After chemical pre-treatment, the samples are burnt to produce carbon dioxide and nitrogen.A small amount of this gas is bled into a mass spectrometer where the stable isotope ratios of carbon and nitrogen are measured.In AMS the radiocarbon atoms are directly detected instead of waiting for them to decay.Sample sizes are thus typically 1000 times smaller allowing a much greater choice of samples and enabling very selective chemical pre-treatment.
-counting method are the much greater sensitivity of the measurement.
-counting method) or by directly counting the radiocarbon atoms using a method called Accelerator Mass Spectrometry (AMS).
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.
The sample is put into the ion source either as graphite or as carbon dioxide.
It is ionised by bombarding it with caesium ions and then focused into fast-moving beam (energy typically 25ke V).