Mass spectrometry

Stages of mass spectrometry

• vaporization

• Ionization

• Acceleration

• Deflection

• Detection

The formation of molecular ions

When the vaporized organic sample passes into the ionization chamber of a mass spectrometer, it is bombarded by a stream of high energy electrons. These electrons have high enough energy to knock an electron off an organic molecule to form a positive ion. This ion is called the molecular ion or sometimes the parent ion

Fragmentation of the molecular ion

• The molecular ions are energetically unstable and some will break into smaller fragments, in the simplest case the molecular ion breaks into two parts - one of which is another positive ion and the other is an uncharged free radical

• The uncharged radical wont produce a line on the mass spectrum and they will get lost in the machine and then removed by vacuum pump

• The ion will travel through the mass spectrometer just like any other positive, being accelerated by the electric field and deflected by the magnetic field

• The lighter ions are deflected more than the heavier ions and so a spectrum of ions of different mass/charge ratios is produced

The molecular ion peak and the base peak

In the mass spectrum of pentane, the line produced by the heaviest ion passing through the machine is due to the molecular ion

The tallest line in the stick diagram is called the base peak. This is usually given an arbitrary height of 100 and the height of everything else is measured relative to this, it is the tallest peak because it represents the most common fragment ion to be formed - either because there are several ways in which it could be produced during fragmentation of the parent ion, or because it is a more stable ion

Using mass spectroscopy to find the Mr of compounds

One of the peaks shows a unique molecular formula, so there is only one combination of atoms that has that Mr

The low resolution of mass spectrometry does not allow us to determine the Mr of compounds but high resolution spectrometry tells us the specific molecular formula of a compound

Using mass spectroscope to find the Mr of compounds

The first species formed is called the molecular ion

The signals for the molecular ion gives the Mr of the compound - this is the peak with the greatest m/z

Using mass spectroscopy to find the molecular formula of compounds

• high resolution mass spectrometers measure the m/z values to enough accuracy to find the molecular formula

• If more than one compound has the same Mr, high resolution spectrometer can show us what molecular formula they have exactly