‘Measuring isotopes: counting the atoms’ explores how isotopes are measured. For stable isotopes, atoms of each isotope are counted using a mass spectrometer. This turns atoms into charged ions and separates them into the different isotopic species using a mass filter. Precise measurements of isotopic abundance can be achieved in a few minutes or hours. Mass spectrometry could be used for radioactive isotopes, but for short-lived isotopes, their low abundance often makes them difficult to detect. The alternative is to use nuclear spectroscopy or counting methods to detect the characteristic energy released by the radioactive decay of a particular isotope, but these can be much longer processes.