Oxalic acid standard , radiocarbon

Figure 6. Radiocarbon content of modern pre-bomh marine shell from the west coast of North and South America, Data from Refs. 73 and 74. Values have been expressed with respect to 0.95 NBS oxalic acid standard and corrected in light of their values.

Radiocarbon values are expressed with respect to 0.95 NBS oxalic acid standard. ... 

As the international standard of A is the applied standard of the US National Bureau of Standards (NBS), namely the oxalic acid with radioactivity of 14.3 decays per minute per 1 g of carbon (1.176-10 °% C). However, most labs use in their calculations activity of the internationally accepted modem radiocarbon standard. This standard is defined as specific activity of in wood tissue, which grew in 1950. During assimilation, carbon is subjected to isotopic fractionating (see below), which results in its enrichment with light isotope. In order to eliminate the effect of isotopic fractionating, beside the radiocarbon activity is also determined isotopic ratio and a correction is introduced for average value 6 C... 

The presentation of radiocarbon measurements has been rigidly formalized. All measurements are made relative to a modern standard of oxalic acid, the activity of which has been related to the theoretical... 

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