The mass spectrum is a visual graph that shows a Intensity vs Mass plot, like that shown in the picture. This is an actual mass spectrometer diagram for Rubidium. The mass of the isotopes of the Rb element is along the x-axis, while the y-axis is the relative abundance. The atomic mass is determined to be:
∑(Mass×Intensity) = 85(1-0.38) + 87(0.38) = 85.76 amu
∑(Mass×Intensity) = 85(1-0.38) + 87(0.38) = 85.76 amu