Organic Chemistry Text Book (CHEM 3401 and 3402)

Since a mass spectrometer separates and detects ions of slightly different masses, it easily distinguishes different isotopes of a given element. This is manifested most dramatically for compounds containing bromine and chlorine, as illustrated by the following examples. Since molecules of bromine have only two atoms, the spectrum on the left will come as a surprise if a single atomic mass of 80 amu is assumed for Br. The five peaks in this spectrum demonstrate clearly that natural bromine consists of a nearly 50:50 mixture of isotopes having atomic masses of 79 and 81 amu respectively. Thus, the bromine molecule may be composed of two ⁷⁹Br atoms (mass 158 amu), two ⁸¹Br atoms (mass 162 amu) or the more probable combination of ⁷⁹Br-⁸¹Br (mass 160 amu). Fragmentation of Br₂ to a bromine cation then gives rise to equal sized ion peaks at 79 and 81 amu.

 

bromine		vinyl chloride		methylene chloride

 

The center and right hand spectra show that chlorine is also composed of two isotopes, the more abundant having a mass of 35 amu, and the minor isotope a mass 37 amu. The precise isotopic composition of chlorine and bromine is:
        Chlorine:   75.77% ³⁵Cl and 24.23% ³⁷Cl
        Bromine:   50.50% ⁷⁹Br and 49.50% ⁸¹Br

The presence of chlorine or bromine in a molecule or ion is easily detected by noticing the intensity ratios of ions differing by 2 amu. In the case of methylene chloride, the molecular ion consists of three peaks at m/z=84, 86 & 88 amu, and their diminishing intensities may be calculated from the natural abundances given above. Loss of a chlorine atom gives two isotopic fragment ions at m/z=49 & 51amu, clearly incorporating a single chlorine atom. Fluorine and iodine, by contrast, are monoisotopic, having masses of 19 amu and 127 amu respectively. It should be noted that the presence of halogen atoms in a molecule or fragment ion does not change the odd-even mass rules given above. 

To make use of a calculator that predicts the isotope clusters for different combinations of chlorine, bromine and other elements Click Here. This application was developed at Colby College.

Two other common elements having useful isotope signatures are carbon, ¹³C is 1.1% natural abundance, and sulfur, ³³S and ³⁴S are 0.76% and 4.22% natural abundance respectively. For example, the small m/z=99 amu peak in the spectrum of 4-methyl-3-pentene-2-one (above) is due to the presence of a single ¹³C atom in the molecular ion. Although less important in this respect, ¹⁵N and ¹⁸O also make small contributions to higher mass satellites of molecular ions incorporating these elements. 

The calculator on the right may be used to calculate the isotope contributions to ion abundances 1 and 2 amu greater than the molecular ion (M). Simply enter an appropriate subscript number to the right of each symbol, leaving those elements not present blank, and press the "Calculate" button. The numbers displayed in the M+1 and M+2 boxes are relative to M being set at 100%.