Retention Mechanism for Ion-Pair Chromatography with Chaotropic Reagents. From Ion-Pair Chromatography toward a Unified Salt Chromatography.

Abstract

The breakthrough of chaotropic mobile phase modifiers in reversed-phase high-performance liquid chromatography (RP-HPLC) is due to their strong potential to provide adequate retention of ionic analytes without the blamed semipermanent modification of the chromatographic packing often connected to the use of classical ion-pair reagents. The lack of a physicochemical framework that is able to unify eclectic experimental evidence concerning the use of a wide gamut of ionic additives in RP-HPLC is the primary motive force for recent theoretical efforts to model their behavior. The time-honored solvophobic theory cannot properly explain salt effects. Its theoretical basis was recently questioned by breaking experimental evidence at variance with the textbook knowledge of ionic solutions interfaces; meanwhile, a recently proved extended thermodynamic approach to ion-pair chromatography (IPC) is challenged by the breakthrough of neoteric ionic additives whose behavior questions the rigidity of previous retention schemes and bridges salting chromatographic phenomena to IPC. Building on these research needs, the aims of this review are (1) to illustrate a comprehensive theory of analyte retention in the presence of any kind of electrolytes (hydrophobic ions, chaotropes, kosmotropes, ionic liquids [ILs]) to capture and rationalize the main salting effects and to support their strong practical impact for the separation of organic and inorganic ions, ionogenic, neutral, and zwitterionic analytes; (2) to explain why ion-specific salting chromatographic effects that represent a diachronic scientific consideration were not satisfactorily explained in the rubric of the solvophobic theory; and (3) to highlight the eligibility of chromatography as a basic technique that is able to clarify the currently hotly debated behavior of ions at water interfaces. The practical impact of chaotropic chromatography will also be detailed, and urgent research needs and suggestions will be illustrated.