Postcolumn Techniques: A Critical Perspective for Ion Chromatography

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Abstract
Part I. General Overview and Systems with Suppressors Postcolumn techniques represent a particularly fast-growing area in ion chromatography (IC). The developments since 1985 are surveyed in this paper. A general overview of reagent introduction means, including membrane devices and reaction conduit designs—especially various geometrically deformed open tubular designs—is provided. Developments in ion exchange suppressor designs and electrochemically regenerated membrane suppressors are critically assessed. Response linearity in IC is discussed. Several new suppression schemes utilizing novel and imaginative chemistry are described and evaluated. Replacement ion chromatography, during which the analyte ion or the associated counterion is replaced with a more sensitively detected ion, is reviewed in detail. All other postcolumn techniques are discussed in the second part of this paper. Part II. Determination of Metals, Various Ionic and Ionizable Species This review discusses the direct and indirect absorptiometric, fluorometric, and electrochemical detection of metal species, including rare earth elements, alkyltin, and alkyllead compounds. Among nonmetallic cations, the determination of various amines (primary, secondary, and tertiary), NH4+, guanidino compounds, quaternary ammonium drugs and surfactants, polyamines, and thiolbearing amino acids is discussed. The determination of anionic species includes the catalytic determination of I-/Br-, solid phase reaction detection of SO42-, direct and indirect fluorometriccolorimetric detection of a large variety of phosphorus anions, carboxylic acids (including straight chain, hydroxy-, keto-, ascorbic acid, sialic acids, bile acids, kynurenic acid, aminolevulinic acid), phenols, and lipophilic anions, etc. The utility of simple pH modification of the column effluent is discussed. Sugars, which are ionized at high pH, are increasingly being separated by anion exchange chromatography. Thus, the separation and PCR detection of various sugars are considered. Finally, the separation of formaldehyde on ion-exchange media and its PCR detection is described. Part III. New Methods on t he Horizon The final part of this three-part review discusses chemiluminescence detection, including those involving Ru(bpy)33+, luminol, lucigenin, and in particular, peroxyoxalate. Photolytic methods for derivatization before electrochemical or fluorescence detection are described. Spectrophotoelectrochemical detection is suggested as an important avenue for exploration. The use of postcolumn reactors built of metal tubes, wires and powder for selective reductions, electrochemical generation of reagents, fluorescence enhancement by inclusion complex formation by cyclodextrins, the coupling of reversed-phase liquid chromatography to electron capture detectors, and post separation zone compression to sharpen otherwise broad chromatographic peaks are critically reviewed.