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Background samples analyzed are usually geological in origin of infinite age such as coal, lignite, and limestone.
A radiocarbon measurement is termed a conventional radiocarbon age (CRA).
The principal modern standard used by radiocarbon dating labs was the Oxalic Acid I obtained from the National Institute of Standards and Technology in Maryland. Around 95% of the radiocarbon activity of Oxalic Acid I is equal to the measured radiocarbon activity of the absolute radiocarbon standard—a wood in 1890 unaffected by fossil fuel effects.
When the stocks of Oxalic Acid I were almost fully consumed, another standard was made from a crop of 1977 French beet molasses.
Plants and animals assimilate carbon 14 from carbon dioxide throughout their lifetimes.