ABSTRACT
J.J. Thompson in 1913 rst used mass spectrometry (MS) to demonstrate that neon gas consisted of a mixture of nonradioactive isotopes, 20Ne and 22Ne. The atomic weight of neon listed in a modern periodic table is 20.18. Thompson obtained two peaks in the mass spectrum of neon, at masses of 20 and 22 with a roughly 10:1 intensity ratio, but no peak at mass 20.18. This work was revolutionary because it demonstrated that elements existed as isotopes with different atomic weights and simultaneously explained why the apparent atomic weight of an element based on chemical reactions was not a whole number. Neon in fact has three natural isotopes, but 21Ne is present in much smaller amounts than the other two isotopes. In 1923, Francis W. Aston used a higher-resolution instrument he designed to determine the atomic weights of the elements and the isotope ratios of each particular element. This was extremely useful to inorganic chemists and helped solve many of the problems concerning the position of elements in the periodic table at that time. During World War II, Nier at the University of Minnesota developed the high-resolution double-focusing instrument that permitted the analysis and separation of 235U from 238U, aiding in the development of the atomic bomb.
