We report new values from Penning trap single ion mass spectrometry for the atomic masses of ${}^{133}\mathrm{Cs}$, ${}^{87}\mathrm{Rb}$, ${}^{85}\mathrm{Rb}$, and ${}^{23}\mathrm{Na}$ with uncertainties $\le 0.2\mathrm{ppb}$, a factor of 100 improvement over the accuracy of previously measured values. We have found $M{(}^{133}\mathrm{Cs})=132.905\mathrm{}451931(27)u$, $M{(}^{87}\mathrm{Rb})=86.909\mathrm{}180520(15)u$, $M{(}^{85}\mathrm{Rb})=84.911\mathrm{}789732(14)u$, and $M{(}^{23}\mathrm{Na})=22.989\mathrm{}7692807(28)u$. These values are important for new ppb-level determinations of the molar Planck constant $N_{A}h$ and the fine-structure constant α. With the measurement of ${}^{133}\mathrm{Cs}$ we have increased the mass range for sub-ppb measurements by a factor of 3. From $M{(}^{133}\mathrm{Cs})$ and other values we derive ${\alpha }^{-1\mathrm{}}=137.035\mathrm{}9922\left(40\right)$.

• Received 19 April 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.4510