  The metallicity evolution and ionization history of the universe must leave
its imprint on the Cosmic Microwave Background through resonant scattering of
CMB photons by atoms, ions and molecules. These transitions partially erase
original temperature anisotropies of the CMB, and also generate new
fluctuations. In this paper we propose a method to determine the abundance of
these heavy species in low density (over-densities less than $10^4-10^5$)
optically thin regions of the universe by using the unprecedented sensitivity
of current and future CMB experiments. In particular, we focus our analysis on
the sensitivity of the PLANCK HFI detectors in four spectral bands. We also
present results for l=220 and 810 which are of interest for balloon and
ground-based instruments, like ACT, APEX and SPT. We use the fine-structure
transitions of atoms and ions as a source of frequency dependent optical depth
($\tau_{\nu}$). These transitions give different contributions to the power
spectrum of CMB in different observing channels. By comparing results from
those channels, it is possible to {\it avoid} the limit imposed by the cosmic
variance and to extract information about the abundance of corresponding
species at the redshift of scattering. For PLANCK HFI we will be able to get
strong constraints ($10^{-4}-10^{-2}$ solar fraction) on the abundances of
neutral atoms like C, O, Si, S, and Fe in the redshift range 1-50.
Fine-structure transitions of ions like CII, NII or OIII set similar limits in
the very important redshift range 3-25 and can be used to probe the ionization
history of the universe. Foregrounds and other frequency dependent contaminants
may set a serious limitation for this method.
