A radioactive tracer, radiotracer, or radioactive label is a synthetic derivative of a natural compound in which one or more atoms have been replaced by a radionuclide (a radioactive atom). 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 or radiotracing is thus the radioactive form of isotopic labeling. In biological contexts, experiments that use radioisotope tracers are sometimes called radioisotope feeding experiments.
Radioisotopes of hydrogen, carbon, phosphorus, sulfur, 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 H, H and H, with the mass number superscripted to the 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 in large amounts. 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,
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,
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 (hydrogen-3) is produced by neutron irradiation of Li:
Tritium has a half-life 4500±8 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
C decays by positron emission with a half-life of ca. 20 min. C is one of the isotopes often used in positron emission tomography.
C 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 C for tracer studies. Instead it is made by neutron irradiation of the isotope C which occurs naturally in carbon at about the 1.1% level. C has been used extensively to trace the progress of organic molecules through metabolic pathways.
Nitrogen
N decays by positron emission with a half-life of 9.97 min. It is produced by the nuclear reaction
N is used in positron emission tomography (PET scan).
Oxygen
O decays by positron emission with a half-life of 122 seconds. It is used in positron emission tomography.
Fluorine
F decays predominantly by β emission, with a half-life of 109.8 min. It is made by proton bombardment of O 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) for application in PET scans.
Phosphorus
P is made by neutron bombardment of S
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.
P is made in relatively low yield by neutron bombardment of P. It is also a beta-emitter, with a half-life of 25.4 days. Though more expensive than P, 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
S is made by neutron bombardment of Cl
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 S can also be used to trace a phosphate group.
Technetium
Main article: 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 Mo.
- Mo → Tc +
e
+
ν
e
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 Tc generators use column chromatography, in which Mo in the form of molybdate, MoO4 is adsorbed onto acid alumina (Al2O3). When the Mo 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 Mo elutes the soluble Tc, resulting in a saline solution containing the Tc as the dissolved sodium salt of the pertechnetate. The pertechnetate is treated with a reducing agent such as Sn and a ligand. Different ligands form coordination complexes which give the technetium enhanced affinity for particular sites in the human body.
Tc 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
Main article: Isotopes of iodineI is produced by proton irradiation of Xe. The caesium isotope produced is unstable and decays to I. The isotope is usually supplied as the iodide and hypoiodate in dilute sodium hydroxide solution, at high isotopic purity. I has also been produced at Oak Ridge National Laboratories by proton bombardment of Te.
I 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.
I 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.
I 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. I 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
Main article: RadiopharmacologyMany other isotopes have been used in specialized radiopharmacological studies. The most widely used is Ga for gallium scans. Ga is used because, like Tc, it is a gamma-ray emitter and various ligands can be attached to the Ga 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
See also: Nuclear medicine, List of PET radiotracers, and Radionuclides associated with hydraulic fracturingIn metabolism research, tritium and C-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 C 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 Tc 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 C 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 C 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 (NRC) 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
- Rennie MJ (November 1999). "An introduction to the use of tracers in nutrition and metabolism". The Proceedings of the Nutrition Society. 58 (4): 935–44. doi:10.1017/S002966519900124X. PMID 10817161.
- ^ Reis, John C. (1976). Environmental Control in Petroleum Engineering. Gulf Professional Publishers.
- ^ Fowler J. S. and Wolf A. P. (1982) The synthesis of carbon-11, fluorine-18 and nitrogen-13 labeled radiotracers for biomedical applications. Nucl. Sci. Ser. Natl Acad. Sci. Natl Res. Council Monogr. 1982.
- Lucas LL, Unterweger MP (2000). "Comprehensive Review and Critical Evaluation of the Half-Life of Tritium" (PDF). Journal of Research of the National Institute of Standards and Technology. 105 (4): 541–9. doi:10.6028/jres.105.043. PMC 4877155. PMID 27551621. Archived from the original (PDF) on 2011-10-17.
- Kim SH, Kelly PB, Clifford AJ (April 2010). "Calculating radiation exposures during use of (14)C-labeled nutrients, food components, and biopharmaceuticals to quantify metabolic behavior in humans". Journal of Agricultural and Food Chemistry. 58 (8): 4632–7. doi:10.1021/jf100113c. PMC 2857889. PMID 20349979.
- I-123 fact sheet
- Hupf HB, Eldridge JS, Beaver JE (April 1968). "Production of iodine-123 for medical applications". The International Journal of Applied Radiation and Isotopes. 19 (4): 345–51. doi:10.1016/0020-708X(68)90178-6. PMID 5650883.
- Gilby ED, Jeffcoate SL, Edwards R (July 1973). "125-Iodine tracers for steroid radioimmunoassay". The Journal of Endocrinology. 58 (1): xx. PMID 4578967.
- Kraegen EW, Jenkins AB, Storlien LH, Chisholm DJ (1990). "Tracer studies of in vivo insulin action and glucose metabolism in individual peripheral tissues". Hormone and Metabolic Research. Supplement Series. 24: 41–8. PMID 2272625.
- Magkos F, Sidossis LS (September 2004). "Measuring very low density lipoprotein-triglyceride kinetics in man in vivo: how different the various methods really are". Current Opinion in Clinical Nutrition and Metabolic Care. 7 (5): 547–55. doi:10.1097/00075197-200409000-00007. PMID 15295275. S2CID 26085364.
- Peeters M (1998). "Urea breath test: a diagnostic tool in the management of Helicobacter pylori-related gastrointestinal diseases". Acta Gastro-Enterologica Belgica. 61 (3): 332–5. PMID 9795467.
- ^ Whitten JE, Courtemanche SR, Jones AR, Penrod RE, Fogl DB, Division of Industrial and Medical Nuclear Safety, Office of Nuclear Material Safety and Safeguards (June 2000). "Consolidated Guidance About Materials Licenses: Program-Specific Guidance About Well Logging, Tracer, and Field Flood Study Licenses (NUREG-1556, Volume 14)". US Nuclear Regulatory Commission. Retrieved 19 April 2012.
labeled Frac Sand...Sc-46, Br-82, Ag-110m, Sb-124, Ir-192
- Radiation Protection and the Management of Radioactive Waste in the Oil and Gas Industry (PDF) (Report). International Atomic Energy Agency. 2003. pp. 39–40. Retrieved 20 May 2012.
Beta emitters, including H and C, may be used when it is feasible to use sampling techniques to detect the presence of the radiotracer, or when changes in activity concentration can be used as indicators of the properties of interest in the system. Gamma emitters, such as Sc, La, Mn, Na, Sb, Ir, Tc, I, Ag, Ar and Xe are used extensively because of the ease with which they can be identified and measured. ... In order to aid the detection of any spillage of solutions of the 'soft' beta emitters, they are sometimes spiked with a short half-life gamma emitter such as Br...
External links
Library resources aboutRadioactive tracers
- 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