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Due to the distinctive mass spectral patterns caused by the presence of chlorine and bromine in a molecule, interpretation of a mass spectrum can be much easier if the results of the relative isotopic concentrations are known. The following table provides peak intensities (relative to the molecular ion (M+) at an intensity normalized to 100 percent) for various combinations of chlorine and bromine atoms, assuming the absence of all other elements except carbon and hydrogen.1-4 The mass abundance calculations were based upon the most recent atomic mass data.1
Citation
CRC Handbook of Chemistry and Physics - 96th Edition