Telescope Reviews: Variable isotopic percentages?

Lets begin with my thought experiment. Imagine that there is a quasar at a distance of twelve billion light years from our position and that clouds of intergalactic gas lay along our line of sight to that quasar at intervals of a billion light years. Ignoring the various routes of nuclear synthesis and positing that a large enough homogeneous portion of the universe existed at the earlier times in theory would there be a variation in the isotopes seen for the elements if some of those isotopes were radioactive? If so could such an experiment be used to detect whether for example if helium three or carbon thirteen are radioactive with extremely long half lives? I realize that the spectral lines are nearly identical for the lightest element's isotopes and that the intergalactic gas would be in motion.