Eluants in Cation Exchange Chromatography

Divalent cations such as alkaline-earth metals exhibit a much higher affinity towards the stationary phase of strong acid cation esodiangers and, thus, caimot be eluted with dilute mineral acids. However, increasing the add concentration in the mobile phase is impossible because of the following reasons  

In his pioneering paper. Small [1] suggested silver nitrate, which also exhibits a high afimity towards the stationary phase, as an eluant for alkaline-earth metals. However, this system required a suppressor colunm in the chloride form to predpitate the silver as insoluble silver chloride. The apphcation of this eluant quickly proved to be disadvantageous, because peak broadening and high backpressure from the suppressor column are assodated with the predpitation of silver chloride. 

In combination with a membrane-based suppressor (see Section 4.3) a mixture of 2,3-diaminopropionic add (DAP) and hydrochloric acid is frequently used for the separation of alkaline-earth metals. Using this eluant provides the advantage of being able to adjust the elution power via the dissociation equilibrium of 2,3-diaminopropionic acid see Eq. (104) in Section 4.1.1. With pJC = 1.33 for the dissociation of the carboxyl group, DAP may exist in the mobile phase as a monovalent cation, a divalent cation, or a mixture of both. In any case, the product of the suppressor reaction is the zwitterionic form, which has no intrinsic conductance  

In non-suppressed ion chromatography systems, mixtures of ethylenediamine and aliphatic dicarboxylic adds such as tartaric add or oxalic add are typically employed as an eluant for the separation of alkaltne-earth metals. Those mixtures have a relatively high intrinsic conductance, so that chromatographic signals are registered as negative peaks. 

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