I just began studying mass spectroscopy and I just seem to understand why certain peaks are not indicted in mass spectrum graph. Like lets say u have methyl or ethyl bromide. Usually, there is two equal peaks at right most side indicate 1:1 ratio of Br -79 and 89 isotopes. But this is only when u assume all carbon atom is C-12? Why does not graph show peak with combination of C-13 and bromine isotopes? None of the examples online indicate this. Pls help!!!