Suppressed Anion Chromatography

Modern methods of chemical conductivity suppression utilize much smaller and more efficient devices than the original stripper columns, but the basic principles are largely unchanged. The reactions for suppressed anion chromatography are as follows  

The basic eluent (OH ) is neutralized by H from the cation exchanger of the suppressor to form water. A sample zone passing through the suppressor is converted from the sodium as the counterion (Na OH ) to the more highly conducting hydrogen counterion (H X ). 

The major devices for suppressed conductivity detection in ion chromatography have been reviewed [2], These are described in chronological order in the following sections. 

The fiber suppressor [3, 4] was the first device based on the use of an ion-exchange membrane. It consisted of a long, hoUow fiber made of a semi-permeable ion-exchange material. Column effluent containing zones of separated sample ions passed through the hoUow center of the fiber. Here the sodium counterion was exchanged for from the membrane. The outside of the hollow fiber 

A flat membrane suppressor from Dionex, known as the Micro-Membrane Suppressor (MMS), had a much higher capacity and lower dead volume than previous devices and was able to operate around the clock with minimal attention. 

The eluent screen is of fine mesh to promote the suppression reaction while occupying a very low volume. The ion-exchange membranes on either side of this screen 

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Madden, J. E. and Haddad, P. R., Critical comparison of retention models for the optimization of the separation of anions in ion chromatography II. Suppressed anion chromatography using carbonate eluents, /. Chromatogr. A, 850, 29, 1999. 

Lamb, J. D. and Drake, P. A. (1989) Chemically-Suppressed Anion Chromatography Based On Macrocycle-Cation Complexation, J. Chromatogr. 482, 367-380. ... 

Shintani and Dasgupta [32] have reported that post-suppression membrane-based ion exchange chromatography with fluorescence detection permits detection limits superior to those obtained by conductivity detection in hydroxide eluent suppressed anion chromatography... 

In suppressed anion chromatography, the effluent from the ion exchange column comes into contact with a cation-exchange device (Catex-H" ) just before the liquid stream passes into the detector. This causes the following reactions to occur. 

The major eluents that have been used for suppressed anion chromatography are listed in Table 6.6. These are all basic eluents that produce a suppressor product that is a very weak acid and therefore one that has a very low conductivity. The ultimate eluent in terms of suppressed conductivity detection is the hydroxide ion. which gives water as the suppressor product. 

It is appropriate to refer here to the development of non-suppressed ion chromatography. A simple chromatographic system for anions which uses a conductivity detector but requires no suppressor column has been described by Fritz and co-workers.28 The anions are separated on a column of macroporous anion exchange resin which has a very low capacity, so that only a very dilute solution (ca 10 4M) of an aromatic organic acid salt (e.g. sodium phthalate) is required as the eluant. The low conductance of the eluant eliminates the need for a suppressor column and the separated anions can be detected by electrical conductance. In general, however, non-suppressed ion chromatography is an order of magnitude less sensitive than the suppressed mode. 

Nowak, M. and Seubert, A., Application of experimental design for the characterization of a novel elution system for high capacity anion chromatography with suppressed conductivity detection, /. Chromatogr. A, 855, 91,1999. 

Other eluent systems in suppressed ion chromatography are typically chosen based on specific separation requirements. For routine analysis of monovalent and divalent anions, carbonate-based eluents represent a reasonable alternative to hydroxide-based eluent systems. Carbonate eluents are simple to prepare and can be useful in cases where anion analysis is only occasionally performed. It must be kept in mind, however, that carbonate lowers the detection sensitivity for anionic species and introduces significant nonlinearity into the analysis. ... 

Amin M., Lim L. W Takeuchi T. Determination of common inorganic anions and cations by non-suppressed ion chromatography with column switching. Journal of Chromatography A 2008 1182 (2) 169-175. 

In suppressed-ion anion chromatography (Figure 26-4a), a mixture of anions is separated by ion exchange and detected by electrical conductivity. The key feature of suppressed-ion chromatography is removal of unwanted electrolyte prior to conductivity measurement. 

Figure 26-4 Schematic illustrations of (a) suppressed-ion anion chromatography and (b) suppressed-ion cation chromatography. 

Ion-Exchange and Ion Chromatography 26-1. State the purpose of the separator and suppressor in suppressed-ion chromatography. For cation chromatography, why is the suppressor an anion-exchange membrane ... 

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 suppressed ion chromatography, anions are separated on a separator column that contains a low-capacity anion-exchange resin. A dilute solution of a base, such as sodium carbonate/sodium bicarbonate or sodium hydroxide is used as the eluent. Immediately following the anion-exchange separator column, a cation-exchange unit (called the suppressor) is used to convert the eluent to molecular carbonic acid. 

For non-suppressed ion chromatography to be successful, the ion exchanger used in the separation column must have a low exchange capacity and a very dilute eluent must be used. In the separation of anions, the resin must have an exchange capacity between about 0.005 mequiv/g and 0.10 mequiv/g. Typical eluents are 1.0 x 10 M solutions of sodium or potassium salts of benzoic acid, hydroxybenzoic acid, or phtha-lic acid. These eluents are sufficiently dilute that the background conductivity is quite... 

The eluent used in anion chromatography contains an eluent anion, E". Usually Na" or will be the cation associated with E". The eluent anion must be compatible with the detection method used. For conductivity, the detection E should have either a significantly lower conductivity than the sample ions or be capable of being converted to a non-ionic form by a chemical suppression system. When spectrophoto-metric detection is employed, E will often be chosen for its ability to absorb strongly in the UV or visible spectral region. The concentration of E in the eluent will depend on the properties of the ion exchanger used and on the types of anions to be separated. Factors involved in the selection of a suitable eluent are discussed later. 

For anion chromatography, and especially for non-suppressed IC, it is necessary to have resins of low exchange capacity. Earlier work centered on functionalization of macroporous resins produced by Rohm and Haas. 

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