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This value is not reported but it is used to produce the correct “Conventional Radiocarbon Age”.

Important: Reporting conventions using “Conventional Radiocarbon Age” terminology indicate the result has been corrected for isotopic fractionation.– Beta also measures a second δ13C value in an isotope ratio mass spectrometer (IRMS δ13C).

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).

Conversely the inorganic carbon dissolved in the oceans is generally 0.7% enriched in 13C relative to atmospheric carbon dioxide.

The Conventional Radiocarbon Age BP is calculated using the radiocarbon decay equationt=-8033 ln(Asn/Aon)Where -8033 represents the mean lifetime of 14C (Stuiver and Polach, 1977).

Aon is the activity in counts per minute of the modern standard, Asn is the equivalent cpm for the sample. A CRA embraces the following recommended conventions: About AMS Dating Accelerator Mass Spectrometry (AMS) dating involves accelerating ions to extraordinarily high kinetic energies followed by mass analysis.

Radiocarbon users should always be aware whether the δ13C they are using in their research is the IRMS δ13C or the AMS δ13C.Atoms of larger or smaller mass may be favored in such a situation.If, however, the entire sample is converted completely from one form to another (e.g.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.

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.