The application of ISO 16620 to determine “biobased carbon content” is done by deriving a ratio of the amount of radiocarbon in an unknown sample to that of a modern reference standard. This ratio is calculated as a percentage with the units “pMC” (percent modern carbon) and is cited as the analytical measure. If the material being analyzed is a mixture of present-day radiocarbon and fossil carbon (containing no radiocarbon), then the pMC value obtained correlates directly to the amount of biomass or animal-derived carbon in the sample.
A material derived 100% from present-day soybeans will give a radiocarbon signature near 102 pMC. If it is diluted with 15% petroleum carbon, it will give a radiocarbon signature near 87 pMC. The “biobased carbon content” of a material is calculated by multiplying this pMC value by a correction factor to adjust for bomb carbon effect.
The ISO 16620-2 standard published on April 1, 2015 cites a value of 105 pMC for the adjustment reference (i.e. divide pMC by 1.05 or multiply pMC by 0.95). An alternative value may be referenced in the presence of empirical evidence. The 2015 value of 102 pMC (divide by 1.02 or multiply by 0.98) was empirically derived. It has now been well documented that the natural value is decreasing at a rate of 0.5 pMC each year. Therefore, starting January 2 each year the previous adjustment factor is reduced by 0.5 pMC (e.g. 101.5 for 2016 and 101.0 for 2017).
ISO 16620-2 allows for results to be reported as biobased carbon to TOTAL ORGANIC CARBON or to TOTAL CARBON. The final value is cited as % Biobased Carbon as a Fraction of Total Carbon or % Biobased Carbon as a Fraction of Total Organic Carbon depending upon the goal of the analysis and assumes all the components within the analyzed material were either present-day living (within the last decade) or fossil in origin.