that are involved in structural modifications, it is likely that the temperature dependences of their retention factors do not follow this rule. It is the aim of this paper to report some particular cases when compounds involved in tautomeric interconversion have a different retention behavior with temperature: a deviation from the linearity of ln k on 1/T, or, in certain temperature ranges, temperature increase leading to a retention increase. Examples of compounds exhibiting deviation from the van't Hoff temperature dependence are piroxicam, drotaverine, vincamine, and epivincamine. A simple thermodynamic model relying on tautomeric equilibria in mobile phase is proposed for these compounds, which gives a polynomial dependence between ln k and 1/T.