Radioactive tracer

Source: Wikipedia, the free encyclopedia.

A radioactive tracer, radiotracer, or radioactive label is 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natural compound in which one or more atoms have been replaced by a radionuclide (a radioactive atom).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

isotopic label
.

Methodology

is an example of a radioactive isotope.

The principle behind the use of radioactive tracers is that an

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

  1. 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.
  2. 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,

13C + n14C

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 + n32P + p

Neutron irradiation is performed in a

linear accelerator.[3]

Tracer isotopes

Hydrogen

Tritium (hydrogen-3) is produced by neutron irradiation of 6Li:

6Li + n4He + 3H

Tritium has a half-life 4500±8 days (approximately 12.32 years)[4] 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.[3]

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.[5]

Nitrogen

13N decays by positron emission with a half-life of 9.97 min. It is produced by the nuclear reaction

1H + 16O13N + 4He

13N is used in positron emission tomography (PET scan).

Oxygen

15O decays by positron emission with a half-life of 122 sec. It is used in positron emission tomography.

Fluorine

fluorodeoxyglucose (FDG) for application in PET scans.[3]

Phosphorus

32P is made by neutron bombardment of 32S

32S + n32P + 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

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
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 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 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.[6] 123I has also been produced at Oak Ridge National Laboratories by proton bombardment of 123Te.[7]

123I decays by

(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.[8]

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

In

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.[10]

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 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.[11]

In

xenon-133 are also used extensively because they are easily identified and measured.[13]

References

  1. .
  2. ^ a b Reis, John C. (1976). Environmental Control in Petroleum Engineering. Gulf Professional Publishers.
  3. ^ a b c 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.
  4. PMID 27551621. Archived from the original
    (PDF) on 2011-10-17.
  5. .
  6. ^ I-123 fact sheet[permanent dead link]
  7. PMID 5650883
    .
  8. .
  9. .
  10. .
  11. .
  12. ^ a b 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
  13. ^ 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 3H and 14C, 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 46Sc, 140La, 56Mn, 24Na, 124Sb, 192Ir, 99Tcm, 131I, 110Agm, 41Ar and 133Xe 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 82Br...

External links