For isotopes occurring at extremely low levels, accelerator mass spectrometry (AMS) can be used.  For example, the decay rate of the radioisotope C is widely used to date organic materials, but this approach was once limited to relatively large samples no more than a few thousand years old.  AMS extended the range of C dating to about 60,000 years BP, and is about 10 times more sensitive than conventional IRMS.
AMS works by accelerating negative ions through a large (mega-volt) potential, followed by charge exchange and acceleration back to ground. During charge exchange, interfering species can be effectively removed. In addition, the high energy of the beam allows the use of energy-loss detectors, that can distinguish between species with the same mass/charge ratio. Together, these processes allow the analysis of extreme isotope ratios above 10.