Isotopic labeling
![A carbon-13 label was used to determine the mechanism in the 1,2- to 1,3-didehydrobenzene conversion of the phenyl substituted aryne precursor 1 to acenaphthylene.[1]](/Images/godic/202501/21/Benzyneconversioncolouredlabels4129.png")
![Isotopic tracing through reactions in the Pentose Phosphate Pathway. The blue circles indicate a labeled carbon atom, while white circles are an unlabeled carbon atom.[3]](/Images/godic/202501/21/StableIsotopeLabeling24129.png")
![Determining the percent of isotope labeling throughout a reaction. If a 50% labeled and 50% unlabeled metabolite is split in the manner shown, the expected percent of each outcome can be found. The blue circles indicate a labeled atom, while a white circle indicates an unlabeled atom.](/Images/godic/202501/21/StableIsotopeLabeling4129.png")
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.