Radionuclidic purity

Radionuclidic purity is the proportion of the total radioactivity in a preparation that is due to the stated, intended radionuclide, as opposed to any other radioactive species (radionuclidic impurities) that may have formed alongside it during production. It concerns the identity of the radioactive atoms themselves, not their chemical form (that is radiochemical purity) or the mass of the labelled compound (that is specific activity).

Radionuclidic impurities typically arise from side reactions in the target material during cyclotron or reactor production, or from parent/daughter contamination in a generator system. A well-known example is a technetium-99m generator eluate, where a small amount of the parent molybdenum-99 can break through with the technetium-99m and must be kept below regulatory limits; another is actinium-225 production, where the accompanying actinium-227 impurity level is a live manufacturing and regulatory concern. For fluorine-18 FDG, IAEA guidance specifies acceptance criteria including a measured half-life within 105-115 minutes and no less than 99% of gamma emissions attributable to 18F, verified by gamma-ray spectrometry, which distinguishes radionuclides by the characteristic energies they emit.

Radionuclidic purity is a mandatory release test because an unexpected radionuclide contributes unplanned dose to the patient (with its own, possibly very different, half-life and emission type) and can also corrupt quantitative imaging or dosimetry that assumes a single, known decay scheme.