Isotopic tracer
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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.