Radioactive Isotopes
Radioactive isotopes occur as minor constituents in many natural materials, from which they can be concentrated by fractionation procedures. In a very limited number of cases, more significant amounts of a radioactive isotope, for example, radium or radioactive lead, can be found in nature. Most radioactive isotopes in use today, however, are prepared artificially by nuclear reactions. When a high-energy particle, such as a proton, a deuteron, an alpha particle, or a neutron, collides with an atom, a reaction takes place, leading to the formation of a new, unstable compound—a man-made radioactive isotope.
The great usefulness of radioactive isotopes, as we shall see later, is that they can be detected and identified by proper instruments. Biochemists have long recognized the desirability of “tagging” or “labeling” a molecule to permit tracing or keeping track of the “label” and consequently of the molecule as it moves through a reaction or process. Since the radiations emitted by radioactive isotopes can be detected and measured, we can readily follow a molecule tagged with a radioactive atom.
Figure 9 A laboratory technologist preparing dissolved biological materials as part of a study of the uptake of radioactive substances in living organisms. Note the radiation-detection instrument at right.
The earliest biochemical studies employing radioactive isotopes go back to 1924, when George de Hevesy used natural radioactive lead to investigate a biological process. It was only after World War II, however, when artificially made radioactive isotopes were readily available, that the technique of using isotopic tracers became popular.
In our investigations of life processes, we are especially interested in three radioactive isotopes: ³H, the hydrogen atom of mass 3; ¹⁴C, the atom of carbon with atomic weight 14; and ³²P, the atom of phosphorus with atomic weight 32. These radioactive isotopes are important because the corresponding stable isotopes of hydrogen, carbon, and phosphorus are present in practically all cellular components that are important in maintaining life. With the three radioactive isotopes, therefore, we can tag or label the molecules that participate in life processes.
Figure 10 A visiting scientist at an AEC laboratory uses radioactive tritium (³H) to study the effect of radiation on bean chromosomes. The famous scientist, George de Hevesy, also used beans in conducting the first biological studies ever made with radioisotopes.
Hydrogen-3 is a weak beta emitter; that is, it emits beta particles with a very low energy (0.018 Mev[6]) and therefore with a very short range. Carbon-14 is also a weak beta emitter (0.154 Mev), although the beta particles emitted by ¹⁴C have a higher energy and therefore a longer range than those emitted by ³H. The beta particles emitted by phosphorus-32 are quite energetic (1.69 Mev) and have a longer range.