Some radiotracers can be generated by “nature” itself, such as Rn-222: these are called natural radiotracers. They are mainly used to trace processes in environmental, biological and agricultural studies. However, most of the commonly used radioisotopes have short half-lives and so are not present in nature: these are artificial radionuclides. They can be produced with three different methods:

  • nuclear reactor, by neutron induced reactions, (n,γ), (n,p) and (n,α);
  • cyclotron, by charged particle induced reactions;
  • as fission products nuclei, by chemical separation from irradiated uranium.

Then, radionuclides can be used directly or, more often, a compound is created. The compound is qualified with respect to the position of the radiotracer. It can either be specifically, uniformly, nominally or generally labeled.
The optimal solution is specifically labeled compounds, where the radionuclide is in a precise position. If the radionuclide is present in a nearly uniform pattern inside the compounds, it is called uniformly labeled. When the position is specific but it may be not necessarily exclusive, in other

words, when it cannot be assured that the radionuclide is not also found in other positions, the compound is nominally labeled. When the position of the radionuclide is completely random, the compound is called generally labeled. The compound containing the radiotracer can be synthesized with standard synthetic procedures, by chemical synthesis, or it can be synthesized by the action of living organisms or active enzyme molecules, this is called biosynthesis. This latter technique has the advantage of producing quickly natural compounds. One example is the production of labeled molecules with plant photosynthesis: when one of the precursors is substituted with a “labeled” precursor, which can be cheaper than the desired compound, the final products are labeled. However, these compounds have to be separated from complex mixtures.

THE PURITY
The importance of using compounds of known purity for any tracer investigation is widely recognized. The data obtained should be related to the radiochemical tracer compound and not be a consequence of the presence of other impurities. The well-known instability of radiochemicals during storage period has to be taken into account

prior to their use: the decay of the radionuclide, self- or radiolytic decomposition lead to the formation of other elements and compounds.

In order to assure a good radiotracer quality, both nuclidic and radiochemical purity must be considered. Nuclidic purity refers to the fraction of the total activity that is due to the selected radionuclide. The presence of another radioactive isotope of the same element could decrease the purity. Nevertheless, it is not a problem if the measurement allows a clear distinction of the selected nuclide from the others. Commercial radionuclides are available with nuclidic purity above 98% but it decreases with time.
Radiochemical purity is the fraction of the radioactive compound present in the stated chemical form. The radionuclide could be in different oxidation states, or the compound could be labeled in different positions. It is preferable to have a unique chemical form but, if the chemical behavior of the compound is not affected, the presence of different chemical forms may be acceptable. The radiochemical purity also decreases over time due to oxidation, self- or radiolytic decomposition. It is recommended to purify radiotracer compounds prior to use.