ion-exchange chromatography (IEC) - a mode of chromatography in which ionic substances are separated on cationic or anionic sites of the packing. The sample ion (and usually a counterion) will exchange with ions already on the ionogenic group of the packing. Retention is based on the affinity of different ions for the site and on a number of other solution parameters (pH, ionic strength, counterion type, etc.).
ion-exchange capacity - the number of ionic sites on the packing that can take part in the exchange process. Exchange capacity is expressed in mequiv/g; typical strong anion-exchange resin may have 3-5 mequiv/g capacity.
ion-pair chromatography - form of chromatography in which ions in solution can be "paired" or neutralized and separated as an ion pair on a reversed-phase column. Ion-pairing agents are usually ionic compounds that contain a hydrocarbon chain that imparts a certain hydrophobicity so that the ion pair can be retained on a reversed-phase column. Ion-pairing can also occur in normal-phase chromatography when one part of the pair is loaded onto a sorbent, but this technique is not as popular as the RPC technique.