  By combining a recent precise measurement of the ionization energy of
$^{87}$Rb with previous measurements of electronic and hyperfine structure, an
accurate value for the $^{85}\textrm{Rb}-^{87}\textrm{Rb}$ isotope shift of the
5$^2S_{1/2}$ ground state can be determined. In turn, comparison with
additional spectroscopic data makes it possible for the first time to evaluate
isotope shifts for the low-lying excited states, accurate in most cases to
about 1 MHz. In a few cases, the specific mass shift contribution can be
determined in addition to the total shift. This information is particularly
useful for spectroscopic analysis of transitions to Rydberg states, and for
tests of atomic theory.
