The Commission on Isotopic Abundances and Atomic Weights of the International Union of Pure and Applied Chemistry recently recognized that there is confusion about the notation of delta values in isotope studies. The delta value (δ) is used to express the relative difference of a ratio of the numbers (or the amounts) of two isotopes in a specimen compared with that of a reference, commonly an international measurement standard. Results of such isotopic measurements are used in anthropology, atmospheric sciences, biology, chemistry, environmental sciences, food and drug authentication, forensic applications, geochemistry, geology, oceanography, and paleoclimatology.

Tyler B. Coplen, U.S. Geological Survey, Virginia, USA, has clarified the terminology and concepts behind the terms involved in expressing relative differences in stable isotope ratios. His guidelines also provide information on the reporting of relative differences in ratios of volumes, numbers of molecules, and amounts of gases.

• Guidelines and recommended terms for expression of stable-isotope-ratio and gas-ratio measurement results
  T. B. Coplen,
  Rapid Commun. Mass Spectrom. 2011, 25, 2538–2560.
  DOI: 10.1002/rcm.5129