OCR AS Chemistry Unit 2 - Mass Spectrometry

Mass spectrometry can be used to:

• identify unknown compounds,
• determine the abundance of each isotope in an element,
• gain further information about the structure and chemical properties of molecules.

Mass spectrometry also has a wide range of industrial and medical applications today, for example:

• monitoring the breath of patients during surgery whilst under anaesthetic,
• analysing molecules in space,
• detecting traces of toxic chemicals in contaminated marine life,
• detecting banned substances such as steroids in athletics.

A mass spectrometer determines the mass of a molecule or isotope by measuring the mass-to-charge ratio of ions. Although mass spectrometers differ considerably in their operation, the same basic principles occur inside the instrument.

• The sample is introduced via a sample inlet.
• Sample molecules are converted into ions by an ionisation source. Methods of ionising include electron impact, chemical ionisation, an electrospray and even lasers.
• The ions are propelled into a mass analyser.
• The ions are separated according to their mass-to-charge ratio.
• The ions are detected. The mass spectrum is generated by computer software and either printed or shown on-screen.

The mass spectrum produced can be used to find the molecular mass, determine the structural features of a molecule or to find the abundance of isotopes.

A mass spectrum is a graph showing relative or percentage abundance on the y-axis and mass:charge ratio on the x-axis.

The mass to charge ratio in all mass spectra is shown as m/z, m being the mass and z being the charge on the ion.

The heights of the peaks give the relative abundances of the isotopes in the sample.

The molecular ion, M+, is the positive ion formed in mass spectrometry when a molecule loses an electron. It produces the peak with the highest m/z value in the mass spectrum

When a molecule is ionised by being impacted on by an electron, excess energy from the ionisation process can be transferred to the molecular ion, making it vibrate. This causes bonds to weaken and the molecular ion can split into pieces by fragmentation. Fragmentation results in a positive fragment ion and a neutral species.

Fragment ions are often broken up further into smaller fragments. The molecular ion and fragment ions are detected in the mass spectrometer.

Mass spectrometry can be used to determine the structure of an unknown compound and give its precise identity.

Although the molecular ion peak of two isomers will have the same m/z value, the fragmentation patterns will be different.

Each organic compound produces a unique mass spectrum, which can be used as a fingerprint for identification.

When looking at a mass spectrum, fragment peaks appear alongside the more important molecular ion peak. However, these fragment peaks give information about the structure of the compound.

For example, you may see the CH3+ fragment ions with an m/z value of 17 or an OH+ ion with an m/z value of 17.