Precision Penning trap comparison of nondoublets: Atomic masses of H, D, and the neutron

We compare the cyclotron frequency of two single ions in a Penning trap to obtain their mass ratio with a relative uncertainty close to 1×${10}^{\mathrm{-}10}$. We have developed methods, based on a classical implementation of the separated oscillatory fields technique, that extend this precision to mass ratios far from unity (nondoublets). This allows direct determination of the atomic mass of any ion by comparison to a ${\mathrm{C}}^{+}$ ion. Using isotopically selected methane ions, we obtain the masses of H, D, and the neutron with up to 40 times improved precision over accepted values. We have checked our technique with known nondoublet ratios such as M[${\mathrm{N}}_2^{+}$]/M[${\mathrm{N}}^{+}$].