The use of mass spectrometry to analyse the components separated by GC.

The idea

Gas chromatography separates a mixture in time, but on its own it tells you only when each component left the column, and two different compounds can share a retention time. Feeding the outflow straight into a mass spectrometer removes that ambiguity. Each component is ionised as it arrives and its own mass spectrum is recorded: the molecular ion peak gives the relative molecular mass, and the fragmentation pattern acts as a fingerprint that software matches against a database of known spectra. The combination is powerful because the two techniques fail in different ways, and it works on tiny samples, which is why it is used in drug testing in sport, forensic analysis, airport screening and pollution monitoring.

What earns the marks

• The gas chromatography stage separating the components by retention time.
• The mass spectrometer recording a spectrum for each separated component.
• The molecular ion peak giving the relative molecular mass.
• The fragmentation pattern being matched against a database to identify the component.

Common mistakes and how to put them right

• Saying the mass spectrometer separates the mixture, when the column has already done that.

• Identifying a component from its retention time alone and calling that conclusive.

• Using the base peak rather than the molecular ion peak to obtain the relative molecular mass.

• Describing a use of the technique without saying what each of the two stages contributes.

Exam tips

• Answer in two stages, separation and then identification, and say which instrument does which.
• Name the molecular ion peak explicitly whenever relative molecular mass is asked for.
• A named application earns credit only when it is tied to a property of the technique, such as working on very small samples.