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Mass spectrometers are capable of separating and detecting individual ions even those that only differ by a single atomic mass unit. As a result moleucles containing different isotopes can be distinguished.

The number of isotopes
The two peaks in the mass spectrum shows that there are 2 isotopes of boron - with relative isotopic masses of 10 and 11 on the ¹²C scale.

The abundance of the isotopes
The relative sizes of the peaks gives you a direct measure of the relative abundances of the isotopes. You can find the relative abundances by measuring the lines on the diagram. In this case, the two isotopes (with their relative abundances) are:

Boron-10 = 23

Boron-11 = 100

Working out the relative atomic mass
The relative atomic mass, Ar, of Boron can be calculated as follows:
[(23 x 10) + (100 x 11)] / (23 + 100) = 10.8 (to 3 significant figures)

Note:
Based on the relative abundances of the 2 isotopes, one can predict that the A_rof Boron should be closer to 11 than 10 ("weighted" average), instead of a simple average of 10 and 11 to obtain 10.5.

Introduction	Characterisitics of Mass Spectrum	Determination of Isotopic Composition
Determination of Mr	Identification of Substances	Identification of Fragments
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