Of the 98 naturally occurring elements on earth, 18 are known to be radioactive, meaning that they exhibit time-dependent decay to lighter elements, and 80 are known to be stable. Of those 80 stable elements, 54 are known to exhibit isotopic variation, meaning that atoms within the same elemental category have different atomic masses due to variations in neutron number (Section 3.5). Given the dependency of diffusive flux on atomic mass, molecules of the same compound, but composed of different isotopes, will diffuse at different rates and thus segregate into isotopic fractions over time. Analysis of isotopic fractionation provides researchers with one of their most valuable tools for understanding rates of diffusive flux, enzyme-substrate interactions, interactions among metabolic pathways, and the sources and sinks of compounds used for various biogeochemical processes, even extending beyond those defined solely by diffusion. Of particular importance have been analyses of the isotopic composition of CO2 and H2O, given that C, H, and O are among those 54 elements that exhibit stable isotope variation.