Analytical chemistry part 1 interpreting mass spectra

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Detecting elements and compounds

Mass spectrometry is a highly sensitive technique that can be used to help identify compounds. Samples of only a few milligrams are required.

This makes it useful in forensic science. Spectra of samples could be compared, for example, to link a suspect to a weapon that has been fired.

Mass spectrometry is useful in the identification of unknown materials, because the mass spectrum of an unknown sample can be compared with a database of known spectra.


Mass spectroscopy in space

Mass spectrometers have been included onboard several space probes.

During the descent towards the surface of Titan, the mass spectrometer analyzed the atmospheric composition at various heights. It was found that the atmosphere consists mainly of nitrogen and methane. The presence of argon-40 was also detected.

In 2005, the Huygens probe landed on the surface of Titan, the largest moon of Saturn.


Molecular ions

When a compound is analyzed in a mass spectrometer, gaseous molecules are bombarded with high-speed electrons from an electron gun.

M(g) + e- M+(g) + 2e-

The peak with the highest mass-to-charge ratio (m/z) is formed by the molecular ion, and the value of m/z is equal to the relative molecular mass of the compound.

These knock out an electron from some of the molecules, creating molecular ions, which travel to the detector plates:


Molecular ions and peaks

The peak at the highest m/z on the mass spectrum is formed by the heaviest ion that passes through the spectrometer. Unless all molecules of the original substance break up, this corresponds to the molecular ion of the sample substance.

molecular ion peakmass spectrum of paracetamol

40 80 120 160

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m/z


High resolution mass spectroscopy

High resolution mass spectrometry can determine the m/z of a peak to several decimal places. This can distinguish between compounds that have very similar relative molecular masses (Mr) but different empirical formulae.

To integer values, hexane (C6H14) and pentanal (C5H10O) both have a Mr of 86. However, using atomic masses to four decimal places, the Mr of hexane is 86.1106, while the Mr of pentanal is 86.0739.

With a high enough resolution, it is therefore possible to distinguish between hexane and pentanal, which are both colourless liquids.


Determining molecular formulae


What is fragmentation?


Interpreting mass spectra


Mass spectroscopy

Analytical chemistry part 1 interpreting mass spectra

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