90.7 amu

We dunno.

At one time scientists thought it came from comets, but now that we've learned more about the relative isotopic abundances in comets, that is no longer seeming quite so likely.

Separation of a mixture according its isotopic composition.

explain the mechanism of diffusion in the separation of isotopic mixtures

It is approximately equivalent to the number of protons and neutrons in the atom (the mass number) or to the average number allowing for the relative abundances of different isotopes.