Mass Spectra – The Molecular Ion (M+) Peak
This page explains how to find the relative formula mass (relative molecular mass) of an organic compound from its mass spectrum. It also shows how high resolution mass spectra can be used to find the molecular formula for a compound.
Using a Mass Spectrum to Find Relative Formula Mass
The Formation of Molecular Ions
When the vaporised organic sample passes into the ionisation chamber of a mass spectrometer, it is bombarded by a stream of electrons. These electrons have a high enough energy to knock an electron off an organic molecule to form a positive ion. This ion is called the molecular ion.
Note: If you aren't sure about how a mass spectrum is produced, it might be worth taking a quick look at the page describing how a mass spectrometer works.
The molecular ion is often given the symbol M+ or M• – the dot in this second version represents the fact that somewhere in the ion there will be a single unpaired electron. That's one half of what was originally a pair of electrons – the other half is the electron which was removed in the ionisation process.
The molecular ions tend to be unstable and some of them break into smaller fragments. These fragments produce the familiar stick diagram. Fragmentation is irrelevant to what we are talking about on this page – all we're interested in is the molecular ion.
Note: If you are interested in a detailed look at fragmentation patterns you could follow this link.
Using the Molecular Ion to Find the Relative Formula Mass
In the mass spectrum, the heaviest ion (the one with the greatest m/z value) is likely to be the molecular ion. A few compounds have mass spectra which don't contain a molecular ion peak, because all the molecular ions break into fragments. That isn't a problem you are likely to meet at A-level.
For example, in the mass spectrum of pentane, the heaviest ion has an m/z value of 72.