Aston mass spectrographs

Aston mass spectrograph of 1919 that demonstrated that there are two distinct isotopes (by the modem definition) of neon... 

F. W. Aston (Cambridge) discovery, by means of the mass spectrograph, of isotopes in a large number of non-radioactive elements and for enunciation of the whole-number rule. 

Section 1.2 deals with the time period from Dalton to the discovery of isotopes by Soddy and Fajans. Much of the discussion elaborates on the type of material found in introductory chemistry texts. It ends with the discovery of radioactivity by Becquerel and the developments which quickly followed. Section 1.3 starts with the discovery of the concept of isotopes in the early years of the twentieth century and ends with the invention of the mass spectrograph in 1922 by Aston. The literature relating to the work leading up to the 1913 papers by Soddy and Fajans is well and... 

From the Discovery of Isotopes through the Invention of the Mass Spectrograph by Aston... 

Aston, the Mass Spectrograph, the Whole Number Rule... 

HW noted that past atomic number 27 the whole number rule did not work for the atomic weights. They attributed this fact either to the presence of isotopes for all of these elements or to some phenomenon which they did not understand. The first explanation (isotopes) is correct. The mass spectrograph, already mentioned as an invention of Aston, shows that all isotopic masses on the 160 scale (or the very similar 12C scale used today) tend to differ from integer values by less than 0.1%. 

As detailed earlier, Aston had been faced with this problem for the case of neon where he was not convinced by Thomsons s conclusion that there are two neon isotopes of masses 20 and 22. He attempted to show that indeed neon consists of two isotopes by trying to separate the two isotopes using thermal diffusion. The result proved unsatisfactory and he then proceeded to invent the mass spectrograph. 

It should, however, be mentioned that Mulliken s study of the BO system has been followed over the years by many others, An extensive study by Jenkins and McKellar (1932) should be mentioned explicitly. This study involved the long wavelength band of BO. The same method as that used by both Jevons and Mulliken to produce the BO was used in this work. The new (present day) quantum mechanics was used in the theoretical interpretation. Both the vibrational and the rotational isotope effects were observed and agree with theory. One motivation for this work was to determine how well the isotopic ratio of the square roots of the two relevant isotopic masses (10B and nB) agrees with the ratio obtained from Aston s mass spectrometric measurements and hence how well isotopic mass ratios determined from band spectra compare with those obtained using Aston s mass spectrograph. 

Within two decades of inventing the mass spectrograph, Aston succeeded in identifying 212 of the 281 naturally occurring isotopes of all the elements. He realized that atomic weights measured from... 

Figure 1.9 Mass spectrograph from F. W. Aston (1919/20). A - ion source (discharge tube) B, B2 -slits C, C2 - plates of condensator D - valve E - magnet P - photoplate for ion detection. (H. Kienitz (ed.), Massenspektrometrie (1968), Verlag Chemie, Weinheim. Reproduced by permission of Wiley-VCH.)...

Figure 1.11 F. W. Aston with second mass spectrograph (1922).

Francis W. Aston (1877-1945) developed a "mass spectrograph" in 1919 that could separate ions differing in mass by as little as 1 % and focus them onto a photographic plate. Aston immediately found that neon consists of two isotopes (20Ne and 22Ne) and went on to discover 212 of the 281 naturally occurring isotopes. He received the Nobel Prize for chemistry in 1922. 

Aston s work was founded upon accurate measurements of the deflections of charged panicles. These measurements were made in an instrument he devised, the mass spectrograph. Many later instruments were developed following Aston s work, or following die Dempster instrument, which was built before Aston s. The direction-focusing mass-spectrographs and the later velocity-focusing instruments anJ composite instruments facilitated the determination, not only ill tile musses land hence mass numbers) of the isotopes of an element, hut their quantities as well. As a result of the immense amount of research in ihis field, die isotopic composition of the stable elements has been closely determined. And Lilts... 

Aston mastered this new approach to an extremely delicate analysis of the chemical elements and developed it with surprising accuracy. A narrow beam of positive rays was passed into an electro-magnetic field which bent the stream of ions. This deflected beam of rays was then photographed on a sensitized plate. If the stream of ions was composed of atoms of equal mass only one band of light appeared on the plate. Positive rays consisting of atoms of different masses, however, were split into an electric spectrum, the number of bands depending upon the number of isotopes. Even the relative proportion of the isotopes could be determined from the size and darkness of the bands on Aston s mass spectrograph. ... 

Figure 1.9 Mass spectrograph from F. W. Aston (1919/20). A - ion source (discharge tube) B, -...

Thomson s apparatus did not permit very accurate results to be obtained. The method was increased in accuracy by F. W. Aston, also working in the Cavendish Laboratory, arid later by A. J. Dempster of the University of Chicago, and other American physicists. Modern types of mass spectrographs (Fig. 7-3) have an accuracy of about one part in 100,000 and a resolving power of 10,000 or more (that is, they are able to separate ion beams with values of M/n differing by only one part in 10,000). 

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