Interpreting the PastUniversity of California Press. For decades after Libby performed the first radiocarbon dating experiments, the only way to measure the 14 C in a sample was to detect the radioactive decay of individual carbon atoms. 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. Before this can be done, the sample must be treated to remove any contamination and any unwanted constituents. It will therefore have the same proportion of 14 C as the atmosphere, or in the case of marine animals or plants, with the ocean. So, if we find the remains of a dead creature whose C to C ratio is half of what it's supposed to be that is, one C atom for every two trillion C atoms instead of one in every trillion we can assume the creature has been dead for about 5, years since half of the radiocarbon is missing, it takes about 5, years for half of it to decay back into nitrogen. The amount of 14 C in the atmosphere, and therefore in plants and animals, has not always been constant.
This can be done with a thermal diffusion column.
Information for Authors" PDF. Particularly for older samples, it may be useful to enrich the amount of 14 C in the sample before testing. Most carbon atoms have six protons and six neutrons in their nuclei and are called carbon Therefore, if we know the 14 C: 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.