Mass and Relative Abundance of Isotopes

Abstract

The history of the measurement of isotopic masses and abundances parallels closely the history of the development of vacuum and electronic techniques. Today, practically all determinations of isotopic masses and abundances are made with mass spectrometers or mass spectrographs, and the modern versions of these instruments lean heavily upon the latest tech­ niques in the vacuum and electronic fields. The isotopic Tables of 1933 are made up almost exclusively of the mass spectrographic measurements of Aston (1). Both packing fractions and iso­ topic abundances were determined by allowing ion beams to impinge on photographic plates and observing the position and intensities of the lines found on the developed plates. Only in the early work of Dempster (2) was any serious systematic attempt made to measure isotopic abundances by electrical means. The techniques employed by Smyth (3), Bleakney (4), Tate & Smith (5), and others in the studies of impact of gases by electrons were soon applied in isotopic abundance studies, and in 1932 Bleakney (6) and his co-workers began reporting isotopic abundance measurements with an instrument using a common magnetic field for collimating the electron beam and making a mass analysis of the ions produced by electron impact. The use of this type of instrument was extended, by Nier (7) and others, so that by 1942 when the second edition of Aston's book appeared, of the 83 naturalIy occurring elements, 48 had been measured by electrical means. During the war years few isotope abundance measurements were made, but by the end of 1949 electrical abundance measurements had been made in 73 of the 83 elements reported in Tables. By the end of 1950 this number had grown to 78. Whereas in 1930 Aston was the sole worker measuring precise atomic masses, the 1930's saw new workers enter the field. Included among these were Bainbridge (8), Dempster (9), Bainbridge & Jordan (10), Mattauch (11), and Jordan (12). Each name or combination of names is associated with a particular type of instrument. For example, to a mass spectroscopist, a Bainbridge & Jordan instrument means a double focusing mass spectro­ graph employing a 1270 electrostatic analyzer followed by a 600 magnetic analyzer. The 1930's also saw the firm adoption of the doublet for the determination of atomic masses. In this method the mass difference between two atomic or molecular ion fragments having the same mass number is found. The determination of a number of such doublets, at least one of them containing 016, permits the computation of the masses of the various isotopes