Isotopes are variants of a particular chemical element whichdiffer in neutron number, although all isotopes of a given elementhave the same number of protons in each atom. Isotope VariationAnalysis (detection of adulteration in food products or thegeographic origins of products using isoscapes) and Isotopicsubstitution (to determine the mechanism of a chemical reaction viathe kinetic isotope effect) are the most frequently usedapplication for isotopes. Another common application for isotope isisotopic labeling, the use of unusual isotopes as tracers ormarkers in chemical reactions. Normally, atoms of a given elementare indistinguishable from each other. However, by using isotopesof different masses, even different nonradioactive stable isotopescan be distinguished by mass spectrometry or infrared spectroscopy.For example, in 'stable isotope labeling with amino acids in cellculture (SILAC)' stable isotopes are used to quantify proteins. Ifradioactive isotopes are used, they can be detected by theradiation they emit (this is called radioisotopic labeling).