Like bromine, compounds that contain chlorine atoms have a distinct ion pattern on a mass spectrum. The A+2 peak for a monochlorinated compound will be at almost one-third the intensity to the 35Cl peak due to the presence of 37Cl isotope. A compound with two chlorine atoms will show distinct A+2 and A+4 peaks with an approximate ratio of 10:6:1.
The following EI mass spectra are for 3-chloropropanenitrile and (1E)-1,2-dichlorobut-1-ene with nominal masses at 89 and 110 Da, respectively. The top MS, for 3-chloropropanenitrile, shows 2 identifiable ion clusters at m/z 49/51 and 89/91 that contain chlorine. The ion peak at m/z 54 does not contain chlorine.
The MS for (1E)-1,2-dichlorobut-1-ene shows 2 ion clusters at m/z 75/77 and 110/112/114. Only the ion cluster at m/z 110 is dichlorinated.
TIP: Check the fragment ion peaks too for the distinct pattern especially when the molecular ion peak is not visible.