Isotopic substitution is a research area within chemistry and physics that involves the replacement of one or more atoms in a molecule with a different isotope of the same element. This substitution can be used to study the effects of isotopic composition on the physical and chemical properties of molecules, as well as to trace the pathways of chemical reactions and biological processes. Isotopic substitution is commonly used in fields such as geochemistry, environmental science, and materials science to provide insights into the structures and behaviors of molecules and materials.