Many of the molecule studied by NMR contain carbon. Unfortunately, the carbon-12 nucleus does not have a nuclear spin,but the carbon-13 nucleus does due to the presence of an unpalred neutron.Carbon-13 nuclei make up approximately one percent of the carbon nuclei on earth.Therefore,carbon-13 NMR spectroscopy will be less sensitive than hydrogen NMR spectroscopy.
The sensitivity of carbon-13 spectroscopy can be increased by any tecnique which increases the population difference between the lower and upper energy levels,or increases the density of spins in the sample.