Halogen isotopic clusters

Source. From Sparkman, used with permission. 

Consequently, any molecule RCI2 containing two Cl atoms will exhibit masses of R + 70, R + 72, and R + 74 with a relative abundance due to Cl isotopes of 100 66 11, due to the presence of R Cl Cl, R Cl Cl, R Cl Cl, and R Cl Cl. This pattern in the mass spectrum is a CI2 isotope cluster. The same pattern will be seen not only for the molecular ion, but also for any fragment ion that contains both Cl atoms. Similarly, if three chlorine atoms are present (RCI3), a cluster will occur with masses R + 105, R + 107, R + 109, and R + 111 with approximate abundances of 100 98 32 3. 

Similar sets of clusters can be determined for bromine compounds and for compounds containing both Br and Cl. Bromine has two isotopes, Br and Br, and they are approximately equal in abundance. A compound RBr containing 1 Br atom will have masses of R + 79 and R + 81, and they will be about equal in abundance. If 2 Br atoms are present, as in RBr2, there will be masses of R+158, R+160, and R + 162 from R Br Br, R Br Br, R Br Br, R Br Br, and their relative abundances will be 51 100 49, based purely on probabihty. 

Source From Sparkman, O.D., Mass Spectrometry Desk Reference, Global View Publishing. Pittsburgh, PA, 2000. 

---

Of course nowadays exact mass measurement could also distinguish these two molecules, as could a variety of other instrumental techniques. The analysis of isotopic clusters is most useful for detecting the presence of halogens, sulfur, and silicon, all of which have abundant isotopes of two atomic weight units higher, thus leading to relatively large M + 2 peaks. 

Gmelin Handbook of Inorganic Chemistry, System No. 68, Platinum. Main Volume, (a) Part A in Sections 1-6, 1938-1951 (Platinum Metals, Occurrence, History, Preparation, Alloys) (b) Part B The Element in Sections 1-4,1939-1942, Physical and Electrochemical properties of Platinum (Chemical reactions of Pt in Section 4) (c) Part C The Compounds of Platinum. Section 1. Compounds with Noble Gases, H, O, N, Halogens, S, Se, Te, B, C,Si, P, As, Sb, Bi, 1939 (d) Section 2. Compounds with the alkali metals and ammonium, mainly the alkali metal platinum double salts, 1940 . (e) Section 3 Other Compounds, 1940 (f) Part D Complexes with Neutral ligands, 1957. Supplement Volume (g) Part A. Section 1. Technology of Platinum Metals. 1986 (h) Section 2. Isotopes, Atoms, Molecules and Clusters. 1989. 

---