Radioactive tracer
Encyclopedia
A radioactive tracer, also called a radioactive label, is a substance containing a radioisotope that is used to measure the speed of chemical processes and to track the movement of a substance through a natural system such as a cell
or tissue
. A number of different radioactive forms of hydrogen
, carbon
, phosphorus
, sulfur
, and iodine
are commonly used in applications including biochemical assays, metabolism studies, and medical diagnostics.
, who won the 1943 Nobel Prize for Chemistry for his pioneering work using radioactive tracers to study metabolic processes in plants and animals.
, scintillation counter
or other type of radiation detection instrument.
(3H) is a radioactive form of hydrogen that contains one proton and two neutrons in its nucleus. Tritium decays into helium-3
by emission of a low energy beta particle
. The low energy of the emitted particle causes tritium to have low detection efficiency by scintillation counting. However, due to the abundance of hydrogen in organic compounds, tritium is frequently used as a tracer in biochemical
studies.
. Carbon-11 is frequently used as a tracer in positron emission tomography
, an imaging technique that allows for three-dimensional imaging of functional processes in the human body.
Carbon-14
is a radioactive form of carbon that contains eight neutrons and six protons in its nucleus. Carbon-14 decays into nitrogen-14 by emission of a beta particle. Carbon-14 has been used as a tracer in both medical and scientific tests. Carbon-14 is frequently used to trace carbons through metabolic pathways.
is a radioactive form of phosphorus that contains 17 neutrons and 15 protons in its nucleus. Phosphorus-32 decays into sulfur-32 by emission of a beta particle. Phosphorus-32 is frequently used to label amino acids and phosphoproteins and is commonly used to study protein phosphorylation by kinases in biochemistry. Phosphorus-32 emits a relatively high energy beta particle, and a number of safety precautions are needed when working with it.
Phosphorus-33 is a radioactive form of phosphorus that contains 18 neutrons and 15 protons in its nucleus. Phosphorus-33 decays into sulfur-33 by emission of a beta particle. Phosphorus-33 is used similarly to phosphorus-32, except that it emits less energetic beta particles, allowing for higher resolution assays and requiring less usage of safety equipment. Phosphorus-33 is less common and more expensive to produce than phosphorus-32.
group on a given nucleotide
to create a thiophosphate
which has very similar biochemical properties to the original phosphate group.
is a radioactive form of iodine that contains 70 neutrons and 53 protons in its nucleus. Iodine-123 decays into tellurium-123 by electron capture
, producing gamma rays. Iodine-123 is used in nuclear medicine
imaging, specifically to study thyroid
function. Because of its short half-life
, Iodine-123 is the most frequently used isotope in thyroid function studies.
Iodine-125
is a radioactive form of iodine that contains 72 neutrons and 53 protons in its nucleus. Iodine-125 decays into tellurium-125 by electron capture, producing gamma rays. Iodine-125 is frequently used in radioimmunoassay
s because of its relatively long half-life and ability to be detected with high sensitivity by gamma counters.
research, radioactive tracers are frequently used in glucose clamps
to measure rates of glucose uptake
, fatty acid synthesis
, and other metabolic processes. Tritium
and carbon-14
-labeled water and glucose are commonly used tracers for metabolic clamp studies. While radioactive tracers are sometimes still used in human studies, stable isotope
tracers such as carbon-13
are more commonly used in current human clamp studies. Radioactive tracers are also used to study lipoprotein
metabolism in humans and experimental animals.
, tracers are applied in a number of tests, such as technetium-99
in autoradiography and nuclear medicine
, including single photon emission computed tomography (SPECT), positron emission tomography
(PET) and scintigraphy
. The urea breath test
for helicobacter pylori
commonly used a dose of carbon-14 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 the stable isotope carbon-13 has become the preferred method to minimize patient exposure to radiation.
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
or tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
. A number of different radioactive forms of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
, sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, and iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
are commonly used in applications including biochemical assays, metabolism studies, and medical diagnostics.
History
Radioactive tracing was developed by George de HevesyGeorge de Hevesy
George Charles de Hevesy, Georg Karl von Hevesy, was a Hungarian radiochemist and Nobel laureate, recognized in 1943 for his key role in the development of radioactive tracers to study chemical processes such as in the metabolism of animals.- Early years :Hevesy György was born in Budapest,...
, who won the 1943 Nobel Prize for Chemistry for his pioneering work using radioactive tracers to study metabolic processes in plants and animals.
Methodology
Radioactive tracers are compounds that contain one or more radioactive atoms that allows for easy detection and measurement. Tracers are frequently used to track the localization of a specific compound or to trace the path of a compound through a series of chemical reactions. A radioactive tracer is identical in chemical composition to the compound of interest and is administered in minute amounts that do not perturb the experimental system. The tracer behaves in exactly the same way as an unlabeled molecule, but the tracer molecule continually gives off radiation that can be detected with a Geiger counterGeiger counter
A Geiger counter, also called a Geiger–Müller counter, is a type of particle detector that measures ionizing radiation. They detect the emission of nuclear radiation: alpha particles, beta particles or gamma rays. A Geiger counter detects radiation by ionization produced in a low-pressure gas in a...
, scintillation counter
Scintillation counter
A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic , or organic liquid that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube measures the light from the crystal...
or other type of radiation detection instrument.
Tracer isotopes
A number of different radioisotopes are used as radioactive tracers depending on the application.Hydrogen
TritiumTritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
(3H) is a radioactive form of hydrogen that contains one proton and two neutrons in its nucleus. Tritium decays into helium-3
Helium-3
Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...
by emission of a low energy beta particle
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
. The low energy of the emitted particle causes tritium to have low detection efficiency by scintillation counting. However, due to the abundance of hydrogen in organic compounds, tritium is frequently used as a tracer in biochemical
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
studies.
Carbon
Carbon-11 is a radioactive form of carbon that contains five neutrons and six protons in its nucleus. Carbon-11 decays into boron-11 by positron emissionPositron emission
Positron emission or beta plus decay is a type of beta decay in which a proton is converted, via the weak force, to a neutron, releasing a positron and a neutrino....
. Carbon-11 is frequently used as a tracer in positron emission tomography
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
, an imaging technique that allows for three-dimensional imaging of functional processes in the human body.
Carbon-14
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues , to date archaeological, geological, and hydrogeological...
is a radioactive form of carbon that contains eight neutrons and six protons in its nucleus. Carbon-14 decays into nitrogen-14 by emission of a beta particle. Carbon-14 has been used as a tracer in both medical and scientific tests. Carbon-14 is frequently used to trace carbons through metabolic pathways.
Phosphorus
Phosphorus-32Phosphorus-32
Phosphorus-32 is a radioactive isotope of phosphorus. The nucleus of phosphorus-32 contains 15 protons and 17 neutrons, one more neutron than the most common isotope of phosphorus, phosphorus-31...
is a radioactive form of phosphorus that contains 17 neutrons and 15 protons in its nucleus. Phosphorus-32 decays into sulfur-32 by emission of a beta particle. Phosphorus-32 is frequently used to label amino acids and phosphoproteins and is commonly used to study protein phosphorylation by kinases in biochemistry. Phosphorus-32 emits a relatively high energy beta particle, and a number of safety precautions are needed when working with it.
Phosphorus-33 is a radioactive form of phosphorus that contains 18 neutrons and 15 protons in its nucleus. Phosphorus-33 decays into sulfur-33 by emission of a beta particle. Phosphorus-33 is used similarly to phosphorus-32, except that it emits less energetic beta particles, allowing for higher resolution assays and requiring less usage of safety equipment. Phosphorus-33 is less common and more expensive to produce than phosphorus-32.
Sulfur
Sulfur-35 is a radioactive form of sulfur that contains 18 neutrons and 16 protons in its nucleus. Sulfur-35 decays into chlorine-35 by emission of a beta particle. Sulfur-35 is frequently used as a tracer in biochemical experiments, where it is used to label amino acids and nucleic acids containing sulfur. Alternatively, a labeled sulfur can replace an oxygen in a phosphatePhosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
group on a given nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
to create a thiophosphate
Thiophosphate
A thiophosphate is a family of compounds and anions with the general chemical formula PS4-xOx3- . The state of protonation is usually not specified. They could be bound to as many as three protons for the neutral H3PS4-xOx species. Two protons correspond to the related monoanions, and one...
which has very similar biochemical properties to the original phosphate group.
Iodine
Iodine-123Iodine-123
Iodine-123 is a radioactive isotope of iodine used in nuclear medicine imaging, including single photon emission computed tomography . The isotope's half-life is 13.22 hours; the decay by electron capture to tellurium-123 emits gamma radiation with predominant energies of 159 keV and 127 keV...
is a radioactive form of iodine that contains 70 neutrons and 53 protons in its nucleus. Iodine-123 decays into tellurium-123 by electron capture
Electron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
, producing gamma rays. Iodine-123 is used in nuclear medicine
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...
imaging, specifically to study thyroid
Thyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...
function. Because of its short half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
, Iodine-123 is the most frequently used isotope in thyroid function studies.
Iodine-125
Iodine-125
Iodine-125 is a radioisotope of iodine which has uses in biological assays, nuclear medicine imaging and in radiation therapy as brachytherapy to treat prostate cancer and brain tumors. It is the second longest-lived radioisotope of iodine, after iodine-129.Its half-life is around 59 days and it...
is a radioactive form of iodine that contains 72 neutrons and 53 protons in its nucleus. Iodine-125 decays into tellurium-125 by electron capture, producing gamma rays. Iodine-125 is frequently used in radioimmunoassay
Radioimmunoassay
Radioimmunoassay is a very sensitive in vitro assay technique used to measure concentrations of antigens by use of antibodies...
s because of its relatively long half-life and ability to be detected with high sensitivity by gamma counters.
Metabolic Research
In metabolismMetabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
research, radioactive tracers are frequently used in glucose clamps
Glucose clamp technique
Glucose clamp technique is a method for quantifying insulin secretion and resistance. It is used to measure either how well an individual metabolizes glucose or how sensitive an individual is to insulin.Two types of clamps are quite commonly used...
to measure rates of glucose uptake
Glucose uptake
Method of glucose uptake differs throughout tissues depending on two factors; the metabolic needs of the tissue and availability of glucose. The two ways in which glucose uptake can take place are facilitated diffusion and secondary active transport .- Facilitated diffusion :There are over 10...
, fatty acid synthesis
Fatty acid synthesis
Fatty acid synthesis is the creation of fatty acids from acetyl-CoA and malonyl-CoA precursors through action of enzymes called fatty acid synthases...
, and other metabolic processes. Tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
and carbon-14
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues , to date archaeological, geological, and hydrogeological...
-labeled water and glucose are commonly used tracers for metabolic clamp studies. While radioactive tracers are sometimes still used in human studies, stable isotope
Stable isotope
Stable isotopes are chemical isotopes that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay, in theory...
tracers such as carbon-13
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...
are more commonly used in current human clamp studies. Radioactive tracers are also used to study lipoprotein
Lipoprotein
A lipoprotein is a biochemical assembly that contains both proteins and lipids water-bound to the proteins. Many enzymes, transporters, structural proteins, antigens, adhesins, and toxins are lipoproteins...
metabolism in humans and experimental animals.
Diagnostics
In medicineMedicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
, tracers are applied in a number of tests, such as technetium-99
Technetium-99
Technetium-99 is an isotope of technetium which decays with a half-life of 211,000 years to stable ruthenium-99, emitting soft beta rays, but no gamma rays....
in autoradiography and nuclear medicine
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...
, including single photon emission computed tomography (SPECT), positron emission tomography
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
(PET) and scintigraphy
Gamma camera
A gamma camera, also called a scintillation camera or Anger camera, is a device used to image gamma radiation emitting radioisotopes, a technique known as scintigraphy...
. The urea breath test
Urea breath test
The urea breath test is a rapid diagnostic procedure used to identify infections by Helicobacter pylori, a spiral bacterium implicated in gastritis, gastric ulcer, and peptic ulcer disease. It is based upon the ability of H. pylori to convert urea to ammonia and carbon dioxide...
for helicobacter pylori
Helicobacter pylori
Helicobacter pylori , previously named Campylobacter pyloridis, is a Gram-negative, microaerophilic bacterium found in the stomach. It was identified in 1982 by Barry Marshall and Robin Warren, who found that it was present in patients with chronic gastritis and gastric ulcers, conditions that were...
commonly used a dose of carbon-14 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 the stable isotope carbon-13 has become the preferred method to minimize patient exposure to radiation.
External links
- National Isotope Development Center U.S. Government resources for radioisotopes - production, distribution, and information
- Isotope Development & Production for Research and Applications (IDPRA) U.S. Department of Energy program sponsoring isotope production and production research and development