NIER,AL CONTRIBUTION TO ATOMIC WEIGHTS

After completing his Ph.D. at the University of Minnesota, Al Nier too k up a Post-Doctoral Fellowship at Harvard University where he measure d the isotopic composition of 19 elements, with such attention to accu racy that his data have invariably stood the test of time. One of thos e elements was lead, which he measured in a variety of uranium mineral s from the Richards-Baxter collection, and showed that they varied in isotopic composition dependent on their age and chemical composition. This led to his long-standing interest in atomic weights. In 1950 Nier produced some isotopically enriched argon, and used this to calibrate a mass spectrometer to provide argon isotope abundances that were abs olute, in the sense that they were free from all sources of bias. He u sed the same spectrometer to measure the ''absolute'' isotope abundanc es of nine other elements, assuming that the isotopes of those element s behaved in a similar manner to those of argon. In the early 1950s Ni er's interest turned to the measurement of atomic masses. He and his c olleagues built a double-focusing mass spectrometer and this was used to measure the atomic masses of numerous elements with high accuracy. He also departed from the accepted mass spectrographic technique for m easuring atomic masses by using electronic measuring devices rather th an photographic techniques. Atomic masses, together with absolute isot ope abundances, are essential parameters in determining atomic weights , and in some cases, fundamental constants. Professor Nier also made a significant contribution to the International Commission on Atomic We ights. He served as a member of the Commission from 1947-1961, and was a key figure in the unification of the ''chemical'' and ''physical'' scales of atomic weights using carbon-12 as the common base. (Int J Ma ss Spectrom 178 (1998) 1-7) (C) 1998 Elsevier Science B.V.