Radiocarbon dating and methods of low level counting
The carbon isotope ratio can also be used to correct for isotopic fractionation in the radiocarbon measurement.
The carbon dioxide is collected in a glass ampoule or converted to graphite for radiocarbon measurement on the AMS system.
The accelerator is used to help remove ions that might be confused with radiocarbon before the final detection.
These ratios provide useful information on the purity of the sample and clues about the diet and climatic conditions of the living organism.
Measurement of the radioactivity of the sample works very well if the sample is large, but in 9 months less than 0.01% of the radiocarbon ions will decay, so in a reasonable measurement time (typically a few weeks) only a very small proportion of the radiocarbon atoms are detected by this method.
AMS, on the other hand, can in principle detect a much higher proportion (typically about 1% of the total) allowing sample sizes to be smaller by a factor of about a thousand.
The mass of these ions is then measured by the application of magnetic and electric fields.
The measurement of radiocarbon by mass spectrometry is very difficult because its concentration is less than one atom in 1,000,000,000,000.