Properties of the retention time of ionizable analytes in reversed-phase liquid chromatography under organic modifier gradients in different eluent pHs

• Theoretical properties of the retention time of ionizable analytes in HPLC.
• The retention time increases almost linearly with gradient duration when pH is constant.
• The shape of the retention time vs. pH is sigmoid when gradient duration is constant.

This work provides a theoretical basis for an empirical model proposed in Fasoula et al. (2013) [2] for the retention of monoprotic acids/bases during linear organic modifier gradient runs performed with a fixed change of organic content but for different gradient durations and in different eluent pHs. Based on the analytical expressions derived in the above publication for the retention of monoprotic solutes under organic modifier gradients in different mobile phase pHs it was proved that the dependences of retention time of all monoprotic acids/bases upon each of two factors governed the gradient elution under consideration are always the following: practically linear upon gradient duration (for constant pH) and sigmoid upon pH (for constant gradient duration).