Separation of Ions

Ion-exchange chromatography (IEC) #

IEC is a mode of LC that employs a charged stationary-phase surface to separate charged analytes (i.e. ions). Most commonly the mobile phase is based in water, with retention being controlled by the ionic strength.

A distinction is made between weak ion exchangers, where retention depends on the pH of the mobile phase, and strong ion-exchangers, where this is not the case. Weak-anion exchange (WAX) and strong-anion exchange (SAX) are intended to separate anions. WCX and SCX columns are intended for cations.

Efficiency (plate count) in IEC tends to be lower than in other forms of LC.

Ion chromatography is a special form of IEC, especially aimed at the separation of small (inorganic) ions.

Ion-pair and ion-exchange chromatography — Schematic illustration of the equilibria underlying ion-exchange chromatography (IEC, left) and ion-pairing chromatography (IPC, right). All four equilibria that underly IPC occur simultaneously and at the same location in the column. The different equilibria are explained in the main text. A+ denotes analyte cations, C+ counter ions, and X- functional groups on the cation exchanger or ion-pair reagent. Grey “blobs” denote hydrophobic anchor groups or chains.

Ion-pair chromatography (IPC) #

IPC is a way no separate ions as neutral ion pairs. The pair-forming ion is part of the ion-pair reagent (IPR) that is added to the mobile phase. The type and concentration of the IPR is the main factor by which to control retention and selectivity in IPC.

IPC is a complex mechanism that can be affected by many additional parameters, including the mobile-phase composition (aqueous-organic mixture), ionic strength and pH. Using reversed-phase (C18) stationary phases, efficiencies (plate counts) in IPC are typically higher than in IEC.