Suppressor reaction

The second column is called a suppressor column. Its function is to convert the eluent to a less conductive species while converting sample ions to a common form. This system enables conductimetric detection of the sample ions in a low conductivity background. The ion-exchange suppressor reactions are also shown in Figure 1. In the case of anion analysis, sodium carbonate and/or bicarbonate eluent is converted to a weakly conductive dilute carbonic acid while the sample ions are converted to strong-... 

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. 

As the result of the suppressor reaction, strongly conducting mineral acids in the presence of weakly dissociated carbonic acid enter the conductivity cell and are thus easily be detected. 

A second class of available eluents comprises the family of zwitterionic compounds (see Section 3.3.2), which exist in their anionic form at alkaline pH. The product of the suppressor reaction is the zwitterionic form with a correspondingly low intrinsic conductance. Promising experiments were carried out by Irgum [48] with N-substituted... 

With pAa=1.33 for the dissociation of the carboxyl group, DAP may exist in the mobile phase as monovalent cation, divalent cation, or mixture of both, respectively. In any case, the product of the suppressor reaction is the zwitterionic form... 

For continuous regeneration of the suppressor, either potassium hydroxide with a concentration of c = 0.04 mol/L or tetramethylammonium hydroxide with a concentration of c = 0.02 mol/L may be used. The choice of regenerent depends on the type of analyte. Potassium hydroxide is recommended as a regenerent for the analysis of alkali metals. If ammonium is also to be analyzed, it should be noted that the linear range for the determination of this ion is very small when using potassium hydroxide as the regenerent. This is caused by the equilibrium between NH4OH as the suppressor reaction product and the free base NH3. 

In the regenerated state, the membrane exists in the tetrabutylammonium form. The oxonium ions of the organic acids that flow through the interior of the membrane are replaced by tetrabutylammonium ions in the suppressor reaction. Because of the different equivalent conductances of oxonium and tetrabutylammonium ions, the salt formed as the suppressor product has a markedly lower conductivity than the corresponding add form. In the zone of dynamic equilibrium the neutralization reaction between H+ and OH- ions to form water as the suppressor product takes place which is discharged... 

What is the suppressor reaction product for potassium benzoate ... 

WMle non-suppressed and suppressed conductivity detection modes are suitable for detection of alkali and alkaline earth metals, for detection of other metal ions only non-suppressed conductivity detection can be used, because these metals would mostly be transferred by the suppressor reaction into insoluble hydroxides. Thus, spec-trometric detection after suitable post- or precolumn deri-vatization is usually carried out. ... 

Electrochemical suppressors can also be operated without an external chemical regenerant. Hydroniimi or hydroxide ions, required for the suppressor reaction, can be generated from water by means of electrolysis. The respective reactions are given by the Eqs (3.41) and (3.42) ... 

In contrast to conventional chemical suppressors, the suppressor reaction in an SRS is directed by the electrodes. In a micromembrane suppressor, for instance, the chemical regenerant flows countercurrent through both regenerant chambers. Therefore, regenerant ions required for neutralization are provided to the eluent chamber through both membranes. Hydronium ions in an ASRS are exclusively formed at the anode, so that only the ion-exchange membrane in the anodic regenerant chamber is permeable for hydronium ions. Conversely,... 

While both conductivity detection modes are suitable for the detection of alkali and alkaline-earth metals, only nonsuppressed conductivity detection can be used for the analysis of transition metals. The formation of nondissociated metal hydroxides from the suppressor reaction precludes the use of a suppressor system for this type of analysis. Nonsuppressed conductivity detection is also impeded by the presence of complexing agents in the mobile phase these agents are required for separating transition metals. In 1983, Sevenich and Fritz [34] found an eluent mixture suited for nonsuppressed conductivity detection. It was comprised of ethylenediamine and tartaric acid. Ethylenediamine serves as an eluent ion, because it is already fully protonated (EnH ) at pH values < 5. The... 

Because the detection of ahphatic carboxylic acids is usually performed by measuring the electrical conductivity, suppressor systems are also used in ion-exclusion chromatography to chemically reduce the background conductivity of the acid eluent. In the past, modern membrane suppressors were not available, so packed-bed suppressor columns were used such columns contained a cation-exchange resin in the silver form. With a dilute hydrochloric acid eluent, the suppressor reaction is described as follows ... 

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 contrast to a GMMS, the suppressor reaction inside a GSRS is directed by the electrodes. In a GMMS, for instance, the chemical regenerant is directed countercurrent through both regenerant chambers. Therefore, regenerant ions... 

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