By virtue of its radioactive decay, it can be used to explore the mechanism of chemical reactions by tracing the path that the radioisotope follows from reactants to products.
Radioisotopes of hydrogen, carbon, phosphorus, sulfur, and iodine have been used extensively to trace the path of biochemical reactions.
The power of the technique is due to the fact that radioactive decay is much more energetic than chemical reactions.
George de Hevesy won the 1943 Nobel Prize for Chemistry "for his work on the use of isotopes as tracers in the study of chemical processes".
There are two main ways in which radioactive tracers are used The commonly used radioisotopes have short half lives and so do not occur in nature in large amounts.
However, hydrogen atoms are present in all organic compounds, so tritium is frequently used as a tracer in biochemical studies.
It is continuously produced in the upper atmosphere of the earth, so it occurs at a trace level in the environment.
It is produced by the nuclear reaction 13N is used in positron emission tomography (PET scan).
The molybdenum isotope has a half-life of approximately 66 hours (2.75 days), so the generator has a useful life of about two weeks.
Most commercial 99mTc generators use column chromatography, in which 99Mo in the form of molybdate, MoO42− is adsorbed onto acid alumina (Al2O3).
Different ligands form coordination complexes which give the technetium enhanced affinity for particular sites in the human body.
The emitted 159 keV gamma ray is used in single-photon emission computed tomography (SPECT).
125I is frequently used in radioimmunoassays because of its relatively long half-life (59 days) and ability to be detected with high sensitivity by gamma counters.
67Ga is used because, like 99mTc, it is a gamma-ray emitter and various ligands can be attached to the Ga3+ ion, forming a coordination complex which may have selective affinity for particular sites in the human body.
Radioactive tracers are also used to study lipoprotein metabolism in humans and experimental animals.
[10] In medicine, tracers are applied in a number of tests, such as 99mTc in autoradiography and nuclear medicine, including single-photon emission computed tomography (SPECT), positron emission tomography (PET) and scintigraphy.
In recent years, the use of substances enriched in the non-radioactive isotope 13C has become the preferred method, avoiding patient exposure to radioactivity.
In the United States amounts per injection of radionuclide are listed in the US Nuclear Regulatory Commission (NRC) guidelines.
[12] A 2003 publication by the International Atomic Energy Agency confirms the frequent use of most of the tracers above, and says that manganese-56, sodium-24, technetium-99m, silver-110m, argon-41, and xenon-133 are also used extensively because they are easily identified and measured.