A comparison of gradient capacity anion chromatography using macrocycles D-2.2.2 and D-2.2.1 in constant or variable...

The ability of macrocyclic ligands to complex alkali metal cations has been exploited to perform chromatographic separations of anions. Macrocycles adsorbed to reversed phase columns can complex eluent cations, thus generating anion exchange sites. Gradient separations of anions can be performed by changing the column capacity during the course of the separation, either by changing the eluent cation or by changing the column temperature. Gradient anion separations are performed by changing the eluent from sodium hydroxide to lithium hydroxide with the cryptand D-2.2.2, while similar anion separations are achieved with D-2.2.1 by a KOH-LiOH gradient. Since the complexation of cations by macrocycles is exothermic, increasing the column temperature decreases the anion column capacity, allowing temperature gradient separations. The experimentally measured DeltaH values for D-2.2.1 are higher than for D-2.2.2, leading to steeper gradients and thus better separations with D-2.2.1.