Radioanalysis in the life sciences

Two radioanalytical methods described in chapter 17 are applied preferentially in the life sciences, activation analysis and isotope dilution, the latter mainly in combination with the substoichiometric principle. 

Activation analysis can be used for determination of trace elements, in particular heavy metals and essential elements, in various parts or organs, respectively, of plants or animals and man. Making use of the high sensitivity of activation analysis, small samples of the order of several milligrams taken from selected places give information about the concentration of the elements of interest. The results of activation analysis of trace elements also allow conclusions with respect to diseases or malfunctions and are valuable aids to diagnosis. Examples are the determination of Se in man or of trace element concentrations in bones or other parts, with respect to the sufficient supply of essential elements and metabolism. In vivo irradiation has also been proposed. 

Isotope dilution in combination with the substoichiometric principle is applied in various ways. The most important examples are radioimmunoassay for protein analysis and DNA analysis. In radioimmunoassay, radionuclides are used as tracers and immunochemical reactions for isolation. Radioimmunoassay was first described in 1959 by Yalow and Berson, and since then has found very broad application in clinical medicine, in particular for the measurement of serum proteins, hormones, enzymes, viruses, bacterial antigens, drugs and other substances in blood, other body fluids and tissues. Only one drop of blood is needed, and the analysis can be per-fonned automatically. Today more than 10 immunoassays are made annually in the United States. The most important advantages of the method are the high sensitivity and the high specificity. In favourable cases quantities down to 10 g can 

The general procedure of radioimmunoassay is as follows Two aliquots each containing a known mass mo of a labelled protein P are taken. One aliquot is mixed with a much smaller (substoichiometric) mass of an antibody B which forms the complex PB. The latter is isolated and its radioactivity A is measured. The other aliquot of the labelled protein P is mixed with the unknown mass of the protein to be determined and this mixture is also allowed to react with the same amount of antibody B as before. Again, the complex PB is isolated, and its radioactivity Aj is measured. The unknown mass mx is calculated by application of eq. (17.18). 

A great number of variations of radioimmunoassay are possible competitive or non-competitive binding assays and different antibodies may be applied, and various radionuclides may be used for labelling (preferably T, P, and 

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