Mass spectrometry can be used to determine relative molecular mass.

The idea

For a molecular sample, the peak at the highest mass-to-charge value that is not an isotope satellite is the molecular ion peak, formed when the whole molecule loses a single electron. For a singly charged ion its mass-to-charge value equals the relative molecular mass, so reading that peak gives the relative molecular mass directly. The tallest peak, the base peak, is usually a fragment and must not be used. A small peak one mass unit higher comes from the roughly one per cent of carbon present as carbon-13, and it is ignored when quoting relative molecular mass but can be used to estimate the number of carbon atoms.

What earns the marks

• Identifying the molecular ion peak as the peak at the highest mass-to-charge value.
• Stating that its mass-to-charge value equals the relative molecular mass when the ion is singly charged.
• Ignoring the small peak one mass unit above the molecular ion, which arises from carbon-13.
• Distinguishing the molecular ion peak from the base peak or fragment peaks.

Common mistakes and how to put them right

• Using the tallest peak instead of the peak at the highest mass-to-charge value.

• Quoting the peak one mass unit above the molecular ion as the relative molecular mass.

• Forgetting that the reading equals relative molecular mass only for a singly charged ion.

• Treating a fragment peak as evidence of a second compound in the sample.

Exam tips

• Read the spectrum from the right-hand end; the molecular ion is the last significant peak you meet.
• State that the ion is singly charged when you quote relative molecular mass from the axis.
• Use the size of the peak one mass unit above the molecular ion only when the question asks about the number of carbon atoms.