Take the 2-minute tour ×  Say I have a sample of some alkali
metal - Rubidium for example. A sample of Rubidium consists of
approximately 27% $^{87}$Rb and 73% $^{85}$Rb. Does the vapor
pressure of each isotope individual necessarily have to be in
the same proportion? More specifically, assuming I have enough
Rubidium around that I have enough atoms of each isotope to
reach the saturated vapor pressure, is there any physical reason
why the ratio of the vapor pressures would match their relative
abundances?  I guess this is coming from this perspective - the
linear absorption spectrum for a sample of natural rubidium at a
specific fine transition (D2 line - 5S1/2 ->5P3/2 if you are
interested) looks like image below, the $^{87}$Rb peaks (outer)
are clearly smaller than the $^{85}$Rb peaks (inner), and this
amplitude ratio is directly related to their natural abundances.
If the atoms are in a gas, what forces their vapor pressure to
have the same ratio as their relative abundance?  enter image
description here