Basics of HPLC

Ion chromatography is used to separate ionized compounds. There are two types of ion chromatography:

1. Anion retention: the stationary phase is composed of a cationic group and the mobile phase is anionic.
2. Cation retention: The stationary phase is composed of an anionic group and the mobile phase is cationic.

It is always the counter-ion associated with the stationary phase that is exchanged with the ionized compounds to be separated (see figure).

In ion chromatography, the more charged the ions are, the greater their retention will be. Thus in ion chromatography, the sulfate ion SO₄^2- will be better retained than the chloride ion Cl^- and the magnesium ion Mg²⁺ will be better retained than the sodium ion Na⁺. For ions with the same charge, the more voluminous an ion is, the better it will be retained. Thus the NO₃^- anion will elute more slowly than the Cl^- anion.