Radiochemical Techniques

Type of physical science: Chemistry

Field of study: Chemical methods

The radiation emitted by radioactive elements is capable of being detected by nuclear radiation detectors. These elements can be employed in chemistry and other fields of study to analyze and solve problems. Radioactive elements used in this way are called tracers.

Overview

The nucleus of an atom is composed of neutrons and protons. The number of protons contained in the nucleus determines the identity of the element. Two nuclei could have the same number of protons but a different number of neutrons. These two nuclei would be two different isotopes of the same element. For example, the element carbon exhibits two stable isotopes: carbon 12 and carbon 13. Carbon 12 contains six protons and six neutrons, whereas carbon 13 contains six protons and seven neutrons. The number used to label the isotopes corresponds to the sum of the number of protons and neutrons. Carbon 12 and carbon 13 account for 98.89 percent and 1.11 percent of a typical carbon sample, respectively.

Besides these two stable isotopes of carbon, a carbon sample contains trace quantities of several unstable, or radioactive, isotopes. Carbon 14 is one of these. By emitting a radioactive particle, carbon 14 converts itself to nitrogen 14, a more stable element. The radioactivity emitted by the carbon 14 nucleus can be detected by nuclear radiation detectors. Carbon 14 is useful to chemists because it emits radiation.

A number of factors determine nuclear stability. One such factor is the ratio of neutrons to protons, or the N:P ratio. For lighter nuclei, a criterion applicable to stability is an N:P ratio of 1. Carbon 14 is unstable because it has an N:P ratio of 1.3, whereas its decay product, nitrogen 14, has an N:P ratio of exactly 1. This criterion, however, is only a general guideline for nuclear stability, and heavier elements favor a higher N:P ratio.

Another factor that influences nuclear stability is whether an isotope has an odd number of protons or neutrons in its nucleus. There are 157 stable nuclei that have an even number of both neutrons and protons, whereas only 5 stable nuclei contain an odd number of both neutrons and protons. An α (alpha) particle is a particle with an even number of protons (two) and an even number of neutrons (two), identical to a helium nucleus. Nitrogen 14 is a rare example of an element that is stable even though it has an odd number of protons (seven) and an odd number of neutrons (seven).

There are also certain "magic" numbers of neutrons and protons. Thus, nuclei containing 2, 8, 20, 28, 50, 82, or 126 neutrons or protons exhibit unusual nuclear stability. Nuclei that contain a magic number of both neutrons and protons are considered "doubly magic." Calcium 40 is doubly magic because it contains 20 protons and 20 neutrons. Lead 208 is doubly magic because it contains 82 protons and 126 neutrons.

Different unstable nuclei decay at different rates. The unit commonly used in nuclear physics and chemistry to express the rate of nuclear decay is the half-life, which is the amount of time required for half of the atoms present in a sample of radioactive material to decay. The half-life of carbon 14 is 5,730 years. This means that a sample containing one milligram of carbon 14 would contain only half a milligram 5,730 years later, the other half having decayed into nitrogen 14. The half-lives of radioactive isotopes vary tremendously. The half-life of uranium 238 is 2.39 billion years, while that of gold 203 is 5.5 seconds.

In a tracer application, a radioactive isotope of an element replaces a nonradioactive isotope of the same element. If the radioactive isotope acts the same chemically as the original nonradioactive isotope, then the radiation emitted by the radioactive isotope can be used to trace the progress of that element through a certain physical or chemical change. Various radiation detectors, such as Geiger counters and scintillation counters, can be used to detect the radiation produced. The compound containing the radioactive isotope is said to be tagged.

For a radioactive nucleus to be useful as a tracer, it must have a sufficiently long half-life, as most tracer applications require that the radioactivity emitted by the isotope in question remain constant for the duration of the experiment. The isotope should exhibit radioactivity intense enough to be detected by the radiation detector and discernible above the normal background radiation present.

Lastly, the presence of the isotope should not perturb the system to the extent that the system would behave too differently. A difference of this degree is known as the isotope effect. This effect is greatest in isotopes of lighter elements, because the mass difference between the two isotopes is substantial.

Use of tracers in medicine requires that special precautions be taken to prevent injury to the patient from the tracer itself. The most obvious factor to be considered is the effect of the radiation on human subjects. Chemical and radiological purity of the radioactive isotope is also a concern. A small amount of impurity may be allowable in a chemical study employing a tracer. The same impurity in a medical application might be toxic to a human subject.

It is possible to create radioactive isotopes in nuclear reactors or particle accelerators. In such cases, as with naturally occurring radioactive elements, the nucleus absorbs charged particles or neutrons, creating an unstable radioactive nucleus. "Artificial" nuclei created in this manner greatly expand the list of isotopes available for use in tracer studies in all areas of science.

Applications

There are many important applications of radioisotopes as tracers in chemistry and other fields. Some of the more important radioisotopes used in tracer studies are carbon 14, oxygen 18, sodium 24, phosphorus 32, sulfur 35, and iodine 131. All of these radioisotopes are β (beta)-minus emitters, meaning that in each case, the radioactive decay converts a neutron into a proton when the electron is emitted, causing the radioisotope to become a more stable isotope of the element one atomic number higher than the original substance.

One case in which a radioactive isotope could be employed to follow a chemical reaction is the ionization of sodium hydroxide (NaOH), lye, in water. A radioisotope can be used to determine whether the hydrogen atom in sodium hydroxide exchanges with a hydrogen atom on the water molecule. To determine this, we could use water in which tritium, a radioactive isotope of hydrogen, replaces some of the normal hydrogen atoms. The sodium hydroxide would be dissolved in the water labeled with tritium, and the radioactive water (T₂O) would then be evaporated, leaving only the sodium hydroxide solid. If the tritium in the water was exchanged for the hydrogen in the lye, a radiation detector would detect radioactivity in the lye sample. The following chemical equation describes this exchange:

NaOH + T*₂O → NaOT* + H₂O

Another application of tracers is the study of charge transfer between two identical chemical species. In a mixture of tin (II) and tin (IV) ions—ions with a +2 charge and a +4 charge, respectively—could two electrons transfer from a tin (II) ion to a tin (IV) ion to reverse the charges? Radioactive tin 119m (*Sn) used in the following manner could answer this question:

*Sn⁴⁺ + Sn²⁺ → *Sn²⁺ + Sn⁴⁺

Without using a radioactive isotope of tin to distinguish between the two tin species, there would be no way to determine if this exchange took place. Tagging of organic and inorganic compounds is especially important in exchange reactions.

Tracer studies also can help elucidate the mechanism of chemical reactions. An example of this chemical application would be the reaction of lead (IV) oxide (PbO₂) with hydrogen peroxide (H₂O₂) to produce lead (II) oxide, oxygen gas, and water. The equation describing this reaction is as follows:

PbO₂ + H₂O₂ → PbO + H₂O + O₂

The casual observer might postulate that one atom of the oxygen gas is taken from PbO₂ and one from H₂O₂. If the H₂O₂ were tagged with oxygen 18, however, it would reveal that the oxygen produced in the reaction contains two oxygen 18 atoms. This shows that both oxygen atoms originated from the hydrogen peroxide, and thus the mechanism of this reaction is more complex than might be initially presumed. The reaction involves the combination of the hydrogen atoms of H₂O₂ with one of the oxygen atoms of the PbO₂.

In some applications, two different radioactive isotopes may be employed to study the same chemical system or reaction. An example would be an organic compound that contains both carbon and oxygen. The compound could be tagged with both carbon 14 and oxygen 18.

A technique known as a radiometric titration uses radioactive isotopes in a more quantitative way. A titration is a process whereby the amount of a certain substance is determined by the addition of another substance that is known to react with the first substance. The substance added to the original substance is called the standard. The amount of the substance in question can be obtained by knowing the amount of standard added. If the standard added is a radioactive isotope, then the original amount of the substance can be calculated by the amount of radioisotope present in the product of the reaction.

Another quantitative application of radioactive isotopes, similar to radiometric titration, is a technique known as isotopic dilution. In this technique, a standard is prepared that contains a known amount or percentage of the radioactive isotope. This standard is added to a system containing the same chemical substance as the standard. The amount of radioactivity exhibited per unit mass will be reduced in this mixture because of the dilution of the standard by the sample containing the nonradioactive isotope. From the reduced reading of the radioactivity, the original amount of the chemical substance can be determined.

Sometimes radioactive isotopes can be used in concert with other analytical techniques to learn more about a particular chemical system. If a radioisotope is incorporated into several chemical compounds, the mixture can sometimes be separated and analyzed through chromatographic techniques. The separated mixture can then be analyzed quantitatively by using a radiation detector to detect the radioactivity exhibited by each separated fragment of the sample.

The fields of biology and medicine use radioactive isotopes extensively. Certain radioactive isotopes become concentrated in certain areas of the body, making them very useful in diagnostic medicine. The radiation emitted by the radioactive atoms in this area produces an image on a photographic plate or film. One example is the use of radioactive iodine to detect thyroid disease, a technique known as autoradiography. Radioactive iodine has also been employed to diagnose the presence of brain tumors. Radioactive sodium has been employed in the study of diseases of the heart and circulatory system. A radioactive isotope of gold has been used to detect cancerous tumors in the kidneys. Radioactive cobalt inserted into vitamin B12 can be used to monitor the vitamin's uptake, which is related to pernicious anemia.

One particular technique is positron emission tomography (PET), often called a PET scan. This technique uses radioisotopes that decay by emitting beta-plus particles, also known as positrons, hence the name. In positron emission, a proton in the nucleus is converted into a neutron, causing the radioisotope to decay into a more stable isotope of the element one atomic number lower than the original substance. The positron-emitting tracer is introduced into the person being scanned, and the imaging scanner then detects the γ (gamma) rays being emitted by the decaying isotopes and uses them to create a three-dimensional image of the subject. PET is often used in conjunction with computed tomography (CT) or magnetic resonance imaging (MRI).

Radioactive isotopes are employed not only in diagnosis but also in treatment of disease. Sometimes the same radioactive isotope can perform both functions, such as radioactive iodine, which can treat thyroid cancer and certain other thyroid diseases in addition to detecting them. Cobalt 60 is commonly used in cancer therapy, while radioactive phosphorus has been employed in the treatment of leukemia.

There are other techniques that employ radioactive isotopes. One very important application is radiochemical dating, in which a radioactive isotope present in a sample is used to determine the age of the object. This technique is especially important in archaeology, where carbon 14 is commonly used. Cosmic rays from the sun are constantly producing carbon 14 in the atmosphere, resulting in a consistent ratio of carbon 12 to carbon 14. Through respiration, organisms maintain this same ratio of carbon 12 to carbon 14 in their bodies. When an organism dies, it no longer will assimilate carbon 14 from the atmosphere, and as time passes, the ratio of carbon 12 to carbon 14 increases. Researchers can determine the age of an artifact by determining the carbon 12–to–carbon 14 ratio. This method of dating objects is useful only if the artifacts are less than fifty thousand years old.

Another radioactive isotope that can be used for radiochemical dating experiments is potassium 40, which decays to argon 40 via a process known as electron capture. The potassium 40–to–argon 40 ratio determines the age of the object. Objects as old as one million years can be dated by this method. Uranium 238 can also be used to date extremely old objects, as it has a half-life of 4.5 billion years and eventually decays, through a series of decay steps, into lead 208. The ratio of uranium 238 to lead 208 enables researchers to determine the age of objects billions of years old.

Context

George Stoney postulated the existence of the electron as an elementary particle in 1874. In 1897, Joseph John Thomson performed experiments using cathode-ray tubes that demonstrated the existence of the electron. Eugene Goldstein observed a stream of positive rays in a cathode-ray tube in 1886, the first observation of the proton. Paul Dirac proposed the existence of the positron, the antiparticle of the electron, in 1930. Carl David Anderson observed positrons in a cloud chamber in 1932.

Wilhelm Rontgen discovered the existence of x-rays produced by a cathode-ray tube in 1895. Antoine-Henri Becquerel was the first scientist to discover, quite accidentally, that radioactive substances could be emitted from certain elements. In 1899, Becquerel, in France, and S. Meyer, E. von Schweidler, and F. Geisel in Germany showed that certain radioactive particles could be deflected in a magnetic field. This confirmed that certain types of radioactivity consisted of charged particles. These particles were subsequently identified as electrons, or beta-minus particles. The alpha particle's existence was confirmed by Ernest Rutherford in 1903.

Marie Curie performed extensive experiments using radioactive elements and observed the types of radiation they emitted. During the course of these studies, Marie and Pierre Curie discovered the elements polonium and radium. In 1925, Georg de Hevesy and F. Paneth were the first to employ radioactive isotopes as tracers in order to determine the solubility of lead sulfate and lead chromate. The isotopic dilution technique was first employed by Hevesy and Hofer in 1934 but did not receive widespread use until utilized by Rittenberg and Foster in 1940. Since then, tracers and isotopic dilution have developed into very useful techniques in physics, chemistry, biology, medicine, and many other fields. The advent of nuclear reactors and particle accelerators has extended the use of radioactive isotopes as tracers by permitting more radioactive isotopes to be created.

Principal terms

ALPHA PARTICLE: a type of radioactivity corresponding to a helium nucleus, consisting of two protons and two neutrons

BETA-MINUS PARTICLE: an electron that originates in the nucleus of an atom; the electron has a negative electrical charge

BETA-PLUS PARTICLE: the antiparticle of the beta-minus particle, also called a positron; it has the same mass as the beta-minus, but a positive electrical charge

GAMMA RAY: an electromagnetic wave radiation emitted by a radioactive isotope, having very high energy but exhibiting neither mass nor electrical charge

NEUTRON: one of the two main particles contained in the nucleus of an atom; the neutron contains no electrical charge

PROTON: one of the two main particles contained in the nucleus of an atom; the proton is almost the same mass as the neutron and has a positive electrical charge

RADIOACTIVE ISOTOPE: an isotope of an element that emits radioactive particles or rays over a period of time; these can be either naturally occurring or human made

RADIOACTIVITY: the emission of elementary particles or electromagnetic radiation by unstable isotopes

X-RAY: a type of radiation similar to the gamma ray but having slightly lower energy

Bibliography

Chase, Grafton D., and Joseph L. Rabinowitz. Principles of Radioisotope Methodology. Minneapolis: Burgess, 1962. Print. This is a laboratory text containing about sixty experiments concerning the uses of radioisotopes and the techniques of handling and detecting radioisotopes. The introductions to many of the experiments are a good source of material on various applications of tracers and other uses of radioactive isotopes. Chapters 10 and 11 discuss the uses of radioactive isotopes in chemistry and in the biological sciences.

Coleman, Magen E., et al. "The Analysis of Uranium-232: Comparison of Radiochemical Techniques and an Improved Method by Alpha Spectrometry." Journal of Radioanalytical and Nuclear Chemistry 296.1 (2013): 483–87. Print.

Glasstone, Samuel. Sourcebook on Atomic Energy. 2nd ed. Princeton: Van Nostrand, 1958. Print. An extremely good reference book on many different aspects of atomic energy, radiochemistry, and nuclear physics. The first edition of this work was sponsored by the old Atomic Energy Commission (now the Department of Energy). The chapter titled "The Uses of Isotopes and Radiations" would be a particularly useful resource on tracers.

Harvey, Bernard G. Nuclear Chemistry. Englewood Cliffs: Prentice, 1965. Print. A very short work on the various aspects of nuclear chemistry. Chapter 8, "Applications of Nuclear Science," contains a very brief but well-written section on the tracer method, with several specific chemical examples of its use.

L'Annunziata, M. F. Radionuclide Tracers: Their Detection and Measurement. New York: Academic, 1987. Print. This is a guide to the theory and applications of radioactive nuclei. The first chapter has a very suitable introduction to the basic principles of radioactivity and the various interactions that can take place between radiation and matter.

Lehto, Jukka, and Xiaolin Hou. Chemistry and Analysis of Radionuclides. Weinheim: Wiley, 2011. Print.

Oliver, Raymond. Principles of the Use of Radio-Isotope Tracers in Clinical and Research Investigations. Elmsford: Pergamon, 1971. Print. This account outlines the use of radioactive isotopes in the areas of biological and medical research. The text is written in a straightforward manner that should be easy for nontechnical people to understand. It contains brief reviews of nuclear structure and radioactivity in the first and second chapters, respectively.

Overman, Ralph T., and Herbert M. Clark. Radioisotope Techniques. New York: McGraw, 1960. Print. This work is a textbook on techniques involving the use of radioactive isotopes. Parts of this text may be overly technical for the casual reader, while other sections should give the reader an appreciation of the topic.

Pretorius, E. Scott, and Jeffrey A. Solomon. Radiology Secrets Plus. 3rd ed. Philadelphia: Mosby, 2011. Print.

Vertes, Attila, and Istvan Kiss. Nuclear Chemistry. New York: Elsevier, 1987. Print. This work is one volume in a series of monographs titled Topics in Inorganic and General Chemistry. It is an extensive text encompassing 619 pages. One chapter is devoted to radioactive tracing and discusses an interesting technique that combines tracers with Mossbauer spectroscopy.

The Structure of the Atomic Nucleus

Nuclear Reactions and Scattering

Radiation: Interactions with Matter

Radioactive Nuclear Decay and Nuclear Excited States