Chemical applications for radioactive isotopes

The main advantages of using radioactive isotopes in chemical experiments are:

• Radioactivity is readily detected. Methods of detection are sufficiently sensitive to measure extremely small amounts of radioactive substances.
• Studies can be carried out in synthetic chemistry using radiolabelled compounds, e.g. ³H or ¹⁴C.
• Protocols are relatively simple compared with equivalent methods for instrumental chemical analysis.

The main disadvantages are:

• The 'isotope effect'. Molecules containing different isotopes of the same atom may react at slightly different rates and behave in slightly different ways to the natural isotope. The isotope effect is more extreme the smaller the atom and is most important for ³H-labelled compounds of low molecular mass.
• The possiblility of mistaken identity. The presence of radioactivity does not tell you anything about the compound in which radioactivity is present: it could be different from the one in which it was applied, owing to chemical breakdown of a ¹⁴C-containing organic compound.

The main types of experiments are:

• Radiolabelled compounds: the use of radiolabelled compounds in synthetic and tracer studies is important as it allows the scientist to locate the labelled atom, i.e. ¹⁴C, ³H, in, for example, chemical synthesis and laboratory environmental fate (degradation) studies. If using radiolabelled compounds several issues arise and these include deciding upon the radionuclide itself, its position in the molecule, the specific activity, the solvent and cost.
• Radio-dating: the age of plant or mineral samples can be determined by measuring the amount of a radioisotope in the sample. The age of the specimen can be found using t_½ by assuming how much was originally incorporated.
• Medical uses: in radiotherapy the use of gamma radiation from ⁶⁰Co to destroy cancerous cells; ²⁴Na can be introduced into the blood stream to follow the flow of blood and identify obstructions; heart disease can be assessed using ²⁰¹Tl and ⁹⁹Tc where the former concentrates in healthy heart tissue and the latter concentrates in abnormal heart tissue.
• Assays: radioisotopes are used in several quantitative detection methods of value to chemists. Radioimmunoassay is a quantitative method for measurement of a substance (the analyte) using antibodies which bind specifically to that analyte. Isotope dilution analysis works on the assumption that introduced radio labelled molecules will equilibrate with unlabelled molecules present in the sample. The amount of substance initially present can be worked out from the change in specific activity of the radioisotope when it is diluted by the 'cold' material. A method is required whereby the substance can be purified from the sample and sufficient substance must be present for its mass to be measured accurately. Activation analysis is a sensitive technique for the determination of element concentration. It is based upon selectivity inducing radioactivity in some of the atoms of the elements comprising the sample and then selectively measuring the radiations emitted by the radionuclides. After bombardment with suitable nuclear particles, the induced radionuclides are identified or quantitatively measured. Neutron activation analysis is the most common method of analysis.