Mass spectrometers measure the range of possible masses for the particles making up the sample being tested. This results in a spectrum – the mass spectrum – for the substance under study. That spectrum reveals how many isotopes of a given element are to be found in the material. This is known as the isotope’s relative abundance – relative, that is, to the other isotopes found in the sample. Isotopes are atoms of the same element that have different atomic weights, due to varying numbers of neutrons (the neutrally-charged particles found in an atom's nucleus). Depending on the isotopes that make up a particular sample, researchers glean clues about its origin and how it was formed.
This tutorial shows the mass spectrum of seven different elements. Choose among them from the drop-down Menu of Elements. As you can see, gallium, for example, has two different isotopes. One of them – the one with an atomic weight of 69 – is more abundant than the other.