Atomic weights are not constants of Nature! Reading the stories told by the isotopes

The observation of two stable isotopes of neon by J. J. Thompson in 1912 was the first chapter in what would become arguably one of the most significant breakthroughs in Science. Thompson's ingenious yet simple device employed electric and magnetic fields to reveal the existence of isotopes and this original design is the basis for modern instrumentation used in the measurement of isotope abundances. The mass spectrometer quickly became an important tool in the determination of the atomic weights of the elements, quantities that were thought at the time to be Constants of Nature. However, with increasing instrumental precision came the remarkable discovery that not all occurrences of an element had the same atomic weight values because of differences in isotopic composition. Researchers saw this discovery as a remarkable opportunity to use variations in isotopic composition to learn intimate information about physical, nuclear, and biological processes that may affect a particular element. Over the past 100 years technical and instrumental advances have enabled investigations supported by isotope abundance data to have an impact on our scientific understanding of our world and our health and well being in this world. In this talk, I will review the major discoveries that led to the development of modern isotope amount ratio measurement techniques and illustrate the insights made possible by these methods. I will highlight the knowledge we have gained as a result of reading the stories told by the isotopes.