The molecular formula of TCDD is C12H4O2Cl4. Chlorine exists as two isotopes 35Cl (75%) and 37Cl (25%). How many molecular ion peaks are there? What is the mass of the most abundant one?

Because chlorine exists as two isotopes, two molecular ions will be present, and their relative heights will correspond to the abundances of the isotopes. The tallest peak will be 3 times the height of the smaller and will have a mass of 320 g/mol, i.e. corresponding to a Cl mass of 35 g/mol.