ll see what I can do to help downstream.    

The natural abundance background is important to the diagnostic power of many isotopic labeling and low blank trace-element detection techniques as well.  For example, a colleague of mine irradated a nanogram sized particle with so many neutrons that every U235 atom in it was fissioned, and then counted the number of fission tracks that resulted.  He found one, which in turn allowed him to infer U238 abundance (assuming solar system abundance ratios) and hence total U abundance in the particle as well.