text: As a consequence of low receptivity, C NMR spectroscopy suffers from complications not encountered in proton NMR spectroscopy.  Many measures can be implemented to compensate for the low receptivity of this nucleus.  For example, high field magnets with internal bores are capable of accepting larger sample tubes (typically 10 mm in diameter for C NMR versus 5 mm for H NMR).  Relaxation reagents allow more rapid pulsing. A typical relaxation agent is chromium(III) acetylacetonate.  For a typical sample, recording a C NMR spectrum may require several hours, compared to 15–30 minutes for H NMR.  The nuclear dipole is weaker, the difference in energy between alpha and beta states is one-quarter that of proton NMR, and the Boltzmann population difference is correspondingly less.  One final measure to compensate for low receptivity is isotopic enrichment.
  Some NMR probes, called cryoprobes, offer 20x signal enhancement relative to ordinary NMR probes.  In cryoprobes, the RF generating and receiving electronics are maintained at ~ 25K using helium gas, which greatly enhances their sensitivity.  The trade-off is that cryoprobes are costly.