Microamounts of Radioactive Substances

From the previous sections it is evident that radionuclides of high specific activity often represent very small amounts (microamounts, non-weighable amounts 1 pg) of matter, especially if the half-lives are short. Handling of such microamounts requires special precautions, because in the absence of measurable amounts of carriers the radionuclides are microcomponents and their chemical behaviom may be quite different from that observed for macrocomponents. This aspect is of special importance if the system contains liquid/solid, gas/solid or liquid/liquid interfaces. The percentage of radionuclides sorbed on the walls of a container depends on the chemical form (species) of the radionuclide, its concentration and specific activity, and on the properties of the container material. At high specific activity of a radionuclide in solution, the surface of a glass beaker generally offers an excess of surface sorption sites. 

Glass surfaces have an ion-exchange capacity of the order of 10 °mol/cm ( 10 ions/cm ) and a similar number of sorption sites is available for chemisorption. Thus, 100 ml glass beakers have an ion-exchange capacity of about 10 mol coresponding to a concentration of lO mol/1 in 100 ml. Therefore, sorption on 

Chemisorption is very pronouneed if the radionuelide is able to react with the surface. An example is the sorption of hydroxo complexes of tri- and tetravalent elements by the silanol groups on glass surfaces. Sorption increases with the formation of mononuclear hydroxo complexes in solution and deereases with the eondensation to polynuclear complexes at higher pH values. Several measures may be taken to suppress ion exchange or chemisorption of traces of radionuclides on glass surfaees  

The surfaces of plastic materials, such as polyethylene, polypropylene or perspex, do not exhibit ion exchange, but adsorption may be pronounced, in partieular adsorption of organic compounds including organic complexes of radionuclides. 

Sorption of radionuclides on particulates in solution is frequently observed. The particles may be coarsely or finely dispersed. Their surface properties (surface layer, charge, ion-exchange and sorption properties) play a major role. In general, they offer a great number of sorption sites on the surfaee, and microamounts of radionuclides may be found on the surface of these particles instead of in solution. Sorption of radionuclides on colloidal particles leads to formation of radioeolloids (carrier colloids, section 13.4). 

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If macroamounts of other elements are to be separated from microamounts of radioactive substances by precipitation, isotopic or non-isotopic hold-back carriers may be added to suppress coprecipitation of the radioactive substances. 

Radiocolloids are colloidal forms of microamounts of radioactive substances. Their formation was first observed by Paneth (1913) in his research on the separation of 2 °Bi and Po. Radiocolloids can be separated from aqueous solutions by ultrafiltration, centrifugation, dialysis and electrophoresis. They can be detected with high sensitivity by autoradiography. As an example, the autoradiography of a radiocolloid of " " Th is shown in Fig. 13.4. 

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