A radioactive tracer, or radioactive label, is a chemical compound in which
one or more atoms have been replaced by a radioisotope so by virtue of its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. Radiolabeling is thus the radioactive form of isotopic labeling.
Radioisotopes of hydrogen, carbon, phosphorus, sulphur, and iodine have
been used extensively to trace the path of biochemical reactions. A radioactive
tracer can also be used to track the distribution of a substance within a
natural system such as a cell or tissue, or as a flow tracer to track fluid flow.
Radioactive tracers are also used to determine the location of fractures
created by hydraulic fracturing in natural gas production. Radioactive
tracers form the basis of a variety of imaging systems, such as, PET scans,
SPECT scans and technetium scans. Radiocarbon dating uses the naturally
occurring carbon-14 isotope as an isotopic label.
Methodology Isotopes of a chemical element differ
only in the mass number. For example, the isotopes of hydrogen can be written
as 1H, 2H and 3H, with the mass number at top left. When the atomic nucleus of
an isotope is unstable, compounds containing this isotope are radioactive.
Tritium is an example of a radioactive isotope.
The principle behind the use of radioactive tracers is that an atom in a
chemical compound is replaced by another atom, of the same chemical element. The
substituting atom, however, is a radioactive isotope. This process is
often called radioactive labeling. The power of the technique is due to the
fact that radioactive decay is much more energetic than chemical reactions.
Therefore, the radioactive isotope can be present in low concentration and its
presence detected by sensitive radiation detectors such as Geiger counters and
scintillation counters. 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 When a labeled chemical compound
undergoes chemical reactions one or more of the products will contain the
radioactive label. Analysis of what happens to the radioactive isotope
provides detailed information on the mechanism of the chemical reaction.
A radioactive compound is introduced into a living organism and the
radio-isotope provides a means to construct an image showing the way in
which that compound and its reaction products are distributed around the
organism. Production
The commonly used radioisotopes have short half lives and so do not occur in
nature. They are produced by nuclear reactions. One of the most important
processes is absorption of a neutron by an atomic nucleus, in which the mass
number of the element concerned increases by 1 for each neutron
absorbed. For example, 13C + n → 14C
In this case the atomic mass increases, but the element is unchanged. In other
cases the product nucleus is unstable and decays, typically emitting protons,
electrons( beta particle) or alpha particles. When a nucleus loses a proton
the atomic number decreases by 1. For example,
32S + n → 32P + p Neutron irradiation is performed in a
nuclear reactor. The other main method used to synthesize radioisotopes is
proton bombardment. The proton are accelerated to high energy either in a
cyclotron or a linear accelerator. Tracer isotopes
= Hydrogen = Tritium is produced by neutron
irradiation of 6Li 6Li + n → 4He + 3H
Tritium has a half-life 4,500±8 days, 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 y. 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.
= Oxygen = 15O decays by positron emission with a
half-life of 122 sec. It is used in positron emission tomography
= Fluorine = 18F decays by positron emission with a
half-life of 109 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. It is used to make labeled fluorodeoxyglucose 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 + e− + ν
The molybdenum isotope has a half-life of approximately 66 hours, 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. 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 124Xe. 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. A
127 keV gamma ray is also emitted. 125I is frequently used in
radioimmunoassays because of its relatively long half-life 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. Application
In metabolism research, Tritium and 14C-labeled glucose are commonly used in
glucose clamps to measure rates of glucose uptake, fatty acid synthesis,
and other metabolic processes. While radioactive tracers are sometimes still
used in human studies, stable isotope tracers such as 13C are more commonly
used in current human clamp studies. Radioactive tracers are also used to
study lipoprotein metabolism in humans and experimental animals.
In medicine, tracers are applied in a number of tests, such as 99mTc in
autoradiography and nuclear medicine, including single photon emission
computed tomography, positron emission tomography and scintigraphy. The urea
breath test for helicobacter pylori commonly used a dose of 14C labeled urea
to detect h. pylori infection. If the labeled urea was metabolized by h.
pylori in the stomach, the patient's breath would contain labeled carbon
dioxide. 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 hydraulic fracturing, radioactive
tracer isotopes are injected with hydraulic fracturing fluid to determine
the injection profile and location of created fractures. Tracers with
different half-lives are used for each stage of hydraulic fracturing. In the
United States amounts per injection of radionuclide are listed in the US
Nuclear Regulatory Commission guidelines. According to the NRC, some
of the most commonly used tracers include antimony-124, bromine-82,
iodine-125, iodine-131, iridium-192, and scandium-46. 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. References
External links National Isotope Development Center U.S.
Government resources for radioisotopes - production, distribution, and
information Isotope Development & Production for
Research and Applications U.S. Department of Energy program sponsoring
isotope production and production research and development