The Dynamic balances in HPLC: An unconventional application of temperature to optimize ionizable solute separations

Column temperature (CT) is a critical parameter in reversed-phase high-performance liquid chromatography (RP-HPLC), traditionally leveraged to adjust retention times and reduce eluent viscosity. While its effects on analyte diffusion and mass transfer are well established, its potential role in modulating selectivity - especially through indirect pH shifts - remains underexplored. This study investigates the nuanced interplay between CT and selectivity in the context of ionizable compounds, with particular focus on those with pKa values near the working pH range of the mobile phase. We demonstrate that, beyond its classical uses, CT can serve as a powerful tool for fine-tuning chromatographic selectivity by exploiting the temperature dependence of the pH of the eluent. A mathematical model is presented to describe this temperature-pH-retention relationship. The model is validated through a case study involving structural isomers (ortho-, meta-, and para-substituted analogues) of the same compound, where controlled changes in CT led to a reversal in their retention order. This approach holds particular relevance in pharmaceutical impurity profiling, where early elution of impurities - prior to the peak of the active pharmaceutical ingredient (API) - can significantly enhance detectability due to improved resolution and signal-to-noise ratios. The findings highlight a strategic, yet often overlooked, opportunity to leverage CT not just for performance optimization, but as a selectivity-altering parameter in method development.