14c isotope dating
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.A δ13C value, then, represents the per mille (part per thousand) deviation from the PDB standard.PDB refers to the Cretaceous belemnite formation at Peedee in South Carolina, USA.Fractionation also describes variations in the isotopic ratios of carbon brought about by non-natural causes.For example, samples may be fractionated in the laboratory through a variety of means; incomplete conversion of the sample from one stage to another or from one part of the laboratory to another.
The isotopic composition of the sample being measured is expressed as δ13C which represents the parts per thousand difference (per mille) between the sample’s carbon 13 content and the content of the international PDB standard carbonate (Keith et al., 1964; Aitken, 1990).
In Liquid Scintillation Counting, for example, incomplete synthesis of acetylene during lithium carbide preparation may result in a low yield and concurrent fractionation.
Similarly, the transfer of gases in a vacuum system may involve fractionation error if the sample gas is not allowed to equilibrate throughout the total volume.
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.
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This value is not reported but it is used to produce the correct “Conventional Radiocarbon Age”.