Certain elements - Si and Fe being prime examples - are often extremely depleted from the interstellar gas (Jenkins 2003). The explanation for this depletion is that the atoms that are "missing" from the gas phase are located in solid particles - interstellar dust grains. The existence of this solid phase complicates efforts to determine elemental abundances, due to the difficult-to-determine abundance in grains.
The concentration of certain elements in dust grains also creates the possibility of selective transport of those elements through the gas, which could, at least in principle, lead to variations in elemental abundances from point to point in the interstellar medium, and possibly even differences between stellar and interstellar abundances. Dust grains might even deplete deuterium from the gas, providing an alternative explanation for observed variations in D/H ratios.
Observations of interstellar dust were recently reviewed by Whittet (2003) and Draine (2003a). The astrophysics of interstellar dust is a broad subject, encompassing dust grain formation and destruction, charging of dust grains, heating of interstellar gas by photoelectrons from dust, electron transfer from dust grains to metal ions, the optics of interstellar dust, optical luminescence and infrared emission from dust, chemistry on dust grains, and the dynamics of dust grains, including radiation-driven drift of grains relative to gas, and alignment of dust by the magnetic field. Introductions to the astrophysics of dust can be found in Krügel (2003) and Draine (2004).