  We consider element diffusion during the linear growth of structure in the
intergalactic medium (IGM) before the redshift of reionization. The elements
produced during big bang nucleosynthesis (such as D, He-4 or Li-7) condensed in
collapsing regions more readily than hydrogen. Diffusion was most effective
when the IGM was neutral since the collisional cross-section between neutral
species is smaller than the corresponding Coulomb cross-section in an ionized
gas. We find that diffusion led to small deviations in element abundances of up
to 0.1%. Abundance measurements to this level of precision in low-metallicity
environments could provide information about the thermal and ionization history
of the primordial gas during the early formation of structure in the IGM.
