  We present the abundances of N in a sample of 62 stars on the red giant
branch (RGB) in the peculiar globular cluster NGC 1851. The values of [N/Fe]
ratio were obtained by comparing the flux measured in the observed spectra with
that from synthetic spectra for up to about 15 features of CN. This is the
first time that N abundances are obtained for such a large sample of RGB stars
from medium-resolution spectroscopy in this cluster. With these abundances we
provide a chemical tagging of the split red giant branch found from several
studies in NGC 1851. The secondary, reddest sequence on the RGB is populated
almost exclusively by N-rich stars, confirming our previous suggestion based on
Stromgren magnitudes and colours. These giants are also, on average, enriched
in s-process elements such as Ba, and are likely the results of pollution from
low mass stars that experienced episodes of third dredge-up in the asymptotic
giant branch phase.
