The presence of chlorine and bromine in a molecule or fragment can easily be identified due to their distinctive (M+2) isotopic peaks.

Bromine: Bromine has two isotopes one has a mass number of 79 and a natural abundance of 50.7%, and the other has a mass number of 81 and a natural abundance of 49.3%. Due to their natural abundance, they show a ratio of 1:1 intensity for their M and M+2 peaks.

Two bromine atoms: When a compound has two bromine atoms

Coming to the given spectrum:

1,2-dibromoethane has two bromine atoms, there are four possibilities based on the occurrence of the two isotopes.

Since there is one possible arrangement that would give (M), two for (M+2), and one for (M+4), the ratio of peak intensity would be 1:2:1.

Mass Spect:Isotopes Practice Problems