text: Measurement of the abundance of clumped isotopes (doubly substituted isotopologues) of gases has been used in the field of stable isotope geochemistry to trace equilibrium and kinetic processes in the environment inaccessible by analysis of singly substituted isotopologues alone.
  Currently measured doubly substituted isotopologues include:
  *Carbon dioxide: COO
  *Methane: CHD and CHD
  *Oxygen: O and OO
  *Nitrogen: N
  *Nitrous oxide: NNO and NNO