The ratio is measured using an ordinary mass spectrometer.The isotopic composition of the sample being measured is expressed as delta13C which represents the parts per thousand difference (per mille) between the sample carbon 13 content and the content of the international PDB standard carbonate (Keith et al., 1964; Aitken, 1990).

Usually, this is due to lack of attention to detail and incomplete conversion of the sample from one stage to another or from one part of the laboratory to another.

In Liquid Scintillation Counting, for example, incomplete synthesis of acetylene during lithium carbide preparation may result in a low yield and concomitant fractionation.

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.

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”.

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).