Mass spectrometers have become routine laboratory instruments in many disciplines. ‘Measuring isotopes: mass spectrometers’ concentrates on those used to quantify the abundance of different isotopes—gas source isotope ratio, thermal ionization, inductively coupled plasma, and secondary ion mass spectrometers. A mass spectrometer can be used to quantify the concentration of a particular element by monitoring an isotope of that element not overlapped by isotopes of other elements. All mass spectrometers have three essential components: an ion source, a mass filter, and a detector. There are two main types of detector: Faraday detectors measure large signals and a variant of photomultiplier tubes measures small isotope signals.