Suppressor Systems in Ion-Exclusion Chromatography

The high retention of aliphatic monocarboxylic adds ( c 4) and aromatic carboxylic acids can be lowered by adding small amounts of an organic solvent (10-30 mL/L) to the eluent. This blocks adsorption sites on the surface of the stationary phase [9]. Acetonitrile, 2-propanol, or ethanol are particularly suitable. 

If possible, methanol should not be used, because the stationary phase is subjed to strong volume changes when using this solvent 

According to Tanaka and Fritz [10], comparably low backgroimd conductivities were obtained with a benzoic acid eluent, as benzoic acid is only partly dissociated. The acid strength of a dilute benzoic acid solution is sufficient to elute aliphatic carboxylic acids with good peak shapes. 

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  

Because the organic acids to be analyzed do not precipitate with silver, they reach the conductivity cell unchanged. However, this type of suppression has a number of disadvantages  

The detection of aliphatic carboxylic acids usually involves the measurement of the electrical conductivity. In ion-exclusion chromatography, therefore, suppressor systems are used to chemically reduce the background conductivity of the acid that acts as the eluent. Due to a lack of modern membrane suppressors in the past, suppressor columns 

The only possibility to limit the steadily increasing back pressure caused by the continuing precipitation of silver chloride in the suppressor column was to cut off the exhausted part of the cartridge after some days of operation. 

A cation exchange membrane was developed to overcome these disadvantages. This membrane allows continuous regeneration and consists of a sulfonated polyethylene derivative. It is resistant to water-miscible organic solvents and exhibits a high permeability for quaternary ammonium bases such as tetrabutylammonium hydroxide. Hence, the suppression mechanism differs substantially from the process for anion exchange chromatography described in Section 3.3.3. 

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 

A cation exchange membrane was developed to overcome these disadvantages. 

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As described in Sections 3.3.3 and 3.4.3, hollow fiber suppressors no longer represent the state-of-the-art. Thus, a micromembrane suppressor was introduced under the trade name AMMS-MPIC for ion-pair chromatography of anions. Its structure corresponds to the systems developed for ion-exchange and ion-exclusion chromatography. Like the AFS-2, the AMMS-MPIC micromembrane suppressor contains a solvent-resistant membrane that is permeable to quaternary ammonium bases. Regarding the exchange... 

A micromembrane suppressor for ion-exclusion chromatography has been introduced under the trade name AMMS-ICE. Its structure corresponds to the systems developed for anion and cation exchange chromatography (see Sections 3.6.3 and 4.3.3). However, in its mode of operation, it corresponds to the AFS-2 hollow fiber suppressor. An AMMS-ICE micromembrane suppressor also contains membranes that are compatible with water-miscible organic solvents. Therefore, it is used for the analysis of long-chain fatty acids, which are separated on a non polar stationary phase in a weakly acidic medium with methanol or acetonitrile as mobile phase components. In this case, a dilute potassium hydroxide solution is used as the regenerant. With respect to the ion-exchange... 

In order to preclude this problem and the necessary frequent regeneration of the anion system s suppressor column, an ion chromatography exclusion scheme was utilized. Samples collected in a mine environment were reliably concentrated by freeze-drying and then analyzed on an ICE system with dilute hydrochloric acid eluent. The precision of the ICE method was experimentally determined to be 2.5% in a concentration range of 1 to 10 yg/mL. The accuracy was not independently determined but good precision and recovery yield confidence that measured values are within 5% of the true value. No interferences were observed in the ICE system due to strong acids, carbonic acid or other water soluble species present in mine air subject to diesel emissions. 

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