Unusual temperature-retention dependences observed for several benzodiazepines in RP-HPLC using different mobile phase composition

Non-linear van’t Hoff plots were observed for several benzodiazepines over the usual temperature interval
used in RP-LC (10–60 C), when acetonitrile was added in the mobile phase. Such deviation from linearity
was not observed when methanol or isopropanol was added to the mobile phase. Polynomial regressions were applied to fit the experimental data when acetonitrile was used as organic additive in mobile phase. The second-order polynoms may allow finding out the maximum retention depending on temperature, which can be within or out of the normal temperature range used in RP mechanism. A thermodynamic model deriving from the partition of two or more tautomers of the same analyte was proposed to explain this deviation from linearity of van’t Hoff plots.