MS and NMR are complementary elucidation tools. Knowing when to apply the correct tool can facilitate the elucidation process.
Compounds with bromine atoms exhibit a distinct ion pattern on a mass spectrum. The A+2 peak for a monobrominated compound appears at almost identical intensity to the 79Br peak due to the presence of 81Br (~49.3% natural isotope abundance). A compound with two bromines shows a distinct A+4 peak with an approximate ratio of 1:2:1.
The following EI mass spectra are for 3-bromopropanenitrile and (1Z)-1,2-dibromobut-1-ene with nominal masses at 133 and 212 Da, respectively. The top MS shows 3 identified fragments containing bromine (m/z 79/81, 93/95 and 133/135) as noted by the ~1:1 intensity of the ion clusters. The ion peak at m/z 54 does not show a 1:1 ratio since the fragment does not contain a bromine atom.
The MS for (1Z)-1,2-dibromobut-1-ene shows 4 identifiable fragments containing bromine (m/z 117/119, 133/135, 197/199/201 and 212/214/216). In particular, ion clusters at m/z 197/199/201 and 212/214/216 show the distinct ~1:2:1 intensity. This is a result of the varying amounts of the isotopes 79Br and 81Br, that is, the 3 peaks represent 79Br/79Br : 79Br/81Br : 81Br/81Br.
TIP: Check the fragment ion peaks too for the distinct pattern especially when the molecular ion peak is not visible.