Eluents in Cation-Exchange Chromatography

Divalent cations such as alkahne-earth metals exhibit a much higher affinity toward the stationary phase of strong acid cation exchangers and thus cannot be eluted with dilute mineral acids. However, increasing the acid concentration in the mobile phase is impossible because of the following reasons  

In his pioneering paper. Small et al. [1] suggested silver nitrate, which also exhibits a high affinity toward the stationary phase, as an eluent for alkaline-earth metals. However, this system required a suppressor column in the chloride form to precipitate the silver as insoluble silver chloride. The application of this eluent quickly proved to be disadvantageous, because peak broadening and high 

The mixture of w-phenylenediamine-dihydrochloride and hydrochloric acid used later did not gain acceptance. The solution turns piuple upon light absorption because of a slow dimerization of the amine and thus has only a limited stability. 

In combination with a membrane-based suppressor (see Section 4.3) a mixture of 2,3-diaminopropionic acid and hydrochloric acid is frequently used for the separation of alkaUne-earth metals on strong acid cation exchangers. Using this eluent provides the advantage of being able to adjust the elution power via the dissociation equilibriimi of 2,3-diaminopropionic acid see Eq. (4.2). With p/C  

In ion chromatography systems with nonsuppressed conductivity detection, mixtures of ethylenediamine and aliphatic dicarboxyhc acids such as tartaric acid or oxalic acid are typically employed as an eluent for the separation of alkaline-earth metals. Those mixtures have a relatively high intrinsic conductance, so that chromatographic signals are registered as negative peaks. 

While the type of available eluent depends upon the detection method being applied in anion exchange chromatography, a corresponding classification is not necessary in cation exchange chromatography. For the separation of alkali metals, ammonium, and small aliphatic amines, mineral acids such as hydrochloric or nitric acid are typically used as eluents, independent of whether the subsequent conductivity detection is performed with or without chemical suppression. The concentration range lies between 0.002 mol/L and 0.04 mol/L. Bachmann et al. [141] employed cerium(III) nitrate in very low concentrations as the eluent for the indirect fluorescence detection of alkali metals. 

Organic eluents are not very common in these applications. The respective manufacturer recommends the use of pyridinecarboxylic acids such as picolinic acid or isonicot-inic acid only for operating separator columns such as ION-200 and Vydac IC 405. 

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