original_query: What is isotopic labeling and how is it used in tracking the passage of an isotope?
query: What is isotopic labeling and how is it used in tracking the passage of an isotope?
answer: Isotopic labeling is a technique to track the movement of an isotope through a reaction, metabolic pathway, or cell.  A reactant is 'labeled' by replacing atoms with their isotopes.  After the reaction, the isotope's position in the products is measured to determine the sequence it followed.