During the lifetime of an organism, the amount of c14 in the tissues remains at an equilibrium since the loss (through radioactive decay) is balanced by the gain (through uptake via photosynthesis or consumption of organically fixed carbon).However, when the organism dies, the amount of c14 declines such that the longer the time since death the lower the levels of c14 in organic tissue.Most samples require chemical pre-treatment to ensure their purity or to recover particular components of the material.The objective of pre-treatment is to ensure that the carbon being analyzed is native to the sample submitted for dating.Working with several collaboraters, Libby established the natural occurrence of radiocarbon by detecting its radioactivity in methane from the Baltimore sewer.In contrast, methane made from petroleum products had no measurable radioactivity.
Carbon-14 is produced in the upper atmosphere when cosmic rays bombard nitrogen atoms.
A more recent innovation is the direct counting of c14 atoms by accelerator mass spectrometers (AMS).
The sample is converted to graphite and mounted in an ion source from which it is sputtered and accelerated through a magnetic field.
For example, it was once standard practice to simply burn whole bones, but the results were eventually seen to be unreliable.
Chemical methods for separating the organic (collagen) from the inorganic (apatite) components of bone created the opportunity to date both components and compare the results.