Investigation of different counterions and molecular structure effect on the retention behavior of protonated basic solutes in chaotropic chromatography

Abstract This study explores the influence of different counterions (K + , Na + , NH 4 + ) on the retention behavior of protonated basic solutes in chaotropic chromatography with hexafluorophosphate (PF 6 − ) in the mobile phase. The effects were analyzed at varying PF 6 − concentrations and pH values, with experiments conducted at constant ionic strength using corresponding chloride salts (NaCl, KCl, NH 4 Cl) for consistency. Results show that counterions significantly impact the surface potential (Ψ 0 ), affecting retention and elution sequences, enabling optimized separation. An empirical retention model, considering analyte structure and mobile phase composition, was applied. For solutes existing entirely as monocations, retention is primarily influenced by interactions with the electrical double layer on the stationary phase, regardless of ionic strength or pH. However, when analytes exhibit varying ionization states, both electrical double layer interactions and ion pairing with the chaotropic anion affect retention. This study underscores the potential of chaotropic chromatography for versatile separations of protonated basic analytes through careful selection of counterions and chaotropic anions. These findings enhance understanding of retention mechanisms, optimize chromatographic performance, and support the development of advanced analytical methods.