Recent developments in mass spectrometry have made possible the direct detection of many naturally occurring long-lived radioisotopes. Radioactive atoms are present at such low concentrations that the sensitivity of the mass spectrometry has to be increased to detect parts per quadrillion (1015) in a sample. This sensitivity has been achieved, and some of the results taken at Rochester by the Rochester (University), Toronto (University), General Ionex (Corporation) collaboration are listed in the table. All the radioactive isotopes listed in the table are of importance in geochronology, and for 14C and 36C1, sensitivities better than parts per quadrillion (1015) have already been reached. Early work on the stable isotopes of platinum has already reached below parts per billion (109).