Selenium

Selenium is both an essential and toxic element, and plays an important role in environmental analysis as well as in health studies. Sufficient selenium supplementation can protect against heart disease and has a possible role for cancer prevention (31). The absence of selenium correlates with a loss of glutathione peroxidase activity and is associated with damage to cell membranes resulting from accumulation of free radicals. The normal daily dietary intake of selenium is 0.01 and 0.04 ppm. Selenium is accumulated such that the normal concentration in human blood is 95-160 ^g/L (0.015 ppm). In the selenium deficiency, the serum concentration is <40 ^g/L. Selenium toxicity in humans is not known to be a significant problem except in acute overdose cases and selenium is not classified as a human teratogen. Biological and toxicological effects of selenium are strongly dependent on its chemical form (32), thus there is an increasing interest in the differentiation of selenium species, both organic and inorganic.

Analysis of selenium is routinely performed by atomic-absorption spectrometry after the specimen is mixed with matrix modifier (33). The high resolving power of CE makes it potentially valuable for separation and detection of the various selenium species. CZE separation of selenium anions has been the subject of several reports and reviews (23). The preferred way to separate the various species of selenium is the co-electroos-motic mode with direct photometric detection. The absorption properties of selenite (+2 oxidation state) are significantly higher than the selenate (+8 oxidation state) at 200 nm, giving detection limits of 2 and 0.4 ppm, respectively. Indirect photometric detection for the separation of selenate, selinite, selenocystine, and selenomethionine in combination with electrokinetic injection, which selectively injects and concentrates ions, provided highly sensitive detection for inorganic selenium ions. Further, coupling ICP-MS with CE affords specific multi-element detection, in addition to providing extremely sensitive detection of the various metals. Thus, use of CE, specifically CZE, with ICP-MS (32) was applied to selenium speciation providing the possibility to analyze six selenium species in a single run. With this method, detection limits were found to be as 10-20 ^g/L of inorganic species and 3550 ^g/L for organic selenium species, which is within the limits needed to be clinical useful.

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