Aston mass units

Atomic mass is the mass of an atomic particle, i.e. a specific isotope. When expressed in unified atomic mass units, this is called the relative isotopic mass. The word relative is added to denote the fact that all masses are scaled to that of the isotope when set to 12 u. Nominal isotope masses are more commonly used when applying analytical techniques such as Secondary Ion Mass Spectrometry (SIMS) because this significantly simplifies matters without detracting from the information content needed. This represents the number of protons and neutrons within the nucleus, i.e. equal to the atomic mass number (A). Note It was the mass spectrograph constructed by Aston in 1919 (the first mass spectrometer from which SIMS evolved as covered in Section 1.2.1) that confirmed the existence of the isotopes, and allowed for the first time, an accurate means of measuring their relative mass (that relative to H, 0, or more recently C) and distribution. 

Francis William Aston (1877-1945), United Kingdom. For his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the whole-number rule. ... 

Aston discovered in 1923 that every isotopic mass is characterized by a small mass defect that is, the masses of nuclides are not simple multiples of a fundamental unit. Since Aston s discovery, considerable woik has been performed to measure accurately the masses of all the known stable nuclides (mainly by the use of mass spectrometers). Table 1.1 (in Chapter 1) contains the results of this work for the elements commonly found in organic compounds. 

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. 

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