||Chapter 13: Spectroscopy
Mass Spectroscopy (MS)
What should you be able to do with MS ?
- Identify the molecular ion in order to determine the molecular weight
- Determine the presence of common isotopes such as Cl and Br
- More advanced skills allow one to identify fragments ions (but this can be tricky and other spectroscopic methods will likely be easier to use to get
|Mass spectrometry is based on slightly different principles than the other
The physics behind mass spectrometry is that a charged particle passing
through a magnetic field is deflected on a circular path with a radius
that is proportional to the mass to charge ratio, m/e so for +1 ions, the radius of the path is proportional to the mass of the molecule (i.e. the MW).
In an electron impact (EI) mass spectrometer, a high energy beam of
electrons is used to displace an electron from the organic molecule
to form a radical cation known as the molecular
ion. The EI is a high energy process and a lot of energy is transferred to the molecule. As a result, if the molecular ion is unstable then it can fragment
to give other smaller ions.
The collection of ions formed (i.e. molecular ion and various fragment ions) is focused into a beam then accelerated into
the magnetic field and deflected along circular paths according to the
masses of the ions. By adjusting the magnetic field, the ions can be
focused on the detector and recorded generating the mass spectrum.
- Probably the most useful information you should be able to obtain from a
MS spectrum is the molecular weight of the sample.
- This will often be the heaviest ion observed from the sample provided
the molecular ion is stable enough to be observed.
||The ion obtained by the loss of an electron from the molecule
||The most intense peak in the MS, assigned 100% intensity
||Symbol often given to the molecular ion
||+ve charged species (i.e. a cation) with an odd number of electrons (i.e. a radical)
||Lighter cations formed by the decomposition of the molecular ion.
These often correspond to stable carbcations.