Box 4. Determining the Age of Biological Agents
The analysis of isotopes contained in biological agents can yield information on their point of origin, conditions of culture, and time of production. Traditional carbon dating relies on the radioactive decay of carbon-14, which permits dating of organic materials that are at least a few hundred years old. However, relatively large amounts of 14C-labeled carbon dioxide were created during atmospheric nuclear tests in the 1950s and early 1960s (see ref. 1 for example). The decline of 14C reflects the incorporation of the carbon dioxide into the biosphere since the cessation of aboveground testing. This rise and fall of 14C remains measurable in the biosphere, and accordingly, 14C concentrations in materials such as illicit drugs and biological weapons can be matched to this "bomb curve" to assess the average age of their plant and animal starting materials.
Because concentrations of 14C are exceedingly low, about one part in a trillion in environmental samples, only accelerator physics technologies can resolve and quantify such low isotopic abundance's in small samples. Accelerator mass spectrometry (AMS) was originally developed because archaeologists needed a method superior to decay counting for precise measurement of 14C. AMS systems use traditional mass spectrometry methods coupled with a particle accelerator to accelerate ions to energies sufficient for destruction of molecular isobars that would otherwise interfere with the measurement of the isotope of interest. AMS is now the standard method for radiocarbon dating and is ideally suited for forensic determinations of 14C because of its precision and ability to process amounts of material less than 1mg, for example.
The use of AMS to determine the age of biological samples assumes that the calendar ages assigned to the results of these analyses reflect isotopic equilibrium with the organism's growth medium, and that the growth medium represents an average age of its constituents. Due to the fact that a 14C concentration intercepts the bomb curve on both its upward and downward slopes, dates derived by this method have two values. Choosing between them requires other data.
Isotopic 14C signatures may also make it possible to discern between several samples grown on the same medium. For example, cultures grown on media that were produced at different times would have different 14C contents. Therefore, it may be possible to discriminate genetically identical organisms by their 14C signatures.