Ch13 - Mass Spectroscopy

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

Basics

Mass spectrometry is based on slightly different principles than the other spectroscopic methods.

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.

schematic of an electron impact mass spectrometer

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.

Terminology

Molecular ion	The ion obtained by the loss of an electron from the molecule
Base peak	The most intense peak in the MS, assigned 100% intensity
M⁺	Symbol often given to the molecular ion
Radical cation	+ve charged species (i.e. a cation) with an odd number of electrons (i.e. a radical)
Fragment ions	Lighter cations formed by the decomposition of the molecular ion. These often correspond to stable carbcations.