synonyms: delta notation, isotope delta, relative difference of isotope ratios
https://doi.org/10.1351/goldbook.12551
Measure of an isotope ratio in a sample of interest relative to a reference defined by the relation \[\delta = (R_{\rm{sample}} - R_{\rm{standard}})/R_{\rm{standard}}\] where \(R\) is the isotope ratio of the element.
Notes:
- The reference is often an international measurement standard.
- The same relative differences are used to calculate delta values from the abundance of isotopomers and isotopologs, ion currents, frequencies, or optical extinctions.
- Relative isotope ratio differences are usually small and are therefore often expressed in units of parts per mil with symbol ‰.
- Units of parts per thousand (\(\pu{ppt}\), \(\pu{E-3}\)) or parts per million (\(\pu{ppm}\), \(\pu{E-6}\)) are deprecated.