Δ13C

The definition is, in per mille:

${\displaystyle \delta {}^{13}\text{C}=\left(\frac{({}^{13}\text{C}/{}^{12}\text{C}{)}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}}{({}^{13}\text{C}/{}^{12}\text{C}{)}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{a}\mathrm{r}\mathrm{d}}}-1\right)\times 1000}$

where the standard is an established reference material.

The δ¹³C of a given compound can vary based on the sources of the precursor material and the biogeochemical processes it has undergone. For example, carbon dioxide derived from ecosystem respiration can be differentiated from carbon dioxide formed from the combustion of fossil fuels using δ¹³C, as the precursor materials (modern organic matter and petroleum, respectively) have different isotopic values—the basis of the "Suess effect". In the case of photosynthesis, two plants grown adjacently with the same source of carbon dioxide may be isotopically distinguishable due to differing biochemical mechanisms and physiologies preferentially selecting for a given isotope—a process known as "isotopic fractionation".

Reference standard

Reference standards are used for verifying the accuracy of isotope ratio measurements, which are typically performed via isotope ratio mass spectrometry, cavity ring down spectroscopy, tunable laser absorption spectroscopy, or nuclear magnetic resonance.

The initial reference material used to standardize carbon isotope ratios was the "Pee Dee Belemnite" (PDB)—a Cretaceous marine fossil, Belemnitella americana, originating from the Peedee Formation in South Carolina. This material had an anomalously high ¹³C/¹²C ratio (0.0112372), and was established as δ¹³C value of zero.

Due to the high demand for the PDB standard, the supply was ultimately exhausted. Other standards calibrated to the same ratio, including one known as VPDB (for "Vienna PDB"), have replaced the original. The ¹³C/¹²C ratio for VPDB, which the International Atomic Energy Agency (IAEA) defines as a δ¹³C value of zero is 0.011113. The use of different primary reference standards will result in isotope ratios that are incomparable due to the difference in scales. To avoid confusion, isotope ratio measurements typically include a subscript denoting the reference material it was corrected to, such as δ¹³C_PDB or δ¹³C_VPDB.

To prevent the depletion of the supply of VPDB, secondary reference materials with isotope ratios determined in direct comparison to VPDB, such as NBS-19 (available from the National Institute of Standards and Technology, δ¹³C_VPDB = 1.95‰), are commonly used in the laboratory setting for standardizing measurements.