Chemistry Mass spectrometer

Whats the point of a mass spectrometer

• Mass spectrometry was initially used to show the identity of isotopes

• It is now used to calculate molecular masses and characterise new compounds

Explain step 1 ionisation

Gaseous atoms are hit by electrons from an electron gun and are converted into positive ions of 1+ charge

Explain step 2 Acceleration

Ions are charged so can be accelerated by an electric field. Ions are accelerated so that they all have the same kinetic energy.

Explain step 3 Deflection

• Charged particles will be deflected by a magnetic field.

The amount of deflection depends on:

• The mass of the ion Lighter ions are deflected more than heavier ones

• The charge on the ion. Ions with 2 positive charges are deflected more than the ones with only 1 positive charge.

• These two factors are combined into the mass/charge ratio m/z

Most of the ions passing through the mass spectrometer will have a charge of +1 so that the mass/charge ratio will be the same as the mass of an ion

Explain step 4 detection

• by electric or photographic methods to produce a mass spectrum

Give a reason why the mass spectrometer must be operated under vacuum.

To remove the air particles so to avoid the possibility of the ions hitting and being deflected by air molecules. Molecules in the air may also form peaks in the mass spectrum

Points to note abt a mass spectrum grpah

• Position of the peaks gives atomic mass

• Peak intensity gives the relative abundance

Other uses of mass spectrometry

To determine the molecular structure and the molecular mass of a compound

Molecular ion

When a molecule is ionised in the mass spectrometer it forms a molecular ion which can also undergo fragmentation to produce particles of smaller mass.

The final peak of (M+) shows the molecular ion (highest m/z value) and indicates the molecular mass.