Eluents for Anion Exchange Chromatography

The kind of eluent that is used for anion exchange chromatography depends mainly on the detection system being employed. Since, in many cases, the detection of inorganic and organic anions occurs via conductivity detection, the eluents used are classified into two groups  

Regardless of this, the affinities of eluent ions and solute ions must be comparable i.e., as a rule of thumb, divalent solute ions may only be eluted with divalent eluent ions. 

The versatile mixture of sodium carbonate and sodium bicarbonate, on the other hand, finds widespread application, since the elution power and the selectivity resulting there from are determined over a wide range solely by the concentration ratio of these two compounds. A great variety of inorganic and organic anions can be separated with this eluent combination. As the product of the suppressor reaction, the carbonic acid is only weakly dissociated, so that the background conductivity is very low. 

As an alternative to carbonate/bicarbonate systems, amino adds (a-aminocarb-oxylic acids) may be used as an eluent [47,48]. Their dissociation behavior is depicted in Fig. 3-36. At alkaline pH, amino acids exist in the anionic form due to the dissociation of the carboxyl group and, thus, may act as an eluent ion. The product of the suppressor reaction is the zwitterionic form with a correspondingly low background conductance. This depends on the isoelectric point, pi, of the amino acid. 

The residual dissodation of the zwitter ion and the background conductance of the eluent is even lower than for the carbonate/bicarbonate system, if amino acids are selec- 

Eluents of the first kind (for conductivity detection with chemical suppression) include the salts of weak inorganic acids, which exhibit a low background 

33 Eluents for Anion-Exchange Chromatography l45 Cation-exchange connector 

NaHCOa (EG) flow rate 1.5 mL/min detection suppressed conductivity injection voiume  

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In contrast to conventional suppressor columns, hollow fiber suppressors are continuously regenerated, and thus do not require an additional pump system. The reactions that occur across the membrane wall are shown in Fig. 3-37. Since fiber suppressors suited for anion exchange chromatography act as cation exchangers, the eluent cations are exchanged with protons in the regenerent solution. The driving force for the diffusion of protons across the membrane is provided by their subsequent reaction with... 

Ion exchange chromatography is based upon the differential affinity of ions for the stationary phase. The rate of migration of the ion through the column is directly dependent upon the type and concentration of ions that constitute the eluent. Ions with low or moderate affinities for the packing generally prove to be the best eluents. Examples are hydroxide and carbonate eluents for anion separations. 

An especially challenging task is maintaining the selectivity of the method for separation of compounds whose elution time is very short, close to the dead time. In such cases, it is necessary to perform a preliminary review of the planned chromatographic conditions, including the composition of the analyzed material. For example, a typical eluent employed in anion-exchange chromatography (with pH of 8.5) is intended to facilitate the dissociation of separated compounds. Neglecting the time necessary to achieve acid/base equilibrium of substances loaded into the column in a neutral solution can result in their elution in the dead volume. The phenomenon is observed, for example, for MMA(V), whose consecutive dissociation constants are p/sTi 3.6 and p/sT2 8.22 [164]. 

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. 

The general requirement for an anion chromatography eluent is that the eluting anion must have useful affinity for anion-exchange resins. Since it is important to minimize the conductivity of the eluent, the preferred anion is one which has a pKa... 

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