Self-Regenerating Suppressors

Ever since the introduction of membrane-based suppressor systems, attempts have been made to support the ion transport through the ion-exchange membranes with an electric field, thus utilizing the principle of electrodialysis. The basic idea behind this is to enhance suppression by applying an electric field that will impel the ions involved in the suppression reaction to penetrate the ion-exchange membranes. 

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)  

Reactions of this type are common on metal siu aces with a small overpotential such as platinum [105]. Hydroniimi or hydroxide ions formed in those reactions in combination with suitable ion-exchange membranes can be utilized for suppression [106]. The first suppressor based on this principle was developed by Strong and Dasgupta [104]. It housed a spiral-type double membrane having a suppression capacity high enough for hydroxide eluents with a maximum concentration of 0.2 mol/L. The water required for electrolysis was delivered with a peristaltic pump. 

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, 

Close contact between the electrodes and the membrane material is important for a suppressor in which the regenerant ions are formed via electrolysis of water. Because the charge transport in the interior of the suppressor is supported by ion migration via the ion-exchange functions at the membranes, the key for faultless fimctioning is the electrical resistance between the two electrodes the resistance should be as low as possible. 

Tian et al. [77] implemented this idea by constructing a suppressor, in which the resin-packed eluant chamber was separated from the electrodes by cation exchange membranes. Usually, 0.10 mol/L sulfuric acid is used as a static regenerant in this device. According to the authors, the applied electric field supports the removal of sodium ions from the eluant chamber. However, they did not 

6 Suppressor Systems in Anion Exchange Chromatography 2HjO + V2O2 (g) + 2e (70) 

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Self-regenerating suppressor cartridge incorporating an electrolysis cell, (a) For anionic analytes, (b) For cationic analytes. 

FIGURE 6 Effect of p-cyanophenol on the separation of perchlorate. Column 4x250mm lonPac ASII. Flow rate I.OmLmin. Injection volume 25pL. Detection suppressed conductivity utilizing the Anion Self Regenerating Suppressor (4mm), recycle mode. Ion I—perchlorate (20mgL" ). (a) Eluent lOOmM NaOH. (b) Eluent 50 mM NaOH and 5mM p-cyanophenol. 

Fig. 24 Schematic of the self-regenerating suppressor in a recycle mode. (Courtesy of Dionex.)...

Fig. 5. An analysis of a coarse atmospheric aerosol extract by CE and IC [49]. CE conditions a 57 cmX75 xm I.D. capillary, distance to detector, 50 cm. Electrolyte 2.25 mM PMA (pyromel-litic acid), 0.75 mM HMOH (hexamethonium hydroxide), 6.50 mM NaOH and 1.60 mM TEA (triethanolamine), pH 7.7 or 2.0 mM NDC (2,6-naphthalenedicarboxylic acid), 0.5 mM TTAB (tetradecyltrimethylammonium bromide) and 5.0 mM NaOH, pH 10.9 30 kV (PMA) or 20 kV (NDC) pressure injection for 10 s indirect UV detection at 254 nm (PMA) or 280 nm (NDC). IC conditions an IonPac-ASlO column with an IonPac-AGlO guard precolumn conductivity detection using an anion self-regenerating suppressor (ASRS-I) in the recycle mode. Analytes 2, chloride 3, sulfate 5, nitrate 6, oxalate 7, formate 10, hydrocarbonate or carbonate 11, acetate 12, propionate 14, benzoate.

Figure 2.10 Separation of anions in a carbonated apple juice using suppressed ion chromatography. Chromatography conditions column, AS11 with AG11 guard detector, CD20 conductivity detector with the ASRS self-regenerating suppressor in the recycle mode. (Courtesy of Dionex Corporation.)...

In 1992 Dionex introduced a commercial electrochemical suppressor called a Self Regeneration Suppressor, or SRS [6]. The internal design is similar to the membrane suppressor, but the regenerating ion for anion chromatography) is produced by electrolysis of water. This allows the use of very low flow rates for regenerant water and avoids the use of independent chemical feed needed for earlier suppression devices. 

Figure 63. Mechanism of suppression for the Anion Self-Regenerating Suppressor. (From Ref. [2] with permission).

A. Henshall, S. Rabin, J. Statler and J. Stillian, Recent development in ion chromatography detection the self-regenerating suppressor. Am. Lab., 24,20R, 1992. 

Modem suppressors for cation chromatography are both efficient and self-regenerating. The principles are similar to the suppressors for anion chromatography, described in Chapter 6. The mechanism of suppression for a cation self-regenerating suppressor is illustrated in Fig. 7.1 and described in some detail by Rabin et al. [3]. Suppressors for cation chromatography are limited to those cations that do not form precipitates with the hydroxide ions from the suppressor. 

Figure 7.1. Mechanism of suppression for the cation self-regenerating suppressor. fT MSA = Methane-sulfonic acid. From Ret [3] with permission.

FIGURE 4.12 Anion self regenerating suppressor (ASRS) for detection in ion chromatography (hy courtesy... 

The need to periodically regenerate suppressor columns is an inconvenience, especially if one wishes to operate an ion chromatograph in an unattended, automated fashion over long periods. A self regenerating suppressor continually regenerates the element in an ion chromatograph which performs the function of the suppressor... 

J. li, M. Chen, and Y. Zhu, Separation and determination of carbohydrates in drinks by ion chromatography with a self-regenerating suppressor and an evaporative light-scattering detector, /. Chromatogr. A, 1155, 50, 2007. 

In summer 1992, the first commercial electrochemical suppressor system was introduced by Dionex the self-regenerating suppressor [107,108]. Its schematic differs from a micromembrane suppressor (see Figure 3.108) only in the two... 

Figure 3.115 Neutralization reactions in a self-regenerating suppressor for anion analysis.

Depending on the application, a self-regenerating suppressor can be operated in three different modes. 

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