Penning trap measurements of the masses cog Cs-133, Rb-87,Rb-85, and Na-23with uncertainties <= 0.2 ppb

We report new values from Penning trap single ion mass spectrometry for the atomic masses of Cs-133, Rb-87, Rb-85, and Na-23 with uncertainties less t han or equal to 0.2 ppb, a factor of 100 improvement over the accuracy of p reviously measured values. We have found M(Cs-133) = 132.905451931(27)u, M( Rb-87)= 86.909180520(15)u, M(Rb-85) = 84.911789732(14)u, and M(Na-23) = 22. 9897692807(28)u. These values are important for new ppb-level determination s of the molar Planck constant N(A)h and the finestructure constant alpha. With the measurement of Cs-133 We have increased the mass range for sub-ppb measurements by a factor of 3. From M(Cs-133) and other values we derive a lpha(-1) = 137.0359922(40).