Isotope persisted down to the present because their rate of decay is so slow (e.g. uranium-238 and potassium-40). Post-primordial isotopes were created by cosmic ray bombardment as cosmogenic nuclides (e.g., tritium, carbon-14), or by the decay of a radioactive primordial isotope to a radioactive radiogenic nuclide daughter (e.g. uranium to radium). A few isotopes are naturally synthesized as nucleogenic nuclides, by some other natural nuclear reaction, such as when neutrons from natural nuclear fission are absorbed by another atom. As discussed above, only 80 elements have any stable isotopes, and 26 of these have only one stable isotope. Thus, about two-thirds
Reference materials for stable isotope analysis is almost always the isotopologue with no isotopic substitutions. This isCO for carbon dioxide and CH for methane. Standard isotopic reference materials are still required in clumped isotope analysis for measuring the bulk δ values of a sample, which are used to calculate the expected stochastic distribution and subsequently to infer clumped isotope temperatures. However, the clumped isotope composition of most samples are altered in the mass spectrometer during ionization, meaning that post-measurement data correction requires having measured materials of known clumped isotope composition. At a given temperature equilibrium thermodynamics predicts the distribution of isotopes among possible isotopologues, and these