5.   Mass spectrometry

This does not involve an electromagnetic spectrum at all – the spectrum here is a range of masses of molecules or fragments of molecules.

Everything in mass spectrometry occurs in a vacuum. A sample (preferably a gas) is introduced and broken down into charged fragments by electron impact or ‘chemical ionization’. The fragments, accelerated by applying a voltage, pass through a mass selector which separates them by their ratio of mass to charge (m/w or m/z). The separate fragments are detected and measured as ion current. Under constant conditions, a molecule will break up in the same ways, giving a reproducible ‘mass spectrum’, a plot of the relative numbers of fragments vs. their mass/charge ratio.

Early mass spectrometers were large and slow. New designs have produced small and fast-acting instruments which are commonly used as detectors in gas chromatography, producing a new mass spectrum every tenth of a second.