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.
[Illustration: 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.
[Illustration: 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.