Insensitive nuclei enhanced by polarization transfer

Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) is a signal enhancement method used in NMR spectroscopy which involves the transfer of nuclear spin polarization from spins with large Boltzmann population differences to nuclear spins of interest with low Boltzmann population differences. INEPT uses J-coupling for the polarization transfer in contrast to the Nuclear Overhauser Effect (NOE) which arises from dipolar cross-relaxation.

The signal enhancement has two sources:

• The spin population effect increases the signal by a factor of K = ratio of magnetogyric ratios of the nuclei.
• Nuclei with higher magnetogyric ratio generally relax more quickly. Since the rate at which the INEPT transfer can be repeated is limited by the relaxation of these spins (rather than the low sensitivity spins), then the experiment can be repeated more frequently, increasing the signal-to-noise ratio.

As a result, INEPT can enhance the NMR signal by a factor larger than K, while the NOE can increase the signal by a factor of at most 1+K/2. Note that a negative value of K may result in a reduction in signal intensity; historically INEPT was proposed a method to enhance the signal from the limited number of nuclei with negative magnetogyric ratio e.g. ¹⁵N or ²⁹Si.