Most atoms in molecules could occur as different isotopes, resulting in different atomic masses. So, the mass spectrum contains mass signals corresponding to these differences. The positions of mass signals from various isotopes are determined by their mass differences. The relative intensities of the mass signals generally depend on the relative abundance of the isotopes. For example, carbon-containing molecules show an additional peak from 13C, one unit higher than the molecular ion peak. This additional peak is known as the M+1 peak. An increase in the number of carbon atoms gives more chances to include 13C, and so increases the relative intensity of the M+1 peak. The mass spectra of chlorides and bromides exhibit an M+2 signal, as the primary chlorine and bromine isotopes differ by two mass units. The relative intensities of the M+2 peaks helps to identify whether it is chlorine or bromine in the analyte molecule.