Another application is in radiochemistry, where this may refer to isotopic ratios or isotopic abundances. Mathematically, the isotopic abundance is
where N are the number of atoms of the isotope of interest and N is the total number of atoms, while the atomic ratio is
For example, the isotopic ratio of deuterium (D) to hydrogen (H) in heavy water is roughly  (corresponding to an isotopic abundance of 0.00014%).