What is radiochemical purity?
Radiochemical purity refers to the proportion of a radionuclide that is present in the desired chemical form. Even if the correct radionuclide is present (high radionuclidic purity), it may exist in multiple chemical species, not all of which are suitable for imaging.
Radiochemical purity describes the proportion of a radionuclide present in the correct chemical form required for appropriate biodistribution and imaging.
In Tc-99m radiopharmaceuticals, radiochemical purity ensures that technetium is correctly bound to the intended ligand and not present as free pertechnetate or hydrolysed technetium.
High radiochemical purity is essential for correct biodistribution and accurate imaging.
Understanding the physics
Radiochemical purity concerns the chemical form of the radioactive atom, not its nuclear identity.
For example, when Tc-99m is eluted from a generator, it is present as sodium pertechnetate (TcO₄⁻). This form may then be chemically bound to a pharmaceutical compound to target a specific organ.
However, several unwanted chemical species may form:
Free pertechnetate (unbound TcO₄⁻)
Hydrolysed-reduced technetium (insoluble colloidal forms)
Incorrectly labelled complexes
Even though all these species contain Tc-99m, they behave differently in the body.
Free pertechnetate accumulates in the thyroid, salivary glands, and stomach. Hydrolysed technetium may localise in the liver or spleen. These unintended distributions degrade image quality and may increase radiation dose to non-target tissues.
Radiochemical purity is typically assessed using techniques such as thin-layer chromatography (TLC), which separates chemical species and allows measurement of the fraction present in the desired form.
It is important to distinguish radiochemical purity from radionuclidic purity:
Radionuclidic purity asks: “Is this the correct radioactive isotope?”
Radiochemical purity asks: “Is the isotope in the correct chemical form?”
Where this matters clinically
Poor radiochemical purity leads to altered biodistribution and non-diagnostic images. For example, excessive free pertechnetate in a bone scan preparation may result in unwanted thyroid uptake.
Routine radiopharmaceutical quality control ensures radiochemical purity before administration to patients.