Isotopes are variants of a particular chemical element which differ in neutron number, although all isotopes of a given element have the same number of protons in each atom. Isotope Variation Analysis (detection of adulteration in food products or the geographic origins of products using isoscapes) and Isotopic substitution (to determine the mechanism of a chemical reaction via the kinetic isotope effect) are the most frequently used application for isotopes.
Another common application for isotope is isotopic labeling, the use of unusual isotopes as tracers or markers in chemical reactions. Normally, atoms of a given element are indistinguishable from each other. However, by using isotopes of different masses, even different nonradioactive stable isotopes can be distinguished by mass spectrometry or infrared spectroscopy.