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 t½ by assuming how much was originally
- 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.