There are individual monographs for the identity testing of radionuclides for the purpose of radiopharmaceutical preparations.
- Half-life measurement: The preparation is recommended to be tested after appropriate dilution as this would avoid necessary losses. The following equipment must be available during this process: ionizing chamber, Geiger-Muller counter, and a scintillation counter (or a semiconductor detector).
Sufficient radioactivity during this period is required in order to easily detect several estimates of the material’s half-life, and measurements of such should not deviate by more than 5% from what is stated in the specific monograph.
Once the exact radiopharmaceutical has been properly prepared and identified, the purity of the radionuclide must then be observed.
Two ways to specify radionuclide purity:
- Expression of a minimum level of purity: As per individual monographs, the preparation’s gamma-ray spectrum should not vary as much from the standard solution during its entire shelf-life.
- Expression of maximum levels of specific impurities: There are known impurities during the production of the preparation – for example, the presence of thallium-202 (t1/2 = 12.23d) during preparation of thalium-201 (t1/2 = 73.5h).
For the actual radiochemical itself, purity is therefore assessed via analytical techniques such as liquid, paper, thin-layer chromatographs and even electrophoresis. Radioactivity distribution n the chromatogram must be done after or during separation.
The method of choice to measure radiochemical activity depends on the nature of the radiation and the actual separation technique. Normally the amount of radioactive material used in chromatographic support (paper/plate/column) is very small due to the high-sensitivity of detection; caution must be observed during this process, especially with the interpretation of results.
Adding to the complexity of the process of measuring radiochemical purity is the presence of carriers (non-reactive compounds), as risks of the latter reacting with certain radiochemical impurities is ever present; this can further lead to impurity underestimation. For situations that a simple chromatograph fails, high performance liquid chromatography (HPLC) is therefore the next choice.