Clouds, Clumps, Cores & Comets - a Cosmic Chemical Connection?
  We discuss the connection between the chemistry of dense interstellar clouds and those characteristics of cometary matter that could be remnants of it. The chemical evolution observed to occur in molecular clouds is summarized and a model for dense core collapse that can plausibly account for the isotopic fractionation of hydrogen, nitrogen, oxygen and carbon measured in primitive solar system materials is presented. 
S. B. Charnley and S. D. Rodgers (NASA Ames)