Disclosed herein is a method for evaluating and measuring the performance, efficacy and safety of candidate new chemical entities. This method comprises employing a target compound having one or more 12C atoms in the molecule wherein at least one of the 12C atoms is substituted with a 13C atom, The stable labelled target compound is then administered to a test subject following which the target compound and/or one or more of its metabolites are recovered using conventional separation techniques and purified. The resulting isolated material of interest is then combusted in the presence of a petrochemical based carrier, the 12C content of such carrier with respect to naturally occurring 13C being in excess of 99.9 percent. The CO2 resulting from such combustion is then graphitized and the graphitized material is analyzed employing techniques, such as Accelerator Mass Spectrometry (“AMS”), capable of differentiating and counting the carbon atom isotopes (13C vs. 12C), thus allowing quantification of the compound/metabolite of interest.
