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Aston's Mass Spectrograph
From: Science Museum
| By:
Derek Robinson |
EDITOR'S INTRODUCTION |
Isotopes are atoms which share the same chemical properties but have different atomic masses. Francis William Aston established the isotopes of the non-radioactive elements and in 1922 won the Nobel prize for Chemistry for his ground-breaking work. Derek Robinson of the Science Museum tells the story of a scientific pioneer. |
 rancis William Aston (1877-1945) was presented with the Nobel Prize for  Chemistry on 10 December 1922 for his work in establishing the isotopes of the non-radioactive elements. Isotopes are atoms which have the same chemical properties, but different atomic mass. For example, carbon can exist in its common form C12 or the less common form C14. At the award ceremony it was acknowledged that while radioactive elements had readily shown the existence of isotopes, to prove that non-radioactive elements could be a mixture of different isotopes had been far from easy. |
JJ Thomson, himself a Nobel Laureate, had achieved inconclusive experimental results to this end, but his assistant, Aston, used a quartz microbalance and mass spectrograph to identify two isotopes of the element neon. He went on to employ his original mass spectrograph to find the different isotopes of about fifty elements. |
Aston was the second son of a Birmingham metal merchant and farmer. In 1893 he began to study science under William Tilden, Percy Frankland and John Poynting at Mason College, Birmingham (now Birmingham University). Whilst he continued to contribute to basic understanding in this area of research for the ensuing twenty years, it was his appointment by Sir JJ Thomson at the Cavendish Laboratory in Cambridge that led to the discoveries for which Aston is now best remembered. |
Aston was given the task of improving Thomson's apparatus in which a beam of positively charged particles (positive rays) were deflected by a combination of electric and magnetic fields into sharp visible curves, each representing an individual particle's charge-to-mass ratio. Thomson thought that this apparatus gave rigorous proof that all the individual molecules of any given substance had the same mass. This Daltonian belief was rudely shattered in 1912 when Thomson obtained two curves for neon corresponding to masses 20 and 22. Two explanations suggested themselves: if neon had a true atomic weight of 20 (instead of the previously agreed figure of 20.20) then either mass 22 was an unknown compound of neon or a new element, meta-neon. |
Aston was assigned to improve Thomson's original apparatus, with the aim of studying the neon/meta-neon question. He tried to separate the suspected meta-neon by a variety of techniques. To see how well he was succeeding in separating neon and the mysterious substance, he devised the miniature quartz microbalance. The arm of the balance was level only when the case surrounding the balance was filled with gas of known density. Aston then filled it with the unknown gas, and altered its pressure, thereby varying its density until the tiny beam balanced again. Comparing the pressures allowed him to calculate the atomic weight of the unknown gas. The results indicated that the mysterious substance was an element with the same properties as neon, but with a different atomic weight. |
The outbreak of the First World War in 1914 delayed further experiments but on his return to the Cavendish in 1919 Aston attacked the problem from a different direction, building a positive ray or mass spectrograph. Aston improved Thomson's apparatus by using electric and magnetic fields to bring the rays of uniform charge-to-mass ratio to a sharp focus on a photographic plate. Aston devised several methods of calibrating his instrument and in the case of neon obtained mass lines on his photographic plate at 20 and 22 with the intensities of the lines showing that the two particles occurred in the ratio of 10:1, consistent with an average mass of 20.20, the known atomic weight of neon. Neon was thus proven to be isotopic and in the short time before Aston was presented with his Nobel Prize he demonstrated the existence of isotopes in some thirty other gaseous elements. |
Aston's work provided important insights into the structure of the atom and the way different elements are related to each other, and for these reasons he received the ultimate accolade of the scientific community. |
With later more accurate mass spectrographs Aston obtained valuable information on the stability and abundance of particular isotopes. Through this work on the structure of the atom he appreciated the dangers of uncontrollably releasing the huge energy of the nucleus early on, and lived long enough to see his forebodings given awesome substance. Meanwhile his principal experimental tool, the mass spectrograph, has been refined almost beyond recognition, adapted and adopted as an analytical instrument of prime importance in innumerable areas of chemical and biological research and industrial practice. |
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