Suppressor Systems in Cation-Exchange Chromatography

In HPLC peak dispersion is significantly affected by the particle size of the resin. Thus, the band broadening in a suppressor column decreases as the particle diameter of the employed resin decreases. Similarly, the void volume of the suppressor column should be as small as possible to reduce band broadening effects. An optimal suppressor column would therefore have a very low volume and would contain an exchange resin with a very small particle diameter. Both requirements are inconsistent with the analytical practice, which calls for a suppressor column that can be employed at least for one working day without regeneration. This requirement can only be met by suppressor columns with a higher volume. 

According to the procedure described in Section 3.3.3, the regeneration of conventional suppressor columns such as CSC-1 or CSC-2 is performed using sodium hydroxide solution with a concentration of c = 0.5 mol/L. 

conventional suppressor columns have only historical significance. The above-mentioned disadvantages could be surmounted by the development of the CFS hollow 

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. 

The regenerent flow rate should be 2 to 3 mL/min. With eluent concentrations of up to c = 0.005 mol/L, background conductivities of 10 pS/cm at maximum are ensured. 

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The schematics and operation of the CSRS self-regenerating suppressor suitable for cation-exchange chromatography corresponds to the ASRS, which was developed for anion-exchange chromatography (see Section 3.6.4). Both suppressor systems were introduced in 1992 [25,26]. Since then, dynamic suppression capacity has been improved they have developed into the SRS 300 type suppressors now being available on the market The principle schematic of a CSRS only... 

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... 

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... 

Nonsuppressed ion chromatography employs a conventional liquid chromatographic system with a conductivity detector cell connected directly to the outlet end of an ion-exchange separation column. No suppressor unit is required. The successful development of this method was made possible by three principal innovations (i) the use of an anion- or cation-exchange resin of very low capacity (initially 0.007 to 0.04 mequiv (ii) an eluent with a low ionic concentration and hence a low conductivity, and (iii) an eluting ion in the eluent that has a significantly lower equivalent conductance than that of the analyte ions. 

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 ... 

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