  We investigate the gas-phase and grain-surface chemistry in the inner 30 AU
of a typical protoplanetary disk using a new model which calculates the gas
temperature by solving the gas heating and cooling balance and which has an
improved treatment of the UV radiation field. We discuss inner-disk chemistry
in general, obtaining excellent agreement with recent observations which have
probed the material in the inner regions of protoplanetary disks. We also apply
our model to study the isotopic fractionation of carbon. Results show that the
fractionation ratio, 12C/13C, of the system varies with radius and height in
the disk. Different behaviour is seen in the fractionation of different
species. We compare our results with 12C/13C ratios in the Solar System comets,
and find a stark contrast, indicative of reprocessing.
