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This correction factors out error introduced from metabolic and respiratory pathway differences between the modern reference standard material and the sample material. Two δ13C values are measured at Beta Analytic: – One is the value applicable to correct for total fractionation (natural, chemistry and AMS).
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).
This value is representative of the sample itself and is reported. In comparison, reporting the “AMS δ13C” is misleading and open to misinterpretation.
Isotopic fractionation of stable carbon isotopes Carbon-13 (13C) and Carbon-12 (12C) involves alterations in the ratios of isotopic species as a function of their atomic mass as a result of natural biochemical processes.
It is common practice in Carbon-14 laboratories to correct radiocarbon activities for sample fractionation.
The resultant ages are termed “normalized”, meaning the measured activity is modified with respect to -25 o/oo (per mille) with respect to VPBD.
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.Correction factors are available in most C14 texts.A table showing isotopic fractionation of different substances in nature is shown here.Some processes, such as photosynthesis for instance, favour 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).Conversly the inorganic carbon dissolved in the oceans is generally 0.7% enriched in 13C relative to atmospheric carbon dioxide.The ratio is measured using an ordinary mass spectrometer.