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

• Received 14 March 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.1481