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Physical and chemical pretreatments are done on these materials to remove possible contaminants before they are analyzed for their radiocarbon content.The radiocarbon age of a certain sample of unknown age can be determined by measuring its carbon 14 content and comparing the result to the carbon 14 activity in modern and background samples.Radiocarbon, or carbon 14, is an isotope of the element carbon that is unstable and weakly radioactive. Carbon 14 is continually being formed in the upper atmosphere by the effect of cosmic ray neutrons on nitrogen 14 atoms.It is rapidly oxidized in air to form carbon dioxide and enters the global carbon cycle.The CRA conventions include (a) usage of the Libby half-life, (b) usage of Oxalic Acid I or II or any appropriate secondary standard as the modern radiocarbon standard, (c) correction for sample isotopic fractionation to a normalized or base value of -25.0 per mille relative to the ratio of carbon 12/carbon 13 in the carbonate standard VPDB – Cretaceous belemnite formation at Peedee in South Carolina, (d) zero BP (Before Present) is defined as AD 1950, and (e) the assumption that global radiocarbon levels are constant.Standard errors are also reported in a radiocarbon dating result, hence the “±” values.It must be noted though that radiocarbon dating results indicate when the organism was alive but not when a material from that organism was used.There are three principal techniques used to measure carbon 14 content of any given sample— gas proportional counting, liquid scintillation counting, and accelerator mass spectrometry.
Over the years, other secondary radiocarbon standards have been made.
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.
A vial with a sample is passed between two photomultipliers, and only when both devices register the flash of light that a count is made.
Accelerator mass spectrometry (AMS) is a modern radiocarbon dating method that is considered to be the more efficient way to measure radiocarbon content of a sample.
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Radiocarbon dating is essentially a method designed to measure residual radioactivity.