HSC · Topic explainer

How do you read a mass spectrum?

A mass spectrum shows ion abundance against mass-to-charge ratio, written m/z. In a simple organic spectrum, a singly charged molecular ion can reveal the compound’s nominal molecular mass. The base peak is the most abundant ion and is assigned 100% relative intensity. Fragment peaks help you test possible structures.

Worked example

In an example spectrum, the molecular ion of a compound appears at m/z 46. Assuming it carries one positive charge, this indicates a nominal molecular mass of 46. Ethanol, C₂H₆O, is one formula match because 2 × 12 + 6 × 1 + 16 = 46.

Suppose the tallest peak is at m/z 31. That peak could fit a fragment such as CH₂OH⁺. A matching molecular mass or fragment cannot establish a unique structure on its own. Use other spectra or chemical evidence to distinguish plausible compounds.

An illustrative mass spectrum. The base peak at mass-to-charge ratio 31 has relative intensity 100 percent. The molecular ion is marked at mass-to-charge ratio 46.

Official exam practice

2025 HSC Chemistry, Question 36, mass spectroscopy

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Further reading. NESA 2025 Chemistry marking feedback