Stable isotope compositions are commonly reported using the “delta” terminology. Basically, delta is the ratio of the number of atoms of the heavy to the light isotopic form, expressed in parts per thousand deviation from a reference standard. In equation form:
where Rx is the ratio of heavy to light isotope in the sample and Rs is the ratio in the reference standard. Various reference standards have been defined. For stable isotopes of oxygen and hydrogen, the Standard Mean Ocean Water (SMOW) or the Vienna-SMOW (VSMOW) standard is used. The reference for carbon ratios is the Pee Dee Belemnite (PDB) standard, and for nitrogen ratios it is the atmospheric N2.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://www.fort.usgs.gov/resources/spotlight/shorebird_isotopes/isotopes_ratios.asp
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