Eluent suppressed ion chromatography

The main advantages of eluent suppressed ion chromatography (ESIC) are that a wide range of eluents and columns can be used, the wide dynamic range, and the higher sensitivity the main disadvantage is the periodical necessity for suppressor-column regeneration. 

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. ... 

The suppressors in Figure 26-4 also have been replaced by electrolytic units that generate H or OH- necessary to neutralize the eluate and require only H20 as feedstock.6 With electrolytic eluent generation and electrolytic suppression, ion chromatography has been simplified and highly automated. Readily available software can be used to simulate and optimize ion chromatographic separations.7... 

Conductivity detectors respond to all ions. In suppressed-ion chromatography, it is easy to measure analyte because eluenl conductivity is reduced to near 0 by suppression. Suppression also allows us to use eluent concentration gradients. 

In suppressed-ion chromatography, a separator column separates ions of interest, and a suppressor membrane converts eluent into a nonionic form so that analytes can be detected by their conductivity. Alternatively, nonsuppressed ion chromatography uses an ion-exchange column and low-concentration eluent. If the eluent absorbs light, indirect spectrophotometric detection is convenient and sensitive. Ion-pair chromatography utilizes an ionic surfactant in the eluent to make a reversed-phase column function as an ion-exchange column. 

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 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... 

Eluent dips or system peaks and their causes were first described by Gjerde and Fritz [13]. Stevens et al. [3] described the effect of the system peak in suppressed ion chromatography. Called a carbonate dip, the system peak was said to be the absent peak (from the injection) of the carbonic acid that is retained by the unexhausted portion of suppressor column. 

Non-Suppressed Ion Chromatography Table 6.10. Background conductances of eluents l... 

P. R. Haddad and R. C. Foley, Aromatic bases as eluent components for conductivity and indirect ultraviolet detection of inorganic cations in non-suppressed ion chromatography. Anal. Chem., 61, 1435,1989. 

A variety of eluents can be used in the separation of anions although bicarbonate eluents have been used as the mainstay eluent in suppressed ion chromatography. The ideal eluent seems to be hydroxide since, after suppression, it forms water that has virtually zero... 

In ion chromatography, anions are separated by ion exchange and detected by their electrical conductivity. The conductivity of the electrolyte in the eluent is ordinarily high enough to make it difficult or impossible to detect the conductivity change when analyte ions are eluted. Therefore the key feature of suppressed-ion chromatography is removal of unwanted electrolyte prior to conductivity measurement. 

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. 

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