In order to design an experiment where the radiotracer allows to properly track a process or a phenomenon, some basic assumptions have to be listed.

Hypothesis 1

The radioactive isotope of a given element behaves identically as the stable isotopes of the same element.

Answer

This is not exactly true due to the isotopic effect, which is particularly evident with light elements. However, it can be minimized or ignored, if the effects are only between different molecules (intermolecular), so that the intramolecular type of reaction is not involved. When the isotopic effect is intermolecular, it could lead to differences in the initial phase of reaction, that are compensated when the reaction is completed.

Hypothesis 2

The radioactivity does not change the chemical and physical properties of the experimental system.

Answer

If not, the radioactivity would affect the experimental results. To overcome this issue, the amount of activity is set as the minimum that allows to detect a reasonable counting rate. The possible presence of daughter radionuclides should also be taken into account as these should not affect the measurement. For example, if both the radiotracer and the daughter radionuclides are β- emitters, it is not possible to distinguish their contribution to the count rate.

Hypothesis 3

The chemical and physical form of the radiotracer should be the same as the unlabeled compound that undergoes the process.

Answer

Since the radiotracer is used in low concentrations, it is important to consider all the effects that could alter its concentration, for example absorption on surfaces of the container of the system where the radiotracer is injected. This could be prevented by the addition of a carrier, a non-radioactive compound that fill the absorption sites. In particular, it is important to assure that the oxidation state of the radionuclide and the stable nuclide is the same. The radionuclide produced in reactors may have different oxidation states.

Hypothesis 4

The total radioactivity in the sample is due only to the desired radionuclide.

Answer

Radiochemical purity cannot be assumed since the radiotracer compound usually contain a small amount of impurities, also due to the decay of the radiotracer. These impurities are usually in very low concentration but high specific activities. The problem becomes particularly evident with aged tritium or C-14 labeled compounds: due to the short range of the low energy beta-particles that can cause self-decomposition. This is taken into account during data elaboration, by applying proper corrections.

Hypothesis 5

Only the labeled atoms are detected.

Answer

It is important to consider that only the radionuclide itself is followed, and not the original labeled molecule as a whole. Cleavage of the labeled atom and chemical exchange reactions may occur leading to a loss of the labeled compound.

References

  • W. D. Loveland, D. J. Morrissey, G. T. Seaborg, Modern Nuclear Chemistry, 2006, John Wiley & Sons, Inc. DOI:10.1002/0471768626