Due the relative abundance of chlorine and bromine. It is possible to determine which of the halogens is present in a mass spectrum of a organic compound:
The Cl-35, Cl-37 have a relative abundance of 75/25; Br-79 and Br-81 of 50/50.
For that reason, if a compound has a M/M+2 ratio of 75:25 you can be sure the compound has 1 Cl in its structure. Thus, the only possible structure is:
