Comets are among the most primitive bodies left over from the planetesimal-building stage of the solar nebula, and may preserve both presolar as well as nebular material. Studies of their volatile composition can therefore provide clues about the coupled evolution of ices and gases from collapsing molecular clouds to mature planetary systems. Here we show that 1"-3" millimetre-wave aperture synthesis observations of HNC, DCN, and HDO in comet Hale-Bopp reveal arc-like structures offset from the nucleus. The measured (HNC/HCN), (DCN/HCN), and (HDO/H2O) abundance ratios are substantially higher than those on larger scales or than can be accounted for by coma or equilibrium nebular chemistry models, but are similar to the values observed in dense interstellar cloud cores and young stellar objects. We propose that sublimation from millimetre-sized icy grains lifted by jets provides access to volatiles that would otherwise be altered by diffusion through warm surface layers of the nucleus. The (D/H) ratios further suggest that by mass Hale-Bopp, and by inference the outer solar nebula in general, consists of >15-40% largely unprocessed interstellar material.