The discovery of mass spectrometry

The award of the 1922 Nobel chemistry prize to Francis Aston may have surprised some of his contemporaries, as his most significant research was in an area generally regarded as a branch of physics. Yet with a century’s worth of hindsight, the decision seems appropriate. Aston’s work was to have a substantial impact on the next hundred years of chemistry.

Francis William Aston was born in 1877, in Harborne, a village near Birmingham. This region of the English midlands was renowned for its skilled metal-workers, who made nails and needles, buttons and buckles, spurs and stirrups, and thousands of other essential items. Although factories flourished there, much work was still outsourced to small companies, and even to individual artisans with back-yard forges. Both Francis’s grandfathers came from farming backgrounds, but established successful metal-working businesses. And his father – while still owning a farm – became a metal merchant, supplying materials to local manufacturers.

Aston’s work was to have a substantial impact on the next hundred years of chemistry

Francis’s interest in science flowered in this dynamic environment. As a schoolboy he had his own attic laboratory, and in 1893 he entered Birmingham’s Mason College to study for London University’s external examinations. After graduating in 1898 he became a research student there , helping chemistry professor Percy Frankland investigate optically active substances.

From 1900, Aston was employed as a chemist by a nearby brewery, but continued doing research in his home lab. He was fascinated by the luminous phenomena generated when electrical discharges passed through partially evacuated glass tubes. Earlier investigators – including Michael Faraday, Heinrich Geissler, Julius Plücker, Wilhelm Hittorf and William Crookes – had all obtained intriguing results, and Wilhelm Röntgen’s 1895 discovery of x-rays aroused further interest, but many puzzles remained unresolved.

Expert skills

This research was technically demanding. Making complex glass vessels that could protect a near-vacuum against the atmosphere’s pressure was difficult enough. When these containers were pierced by electrical wires – and by valves for admitting and removing gases – maintaining their integrity became even more challenging. But Aston was an expert glass-blower, and had mastered the necessary skills by 1903, when he returned to the college – now part of the new University of Birmingham – as a research assistant to physics professor John Poynting.

Aston’s tasks there included making and operating electrical discharge tubes. He also investigated their peculiarities on his own initiative, and discovered a phenomenon now called the Aston dark space. In 1908, however, a substantial legacy from his father enabled him to spend a year’s holiday travelling around the world via Australia, New Zealand, Canada and the US. While visiting Hawaii, he became one of the first Europeans to try surfing.

In January 1910, he moved to the University of Cambridge’s Cavendish Laboratory, as technical assistant to J J Thomson

Sports were always important to Aston – he performed creditably in tennis tournaments, enjoyed skiing and mountaineering, and once cycled two hundred miles in a day. In later life he often partnered Ernest Rutherford on the golf course, and although the professor’s famously erratic drives must have tried Aston’s patience sorely, the two remained friends. Aston was also an accomplished musician and a skilled photographer – further activities that required an intense concern for precision.

In the autumn of 1909 Aston returned to the University of Birmingham as a science lecturer, but it soon became obvious that he was unsuited to this new career. In January 1910 – on Poynting’s recommendation – he moved to the University of Cambridge’s Cavendish Laboratory, in the more familiar role of technical assistant to J J Thomson. Thomson was a titan of theoretical physics – a Nobel laureate himself, he taught or supervised several further Nobel prize-winners. However, he lacked manual dexterity and needed specialist technicians to build and maintain his apparatus. This was Aston’s function at the Cavendish – until his experimental skills and theoretical insight earned him recognition as an independent investigator.

Rays to ratios

One of Thomson’s major achievements had been proving (in 1895) that the cathode rays produced in discharge tubes were streams of negatively-charged particles – later to be named electrons. Thomson then investigated the analogous anode rays – first observed by Eugen Goldstein in 1886, and shown by Wilhelm Wien in 1897 to consist of positively charged ions. In 1910 Aston joined this project.

The particles they studied were repelled by the anode of an almost-evacuated discharge tube. Having passed through the tube’s perforated cathode, they were diverted by electrical and magnetic fields before hitting a photographic plate or fluorescent screen. In Thomson’s apparatus, the electrical and magnetic fields were applied to the particle-stream at the same point, but at right angles to each other. Under these conditions, the paths of ions having the same charge/mass ratio but different velocities diverged, since the slower-moving ones were more strongly affected by the electrical field. The result was a roughly parabolic line on the detecting medium.

When ions with different charge/mass ratios coexisted in the particle stream, they were steered into a sequence of roughly parallel curves on the detecting medium creating what became known as a mass spectrum. When Aston and Thomson put neon gas into this instrument in 1912, they expected to see lines representing the ions Ne⁺ and Ne²⁺. They did – but close to the Ne⁺ line there was a faint trace they could not explain.

Isotopic intuition

A clue to this anomaly was provided by the English chemist Frederick Soddy, who proposed that the atoms of some elements did not all have identical masses. Soddy eventually succeeded in adding the word isotope to science’s vocabulary – the word was suggested to him by pioneering female doctor Margaret Todd – but initially his suggestion encountered some opposition. Thomson himself was sceptical, though Aston thought their results could indicate the presence of two neon isotopes. However, attempts at separation – by fractional distillation and by gaseous diffusion – were unsuccessful. As Aston stated in his Nobel lecture: ’When the war interrupted the research, it might be said that several independent lines of reasoning pointed to the conclusion that neon was a mixture of isotopes, but none of these could be said to carry absolute conviction.’

Aston spent the first world war at Farnborough, helping the Royal Aircraft Factory improve its aeroplanes, and was fortunate to escape unharmed when one of them crashed. Occasionally, he discussed physics with colleagues there – including Frederick Lindemann, who mistrusted Soddy’s isotope concept, and thought Aston’s neon was probably contaminated by some compound whose ionised molecule had a charge/mass ratio close to that of Ne⁺.

While Aston worked on military aircraft, the Canadian physicist Arthur Dempster was pursuing isotopes at the University of Chicago in the US. Dempster competed his PhD in 1916, and in 1918 built an instrument differing somewhat from Thomson and Aston’s. Initially, Dempster’s results made less impact. But he persevered with the research, and his eventual identification of a uranium isotope with atomic mass 235 would prove to be a vital step towards the exploitation of nuclear energy.

Aston quickly confirmed that neon had isotopes with atomic masses 20 and 22

By the time Aston returned to Cambridge in 1919, Soddy’s isotope concept had been vindicated by measurements of the atomic masses of different lead samples (including radioactive decay products). Nevertheless, to confirm that two neon isotopes did exist, a better instrument was needed. Aston built one, increasing the precision of its measurements from one part in a hundred to one part in a thousand – and eventually, beyond one part in ten thousand.

One of Aston’s improvements to Thomson’s earlier mass spectrograph was to narrow the beam, by passing the positive ions through consecutive slits. Also significant was his decision to divert this beam in one direction by an electrical field, before bending it back in the opposite direction with a magnetic field. The intensities of these fields were adjusted so that particles having the same mass/charge ratio but differing velocities were focussed to a point, rather than tracing a line.

Aston quickly confirmed that neon had isotopes with atomic masses 20 and 22, in proportions that were compatible with neon’s overall atomic mass of 20.2. He also identified two chlorine isotopes having atomic masses of 35 and 37, and further discoveries followed as heavier gases – and eventually solid elements – were ionised and analysed. This work established Aston’s scientific reputation – he became a fellow of Cambridge’s Trinity College in 1920, was elected to the Royal Society in 1921, and received the Nobel Chemistry Prize in 1922. An impressive career trajectory for a former brewery chemist!

A weighty problem

Aston’s discoveries raised profound issues in physics, chemistry and cosmology. Chemists had often wondered why the atomic masses of so many elements were so close to being integers. Now, the discovery of isotopes with (apparently) integral atomic masses suggested new possibilities. In his 1922 Nobel lecture, Aston declared:

’By far the most important result of these measurements is that, with the exception of hydrogen, the weights of the atoms of all the elements measured, and therefore almost certainly of all elements, are whole numbers to the accuracy of experiment, namely, about one part in a thousand.’ [Emphasis added]

This uniformity was more obvious because Aston took one sixteenth of the atomic mass of the oxygen-sixteen isotope – rather than hydrogen’s atomic mass – as his basic unit of measurement. But he also recognised that hydrogen’s anomalous status resulted from the inter-convertibility of mass and energy, in accordance with Einstein’s famous equation.

Aston’s discoveries raised profound issues in physics, chemistry and cosmology

It seemed clear that in the nuclei of all elements – except hydrogen – the mutual repulsion of protons must be resisted by a powerful attractive force if their atoms were to remain stable. Others had already attributed this force to the conversion of a tiny portion of nuclear mass (later called the ‘mass defect’) into binding energy. But the hydrogen nucleus – a lone proton – needed no binding energy, and was therefore proportionally more massive. By 1922 Aston fully appreciated the implications of this fact, stating in his Nobel lecture that

‘… we may consider it absolutely certain that if hydrogen is transformed into helium a certain quantity of mass must be annihilated in the process. The cosmical importance of this conclusion is profound and the possibilities it opens for the future very remarkable, greater in fact than any suggested before by science in the whole history of the human race. … Should the research worker of the future discover some means of releasing this energy in a form which could be employed, the human race will have at its command powers beyond the dreams of scientific fiction.’

Weighing the future

During the 1920s and 30s Aston continued improving his mass spectrograph. But its ever-increasing precision eventually revealed that the atomic masses of isotopes did not, after all, have exactly integral values – each one had its own individual ‘mass defect’. Aston described the ratio of an isotope’s mass defect to its mass number as its ‘packing fraction’, and showed that for any isotope, the size of this fraction was indicative of the relative stability of its nucleus.

In 1932, however, the picture was further complicated when Aston’s Cambridge colleague, James Chadwick, discovered another nuclear particle – the neutron. Soon afterwards, the use of neutron bombardment to split atomic nuclei opened up exciting new possibilities for physicists. But Aston was not involved in these developments, and he ceased doing research following the outbreak of the second world war in 1939.

During the 1940s mass spectrometry advanced rapidly, due to its vital role in the development of nuclear physics (and of nuclear weapons). Its utility was greatly enhanced after the photographic plates employed by earlier investigators were replaced by electronic detectors, permitting more precise measurements of the components of an ion beam.

Since those pioneering days, the mass spectrometer has contributed significantly to many areas of chemical and biological research. It is employed as an analytical tool in numerous industries, and it can extract valuable information from archaeological finds. Recent advances have enabled researchers to vaporise – and ionise – some large and relatively fragile organic molecules, and then subject them to mass spectrum analysis, generating fresh ideas about how such molecules might function in living systems. Further advances may be expected in the years ahead.

Francis Aston died in November 1945. Unmarried and childless, he left substantial bequests to Trinity College and to various scientific institutions – but his greatest legacy was the instrument he had done so much to develop.

Mike Sutton is a historian of science in Newcastle, UK