Isotopes stable, biologically important

Calcium was first recognized as an element in 1808 by Humphry Davy, and the name was given after the Latin for lime calx. Several isotopes of ealcium are known. The stable isotopes are, in order of decreasing natural abundance, " Ca (96.94%), " Ca (2.1%), Ca (0.64%), and Ca (0.145%). Ca is the only isotope with a nuclear spin (/ = ) different from zero, whieh makes it amenable to NMR studies. Ca is a radioactive isotope of some importance ()S decay 8.8 min half life). It has been used in studies of calcium localization and transport in biological systems. 

High-specific activity radiotracers, produced by neutron activation, have been used with great success to study biochemical processes in the small animal model. For example, Se, having a specific activity of 1,000 Cig has been used to advance the discovery of dependent enzymes and other biologically important proteins. Trace-element and mineral nutrition are important aspects of human and animal health. NAA has been used to characterize a wide variety of samples for their elemental content. The basic nutritional requirement at the cellular level can be studied using NAA and radiotracer techniques. NAA is one of the important methods that has been used to study nutritional bio-availability and absorption of essential trace elements in the human using enriched stable isotopes. 

Fig. 7-13. Relative effect of isotopic mass on reaction kinetics, expressed as ]/m2/mt (where m2 and m, are the masses of the nuclides plotted and the most common stable nuclide of the same element, respectively). Biological fractionation is only likely to be important for elements with atomic masses less than — 20, especially in the case of deuterium and tritium.

The role of metals and other elements in the health of humans, animals, and plants continues to be of great interest both on a routine basis and in research. Two recent reviews described the important impact of ICP-MS in biomedical research [227, 228]. ICP-MS provides the ability to measure multiple elements at ultratrace levels rapidly and to measure isotope ratios at low enough concentrations so that stable isotope tracers can be used for biomedical research [229]. Much remains unknown about the role of metals and their speciation on biological activity, in large part as a result of inadequate measurement techniques. 

Use of stable isotope dilution to assay elemental concentrations in biological samples has increased markedly in the recent past. Part of the reason is that use of radioactive isotopes previously employed as tracers has declined, and part is that interest in the effect of various elements on biological systems and their fate once they enter the food chain has increased. This important field is far from the author s area of expertise, so what follows should be considered a rather haphazard survey... 

Food authentication, application of stable isotope tracers, characterization of complex metallobiomolecules, and investigation of their role in biological processes are all areas in which ICP-MS can play a more and more critical role. Speciation will continue to be a very dynamic field of investigation and the integration of speciation and metabolism studies with the use of stable isotopes as species-specific tracers is likely to become a research area of vital importance. On... 

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