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Mass spectrograph

the smallest ‘weighing scales’ of all East Anglia,

Francis William Aston examines apparatus resting on a work bench. Other laboratory apparatus can be seen on the wall behind him.
Francis William Aston, English chemical physicist, c.1920s. Science Museum/ Science and Society Picture Library

While experimenting with how charged atoms and molecules can be deflected by magnetic and electric fields, Cambridge chemist J J Thomson (1856-1940) made an unexpected discovery. The degree of deflection depended on the mass of the particle, but he found that neon produced two traces, rather than one. This suggested that the element was made up of two different kinds of atom which were chemically identical but had a different mass. Today these are known as isotopes. 

Thomson recruited a student, Francis William Aston (1877-1945), who after the First World War Aston constructed his first mass spectrograph, an instrument that can measure the masses and relative concentrations of atoms and molecules in a sample. This apparatus showed that there are indeed two types of neon and also showed that there are isotopes for many other elements. As he improved his mass spectrograph Aston succeeded in identifying 212 of the naturally occurring isotopes. 

Today, mass spectrometry is a powerful analytical method with many applications. These include finding pesticide traces, analysing the breath of anaesthetised patients, exposing drug abuse in athletics, revealing oil deposits and studying complex biological molecules. 

Science Museum

East Anglia
University of Cambridge
Key Individuals
J J Thomson, Francis William Aston,