Accurate Atomic Masses for Fundamental Metrology

We have measured the atomic masses of ten species with a typical accuracy of 10^-10 by comparing the cyclotron frequencies of 20 different pairs of alternately trapped ions. The mass of ²⁸Si provides a possible route to realizing an atomic standard of mass. The neutron separation energies for ¹⁵N and ¹³C determined from these mass measurements suggest the need to recalibrate γ-ray energies in the 2 to 13 MeV range, and may lead to an independent determination of both the fine structure constant α and the molar Planck constant NAh with a precision approaching 10^-7.