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An analytical technique that sorts ions by their mass-to-charge ratio. A sample is converted into ions, those ions are separated according to that ratio, and a detector records how many arrive at each value. The result is a mass spectrum: a plot of intensity against mass-to-charge, conventionally written m/z.

Charge matters as much as mass. An ion carrying two charges is deflected as though it were half as heavy, which is why the measured quantity is a ratio rather than a mass. Reading a spectrum means inferring mass from that ratio, then inferring composition from the pattern of fragments left behind.

Instruments vary in how they separate ions — magnetic sectors bend them, quadrupoles filter them with oscillating fields, time-of-flight analysers simply race them down a tube and measure who arrives first. All of them are answering the same question.

The technique descends from work on positive rays in the early 1900s. Francis Aston built a mass spectrograph in 1919 and used it to show that neon exists as isotopes of different mass, work recognised with the Nobel Prize in Chemistry in 1922.

It is now ordinary laboratory equipment. Mass spectrometry identifies unknown compounds, measures isotope ratios in geology and archaeology, sequences proteins, screens athletes for banned substances, and has flown on spacecraft to sample the atmospheres of other worlds.

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