  Inspired by recent experiments where electron transport was measured across
graphene nanoribbons (GNR) suspended between a metal surface and the tip of a
scanning tunneling microscope [Koch \textit{et al}., Nat. Nanotechnol. {\bf 7},
713 (2012)], we present detailed first-principles simulations of inelastic
electron tunneling spectroscopy (IETS) of long pristine and defected armchair
and zigzag nanoribbons under a range of charge carrier conditions. For the
armchair ribbons we find two robust IETS signals around 169 and 196 mV
corresponding to the D- and G-modes of Raman spectroscopy as well as additional
fingerprints due to various types of defects in the edge passivation. For the
zigzag ribbons we show that the spin state strongly influences the spectrum and
thus propose IETS as an indirect proof of spin polarization.
