Isotopic fractionation of stable carbon isotopes Carbon-13 (13C) and Carbon-12 (12C) refers to the fluctuation in the carbon isotope ratios as a result of natural biochemical processes as a function of their atomic mass (Taylor, 1987).Variations as such are unrelated to time and natural radioactive decay.It is common practice in radiocarbon laboratories to correct radiocarbon activities for sample fractionation.

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This ratio is later used in the calculation of the radiocarbon age and error to correct for isotopic fractionation in nature.

Fractionation during the geochemical transfer of carbon in nature produces variation in the equilibrium distribution of the isotopes of carbon (12C, 13C and 14C).

Craig (1953) first identified that certain biochemical processes alter the equilibrium between the carbon isotopes.

Some processes, such as photosynthesis for instance, favors one isotope over another, so after photosynthesis, the isotope C13 is depleted by 1.8% in comparison to its natural ratios in the atmosphere (Harkness, 1979).

The correction factor must be added or subtracted from the conventional radiocarbon age.*****Beta Analytic’s fees already include δ13C measurements in conjunction with C14 analysis.

The lab also provides δ13C measurements NOT in conjunction with C14 analysis except for water samples.Please email the lab for the rates.*****In order to provide radiocarbon determinations that are both accurate and precise, it is necessary to measure the stable isotopes of 13C and 12C and their ratio.This is performed by extracting a small amount of the CO2 generated during the combustion or acid hydrolysis and measuring the 13C/12C ratio relative to the PDB mass-spectrometry standard.Conversely the inorganic carbon dissolved in the oceans is generally 0.7% enriched in 13C relative to atmospheric carbon dioxide.The extent of isotopic fractionation on the 14C/12C ratio (which must be measured accurately) is approximately double that for the measured 13C/12C ratio.If isotopic fractionation occurs in natural processes, a correction can be made by measuring the ratio of the isotope 13C to the isotope 12C in the sample being dated.