Mass spectra - the m+2 peak
This page explains how the M+2 peak in a mass spectrum arises from the presence of chlorine or bromine atoms in an organic compound. It also deals briefly with the origin of the M+4 peak in compounds containing two chlorine atoms.
Note: Before you start this page, it would be a good idea to have a reasonable understanding about how a mass spectrum is produced and the sort of information you can get from it. If you haven't already done so, explore the mass spectrometry menu before you go on.
The effect of chlorine or bromine atoms on the mass spectrum of an organic compound