10 —
Radiochemical Methods in Analysis
Summary
Principles
Emission of ionizing radiations in radioactive decay, nuclear particle and γ-ray spectrometry.
Quantitative and qualitative analysis by intensity and spectrometric measurements respectively.
Instrumentation
Gas ionization, solid scintillation, liquid scintillation and semiconductor detectors, autoradiography.
Single and multichannel pulse height analysers. Coincidence and anticoincidence circuits.
Applications
Study of chemical pathways in method development. Isotope dilution methods. Radioimmunoassay
very important in biochemistry and medicine. Neutron activation analysis used for trace elements in
geo-chemistry, semiconductor technology, pollution studies and forensic science. Relative precision of
counting 1% if 10^4 counts are recorded. Assessment of pollution by radionuclides.
Disadvantages
Sometimes expensive for tracers or irradiation facilities. Special laboratory and handling facilities
required. Needs highly skilled operators, and complex instrumentation.
Three main features account for the usefulness of radiotracers in analysis. Firstly, a chemical species
may be 'labelled' with a radioactive atom and thus distinguished from non-labelled species of the same
type. Only in exceptional circumstances (notably with tritium labels) does the introduction of the label
materially affect the chemical behaviour of the species. In the special case of tritium the variations in
behaviour (known as isotope effects) arise from the large mass differences between the nuclides
and. Secondly, the radioactive characteristics of an atom are unchanged by its chemical
environment, so that a label can always be detected wherever it appears. Thirdly, the particles and
radiation emitted during radioactive decay are of high energy and may be detected with extreme
sensitivity.