Hydrogen Tritium (hydrogen-3) is produced by neutron irradiation of
6Li: :
6Li +
n →
4He +
3H Tritium has a
half-life days (approximately 12.32 years) and it decays by
beta decay. The
electrons produced have an average energy of 5.7 keV. Because the emitted electrons have relatively low energy, the detection efficiency by scintillation counting is rather low. However, hydrogen atoms are present in all organic compounds, so tritium is frequently used as a tracer in
biochemical studies.
Carbon 11C decays by
positron emission with a half-life of ca. 20 min. 11C is one of the isotopes often used in
positron emission tomography.
14C decays by
beta decay, with a half-life of 5730 years. It is continuously produced in the upper atmosphere of the earth, so it occurs at a trace level in the environment. However, it is not practical to use naturally occurring 14C for tracer studies. Instead it is made by neutron irradiation of the isotope
13C which occurs naturally in carbon at about the 1.1% level. 14C has been used extensively to trace the progress of organic molecules through metabolic pathways.
Nitrogen 13N decays by
positron emission with a half-life of 9.97 min. It is produced by the nuclear reaction :
1H +
16O →
13N +
4He 13N is used in
positron emission tomography (PET scan).
Oxygen 15O decays by positron emission with a half-life of 122 seconds. It is used in positron emission tomography.
Fluorine 18F decays predominantly by β emission, with a half-life of 109.8 min. It is made by proton bombardment of
18O in a cyclotron or
linear particle accelerator. It is an important isotope in the
radiopharmaceutical industry. For example, it is used to make labeled
fluorodeoxyglucose (FDG) or Fluoroalanine|D-3-[18F]fluoroalanine for application in PET scans.
Phosphorus 32P is made by neutron bombardment of
32S :
32S +
n →
32P +
p It decays by beta decay with a half-life of 14.29 days. It is commonly used to study protein phosphorylation by
kinases in biochemistry.
33P is made in relatively low yield by neutron bombardment of
31P. It is also a beta-emitter, with a half-life of 25.4 days. Though more expensive than
32P, the emitted electrons are less energetic, permitting better resolution in, for example, DNA sequencing. Both isotopes are useful for labeling
nucleotides and other species that contain a
phosphate group.
Sulfur 35S is made by neutron bombardment of
35Cl :
35Cl +
n →
35S +
p It decays by beta-decay with a half-life of 87.51 days. It is used to label the sulfur-containing
amino-acids
methionine and
cysteine. When a sulfur atom replaces an oxygen atom in a
phosphate group on a
nucleotide a
thiophosphate is produced, so 35S can also be used to trace a phosphate group.
Technetium 99mTc is a very versatile radioisotope, and is the most commonly used radioisotope tracer in medicine. It is easy to produce in a
technetium-99m generator, by decay of
99Mo. :99Mo → 99mTc + + 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). When the 99Mo decays it forms
pertechnetate TcO4−, which because of its single charge is less tightly bound to the alumina. Pulling normal saline solution through the column of immobilized 99Mo elutes the soluble 99mTc, resulting in a saline solution containing the 99mTc as the dissolved sodium salt of the pertechnetate. The pertechnetate is treated with a
reducing agent such as
Sn2+ and a
ligand. Different ligands form
coordination complexes which give the technetium enhanced affinity for particular sites in the human body. 99mTc decays by gamma emission, with a half-life: 6.01 hours. The short half-life ensures that the body-concentration of the radioisotope falls effectively to zero in a few days.
Iodine 123I is produced by proton irradiation of 124
Xe. The
caesium isotope produced is unstable and decays to 123I. The isotope is usually supplied as the iodide and hypoiodate in dilute sodium hydroxide solution, at high isotopic purity. 123I has also been produced at Oak Ridge National Laboratories by proton bombardment of
123Te. 123I decays by
electron capture with a half-life of 13.22 hours. The emitted 159
keV gamma ray is used in
single-photon emission computed tomography (SPECT). A 127 keV gamma ray is also emitted.
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.
129I is present in the environment as a result of the testing of
nuclear weapons in the atmosphere. It was also produced in the
Chernobyl and
Fukushima disasters. 129I decays with a
half-life of 15.7 million years, with low-energy
beta and
gamma emissions. It is not used as a tracer, though its presence in living organisms, including human beings, can be characterized by measurement of the gamma rays.
Other isotopes Many other isotopes have been used in specialized radiopharmacological studies. The most widely used is
67Ga for
gallium scans. 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. An extensive list of radioactive tracers used in hydraulic fracturing can be found below. ==Applications==