Stable Isotope Facility

Isotope Ratio

The stable isotope concentrations of a molecular compound or material are presented in ratio form as the molar ratio of the heavy-to-light isotopes.  Since this ratio is small, we typically present stable isotope abundances relative to an international standard using delta(δ) notation as:

 δX = (R_sample/R_standard - 1) x 1000, ‰

where δX is the delta value of the sample for element X (H, O, C, etc.) in parts per thousand (permil) (‰) and R is the molar ratio of the heavy (less common) to light (more common) isotope in the sample and in an international standard, respectively. Organic and inorganic standard materials are obtained from the International Atomic Energy Agency (IAEA, reference materials page) and the National Institute of Standards and Technology (NIST, reference materials page) to ensure accurate measurement and reporting of isotope ratios for unknown samples and to facilitate cross-lab comparability.