3. Isotopes
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.
Fig(1) Bromine
Fig(2) Vinyl Chloride
Fig(3) 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.
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