Ion-Exchange Chromatography

In ion-exchange chromatography (IEC) the stationary phase is a cross-linked polymer resin, usually divinylbenzene cross-linked polystyrene, with covalently attached ionic functional groups. As shown here for a styrene–divinylbenzene co-polymer modified for use as an ion-exchange resin, the ion-exchange sites—indicated here by R and shown in blue—are mostly in the para position and are not necessarily bound to all styrene units. The cross-linking is shown in red. The counterions to these fixed charges are mobile and can be displaced by ions that compete more favorably for the exchange sites.

Figure12.52

The photo here shows an example of ion-exchange polymer beads. These beads are approximately 0.30–0.85 mm in diameter. Resins for use in ion-exchange chromatography are smaller, typically 5–11 mm in diameter.

Figure12.52inset

Ion-exchange chromatography is an important technique for the analysis of anions and cations in environmental samples. For example, an IEC analysis for the anions F, Cl, Br, NO2, NO3, PO43–, and SO42– takes approximately 15 minutes. A complete analysis of the same set of anions by a combination of potentiometry and spectrophotometry requires 1–2 days.

Figure12.53

 

Data provided by Jeanette Pope, Department of Geosciences, DePauw University.

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