Francis William Aston, inventor of the mass spectrometer and discoverer of isotopes

September 1, 1877, Birmingham (United Kingdom) – November 20, 1945, London

In 1903 he obtained a scholarship to study at the University of Birmingham and six years later he was a collaborator at the Cavendish Laboratory at the University of Cambridge, invited by Joseph John Thomson, where he worked on the identification of the isotopes of neon and investigated electric discharges in neon. low-pressure tubes.

After the First World War, in 1919, he invented the mass spectrometer. An experimental device that allows charged particles to be separated according to their mass. He also established a rule, named after him, that odd-numbered atomic elements cannot have more than two stable isotopes.

In his early investigations with the mass spectrograph, Aston discovered that passing a sample of pure neon gas through the instrument caused two separate spots to form on the detector, which meant that the gas contained atoms corresponding to two different masses. He, therefore, interpreted his discovery as indicating the existence of two different types of neon atoms, and that both must have the same number of protons since all forms of neon always contain the same number of protons, but a different number of neutrons. As a consequence, their atomic masses must be different.

With this discovery, Aston provided the first experimental proof of the existence of isotopes, i.e. forms of the same atom with an equal number of protons but a different number of neutrons, in two publications: ‘Isotopes’ (1922) and ‘Mass spectrometers and isotopes’ (1933).

He continued to conduct experiments and succeeded in identifying 212 of the 287 isotopes of non-radioactive elements in the periodic table. This led him to join the Royal Society in 1921 and to receive the Nobel Prize in Chemistry in 1922.

He subsequently became a professor at Trinity College, Cambridge, and in 1935 was elected chairman of the Atomic Committee of the Royal Society of Chemistry.

Foundation of Mass Spectrometry

Mass spectrometry is based on a simple principle: when a flow of charged particles is subjected to the action of a magnetic field, it undergoes a deflection; the amplitude of said deviation depends on the mass and the charge of the particles that make up the flow.

The spectrometer or mass spectrograph essentially consists of three parts: the ionization chamber, the deflection chamber, and the detector. In the ionization chamber, the atoms of the substance to be identified receive an excitation energy that makes them lose electrons. Sometimes this energy is achieved simply by heating the sample. As a consequence of the loss of electrons, atoms become positively charged particles called ions.

The ions produced in the ionization chamber then pass into the diversion chamber which is subjected to a strong magnetic field. When the flow of positive ions crosses the chamber, the trajectory of each of them undergoes a deviation due to the effect of the magnetic field; instead of going through the camera in a straight line, they do so following a curve. The degree of curvature of each path depends on the mass and charge of the positive ion; the heavy ions follow a path that does not deviate much from the straight line, while the lighter ones are more deviated.

Leaving the deflection chamber, the positive ions collide with a photographic plate, or similar element, installed in the detector. The detector records the magnitude of the deviations from the straight line experienced by the trajectories of the particles that make up the sample, thus indicating the mass and charge of said particles. Since each element and each atom have a characteristic mass and charge, reading the record collected by the detector allows the atoms present in the sample to be identified.

It currently has multiple applications by allowing the masses of the particles that are part of a sample to be determined with the aim of identifying them. For example, it is used to identify traces of substances found in places where a crime has been committed, when the quantities found are too small to be identified.