| Summary: | Interaction of metal ions, both essential and toxic, with macromolecules is important in biological and environmental systems. The availability of trace metals may govern an organism's nutritional strategy in a given environment. Metal ions are rarely found free in most systems because of their active nature. The transportation of contaminants and nutritionally important metal ions in the environment is facilitated by their ability to bind and form complexes with humic substances and other ligands. Capillary electrophoresis-inductively coupled plasma mass-spectrometry (CE-ICP-MS) has been explored to characterize metal ions bound to various macromolecules because of the rapid separation coupled with high separation efficiency, resolving power and small sample size requirements together with sensitive elemental detection capability afforded by this hyphenated technique. Macromolecules examined here include humic substances, selenium compounds and selenoproteins, and some metal transport and storage proteins. Characterization of metal ion complexed humic substances by CE-ICP-MS is a formidable analytical challenge. Particularly of concern are the potential interactions with the CE capillary wall and the poor concentration detection limit inherent with the CE technique. Thus, sample preconcentration by flow field-flow fractionation was employed. Addition of zwitterions to the separation buffer helped to alleviate analyte-capillary wall interactions. A case study of trace metals bound to soil humic acids (HAs) that were successively extracted from the slopes of a local mountain was performed using the developed method. Trace metals complexed with HAs declined with diminishing oxygen-rich moieties in the HA structure. Analysis of selenoproteins extracted from yeast and selenium-containing compounds obtained from selenized supplement tablets by CE-ICP-MS were investigated. It was demonstrated that bare CE capillaries coupled to ICP-MS are suitable for speciation analysis of low molecular weight selenium compounds using a cationic surfactant separation buffer modifier. However, baseline resolution was not always achieved for selenoproteins owing to analyte adsorption onto the capillary wall. Likewise, speciation of some storage and transport proteins had been carried out with uncoated capillary CE-ICP-MS. Very high concentrations of the samples were employed because of low signal sensitivity. Improvement of the interface is necessary in order for kinetic and binding studies to be attempted.
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