Mass Spectrograph
John Dalton's (1766-1844) atomic theory served scientists remarkably well for nearly a century after its announcement in 1803. But discoveries made in the late 1890s made it clear that Dalton's theory was incorrect or incomplete in some important details. The discovery of radioactivity by French physicist Antoine-Henri Becquerel (1852-1908) was among the most important of these. As scientists began to unravel the nature of this new phenomenon, they realized that atoms were far more complex than Dalton had imagined.
A particularly important discovery was made by English chemist Frederick Soddy (1877-1956) in 1913. During his investigation of naturally-occurring radioactive families, Soddy found that more than one form of an element could exist. These forms all had the same atomic number (and were, therefore, variations of a single element), but had different atomic weights. Soddy gave the name isotopes to these forms. Almost immediately, scientists began to ask whether isotopes might also be found among the stable elements. English physicist Joseph J. Thomson (1856-1940), discoverer of the electron, turned his attention to that question around 1912. After some initial discoveries, Thomson turned this research over to one of his students, Francis Aston.
Francis William Aston was born in Harborne, Birmingham, England, on September 1, 1877. He was educated first at Malvern College and later at Mason's College, Birmingham, later to become Birmingham University. He worked as a brewery chemist from 1900 to 1903 before accepting a research position at Birmingham University. After a world tour in 1909, he was invited by J. J. Thomson to join him at the Cavendish Laboratory at Cambridge. Aston spent the rest of his academic career in various positions at Cambridge. He died there on November 20, 1945.
After discovering the electron in 1897, Thomson decided to study the positively charged canal rays first observed by German physicist Eugen Goldstein (1850-1930) in 1886. He constructed an instrument that accelerated the canal rays through magnetic and electrical fields. The fields caused the rays to travel in a parabolic track whose shape was determined by the mass and velocity of the particles constituting the rays. Thomson found that canal rays produced by the excitation of neon gas formed two distinct tracks in his instrument. He was uncertain as to which of a number of explanations correctly accounted for this observation.
At this point, Thomson asked Aston to take over this line of research. Aston focused his attention at the outset on the improvement of Thomson's original instrument. Over the next decade, he made further refinements on the device until it was eventually able to resolve two tracks with an accuracy of one part in 10,000.
The mass spectrograph (also known as mass spectrometer) that Aston designed exposes a beam of positively charged particles to both an electrical and magnetic field. Aston found a way to modify the magnetic field so that particle beams are separated from each other entirely on the basis of their masses, not their velocities. With his improved instrument, Aston was able to demonstrate in 1919 that the two tracks observed by Thomson in his research were isotopes of neon with masses of 20 and 22 in a ratio of ten parts to one. Over the next two decades, Aston studied all but three of the known stable elements, identifying 212 different isotopes in the process. Eventually, Aston's mass spectrograph design was improved upon by those of A. J. Dempster (1886-1950), K. T. Bainbridge, and A. O. Nier. Today, the mass spectrograph continues to find use in a wide variety of research applications.
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