1. A graduate student is studying bromobenzene, and decides to take an El mass s
ID: 696770 • Letter: 1
Question
1. A graduate student is studying bromobenzene, and decides to take an El mass spectrum of it. A research scientist suggests that he take the mass spectra at 50 eV, 70 eV and 120 eV (i.e., the electrons are accelerated between plates that result in different kinetic energies). The mass spectrum at 70 eV is below. How do you expect the bromobenzene mass spectra to change for El mass spectrometry at 50 eV and 120 eV? Sketch the spectra, highlighting the key differences between the three mass spectra. Explain your reasoning behind any differences between the spectra El mass spectrum of bromobenzene (C6HsBr), taken at 70 eVExplanation / Answer
1. Mass spectral analysis
Shown above is the mass spectrum for bromobenzene at 70 eV
The base peak here is M+ 156 and 158 in 1 :1 ratio due to 79Br and 81Br isotopes presence
When the mass spectra was taken at 50 eV, less fragmentation of the molecule is expected than that at 70 eV. So the M+ peak height would go up and peak at 77 goes down.
When the mass spectra was taken at 120 eV, greater fragmentation of the molecule is expected than that at 70 eV. So the M+ peak height would decrease and peak at 77 increase. The base peak now changes to m/z 77 instead of m/z 156.
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