Isotopic labeling (or
isotopic labelling) is a technique used to track the passage of an
isotope (an
atom with a detectable variation) through a
reaction,
metabolic pathway, or
cell. The
reactant is 'labeled' by replacing specific atoms by their isotope. The reactant is then allowed to undergo the reaction. The position of the isotopes in the
products is measured to determine the sequence the isotopic atom followed in the reaction or the cell's metabolic pathway. The
nuclides used in isotopic labeling may be
stable nuclides or
radionuclides. In the latter case, the labeling is called
radiolabeling.