Isotopic Tracer
An isotopic tracer, (also "isotopic marker" or "isotopic label"), is
used in chemistry and biochemistry to help understand chemical
reactions and interactions. In this technique, one or
more of the atoms of the molecule of interest is substituted for an
atom of the same element, but of a different (often radioactive)
isotope. Because the atom has the same number of protons, it will behave
in almost exactly the same way chemically as other atoms in the compound,
and with few exceptions will not interfere with the reaction under
investigation. The difference in the number of neutrons, however, means
that it can be detected separately from the other atoms of the same element.
NMR typically uses this type of technique to investigate the mechanisms
of chemical reactions (basically trying to find out which starting atom ends
up where after a reaction), because NMR detects not only isotopic
differences, but also gives an indication of the position of the atom.
Mass spectrometry can also be used with this technique, since mass
spectra recorded with sufficiently high resolution can distinguish among
isotopes based on the different masses resulting from the different number
of neutrons.
Autoradiograms of gels in gel electrophoresis can also take advantage of
this approach. In this technique, radioactive isotopes are used. The
radiation emitted by compounds containing the radioactive isotopes darkens a
piece of photographic film, recording the position of these compounds
relative to one another in the gel.
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