Sensitivity 

Great article!  Very useful.

One thing that seems missing is discussion of how sensitive these things are and what sample size they need.

I was speculating about the recent Alexander Litvinenko poisoning with Polonium-210 and presuming that one could use a Mass Spectrometer to date the age of the Polonium-210 as it decays to Lead-206 with a half-life of only 138 days.  One might thereby deduce on which day the Polonium had been created (it's a synthetic isotope).  However the question of how big a sample one might need - and how sensitive the results might be came up and I was unable to find an answer.