Commonly used radioisotopes in biological studies

Not every radioisotope will be suitable for a particular autoradiography procedure. The criteria for determining suitability are:

1. Nature of emission. Particle emitters are preferred. Gamma and X-ray emitters have very poor efficiency. Electron emitters are usually the most favorable.

2. Particle energy. Generally the higher the energy the lower is the efficiency of detection. This is because high-energy particles have a tendency to escape from the sample and emulsion. (3 Particles with energies above 0.5 MeV are usually very difficult to detect. A 'high' energy will also mean that the particle will travel further before forming a latent image, hence resolution will also be poor.

3. Half-life. If the half-life of the radioisotope used is too short (less than 30 days), most of the radioactivity will disappear during sample processing. On the other hand, use of isotopes with very long half-lives will mean a very low specific activity (few radioactive decay events per unit time) and hence the exposure time would have to be lengthened unduly to collect the requisite number of latent images in the emulsion.

Some of the commonly used radioisotopes in biology are listed in Table 1. The exact choice of a particular radionuclide is dependent upon its biological relevance. The final decision in determining a suitable isotope should be made keeping in mind the nature of the biological process under study. The radiolabeled biomolecule prepared has to be purified, checked for specific radioactivity and assayed for biological activity before use for autoradiography.

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