The relative abundances of the isotopes of an element is determined using mass spectrometry. A mass spectrometry isotope distribution graph is needed as shown in the picture. This is for the isotopes of Zirconium. The x-axis is the individual mass, while the y-axis is the relative abundance. For this example, the atomic weight of Zirconium is calculated as:
Atomic weight = 90 amu(50%) + 91 amu(11%) + 92 amu(18%) + 94 amu(19%) + 96 amu(2%) = 91.35 amu
