22.2.6—The presence of bromine and chlorine atoms

A compound containing chlorine or bromine can show an M+2 peak because ³⁷Cl and ⁸¹Br are two mass units above the common isotopes. The relative M:M+2 pattern carries the evidence.

One chlorine gives an approximate 3:1 M:M+2 pattern, while one bromine gives about 1:1. Multiple atoms create larger isotope clusters, so inspect the whole pattern.

A molecular ion with nearly equal M and M+2 peaks suggests one bromine atom; a 3:1 pair suggests one chlorine atom.

M+2 does not prove which halogen from mass difference alone. Use abundance ratios and consider more than one halogen.