Dempster mass spectrometer

A quite different type of mass spectrometer - the first 180° magnetic sector field mass spectrometer (see Figure 1.7), with directional focusing of ions for isotope analysis, was constructed by Dempster, independently of other instrumental developments in mass spectrometry, in 1918. 

Figure 1.7 Mass spectrometer from A.]. Dempster (1918). A - ion source - electromagnet ...

Figure 1.7 Mass spectrometer from A.f. Dempster (1918). A - ion source B - eiectromagnet C - Faraday cup D - eiectrometer. (H. Kienitz (ed.), Massenspektrometrie (1968), Veriag Chemie, Weinheim. Reproduced by permission of Wiiey-VCH.)...

The basic principle underlying mass spectrometry was formulated by J. J. Thomson (the discoverer of the electron) early in the century. Working with cathode ray tubes, he was able to separate two types of particles, each with a slightly different mass, from a beam of neon ions, thereby proving the existence of isotopes. (Isotopes are atoms of the same element that have slightly different atomic masses due to the presence of differing numbers of neutrons in the nucleus.) The first mass spectrometers were built in 1919 by F. W. Aston and A. J. Dempster. 

Electro-magnetic separation is the only method by which the isotopes of an element can be completely separated from one another. The separation is carried out by the mass spectrograph which works on the principle elaborated earlier for the mass spectrometer (Chapter 13). Working of the Dempster s mass spectrograph diagramatically represented by Figure 19.4, illustrates the basics of electromagnetic separation of isotopes. 

Following the pioneer work of Aston and Dempster, there were rather limited advances made in the field of isotopic abundance measurements until 1935 when A. 0. Nier (85) introduced a number of improvements to mass spectrometer design. Since then, many more refinements have been made, particularly with regard to more efficient ion sources and to more sensitive ion detectors, so much so, that the mass spectrometer has become an extremely sensitive instrument for the detection of practically all of the elements, and its sensitivity remains second only to that of radioactive counting techniques. 

Figure 9.16 Early mass spectrometer designs, (a) Aston, 1919 (b) Dempster, 1918 ...

Figure 9.18 Early mass spectrometer designs, (a) Aston, 1919 (b) Dempster, 1918 (c) Mattauch-Herzog, 1935 (d) Bainbridge, 1933. In each case, B signifies the magnetic fieid. Spectrometers (c) and (d) are dispersive mass spectrometers (c) the Mattauch-Herzog design is aiso a doubie-sector instrument, using an electric sector before the magnetic field. Modern versions of these spectrometers are still in use, with the photoplate replaced by one of the modern detectors discussed subsequently.

Electron ionization, first modern mass spectrometer A. Dempster [49]... 

Mass spectrographs were also built in the United States by A. J. Dempster and K. T. Bainbridge. The mass spectrograph has been succeeded by the mass spectrometer, in which the intensity of the separated ion beams are measured electrically. These instruments are now widely used in the determination of molecular structure (Chapter 13). The term relative atomic mass is now used in place of atomic weight, and isotopic masses are measured on the = 12.0000 scale. Aston himself soon discovered that small deviations from the whole-number rule are the norm. 

Mass spectroscopy employs the method developed by Aston and Dempster (Chapter 11). Before World War II mass spectrographs and mass spectrometers were used almost exclusively to determine the masses and abundances of isotopes. However, it became clear that not only could atoms be ionised in the mass... 

Historically, mass spectroscopy was developed to separate atoms and to determine the masses of isotopes accurately [1]. The predecessor of magnetic sector field mass spectrometers was built in 1918 by Dempster [2]. Later, some of the first instruments for the analysis of molecules [3] were used by analytical chemists in refineries to determine hydrocarbons (the first article in the first issue of the journal Analytical Chemistry dealt with mass spectrometry of a mixture of hydrocarbons [4]). Combinations of infrared and mass spectroscopy were reported shortly afterwards [5]. 

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