Chromatographic Separations with Selected Supported Chelating Agents

Abstract

Chelex is another example of a commercially available supported chelating agent. The material was available from "io-Rad and was used to purify compounds, most notably transition metal ions. [ ] The preference for transition metal ions, e.g., copper II or iron II ions, over such univalent metal ions as sodium or potassium is said to be about to [ ]. The material consists of a styrene-divinylbenzene copolymer to which is attached iminodiacetic acid moieties.In addition, Chelex can be used for purification of DN" [ ]. Though the mechanism of action seems uncertain, a plausible suggestion is that Chelex protects DN" from the effect of heating that is used to release DN" from cells. In addition, Chelex can sequester magnesium and heavy metal ions that would adversely affect the DN". "fter treatment, DN" and RN" remain in the aqueous supernatant above the Chelex sample.Supported chelating agents serve three major functions, i.e., concentration, elimination, and recycling metal ions. Concentration of metal ions is a significant function that enhances analytical capabilities.Measuring the concentration of trace metals in dilute sea water is a significant analytical challenge. Using supported chelators allows an analyst to concentrate transition metals present in trace amounts from a large volume of sea water onto a solid in, say, a chromatography column, then eluting with dilute nitric acid to obtain a suitably concentrated solution. The technique can also be used to eliminate matrix effects. Unfortunately, Chelex seems to be less effective in sea water than in fresh water [ ]."lso, users of Chelex may encounter problems with certain natural waters that are colored with samples of naturally occurring chelating agents such as soil organic acids e.g., fulvic or humic acids that provide competition for metal ions and reduce the effectiveness of Chelex [ , ]. Ryan and Weber [ ] studied this problem and made pre-concentration comparisons of trace levels of copper ion in standard solutions comparing Chelexwith three different chelating agents supported on silica. The reagents tested were N-propylethylenediamine, the bis-dithiobicarbonate of that compound, and -hydroxyquinoline. The results indicated that in the presence of organic compounds typical organic-rich natural waters reduced the effectiveness of Chelexin comparison with the three silica-supported agents. In general, immobilizedquinolinol was the most effective, as prepared by a modified Hill procedure [ ].It is not the purpose of this review to criticize use of a successful commercial supported chelator rather one may note that no such product can be expected to be highly effective under all conditions and circumstances, so it seems appropriate to consider examples of custom-made agents.