The atomic masses of both ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ have been measured with a Penning trap mass spectrometer that utilizes a frequency-shift detector to observe ion cyclotron resonances. Present resolution exceeds 1 part in ${10}^9$ and is limited only by the stability of the magnetic field. The leading systematic shift (at ≲1 part in ${10}^{10}$) is due to the residual quadratic B field dependence. The atomic masses have been combined to yield Δ${\mathit{Mc}}^2$${(}^3$H${\mathrm{-}}^3$He)=18 590.1(1.7) eV. The excellent agreement with recent results from β spectrometers lends strong support for new limits on the neutrino’s rest mass.

• Received 19 October 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.2888