Insensitive nuclei enhanced by polarization transfer

[1] INEPT uses J-coupling for the polarization transfer in contrast to Nuclear Overhauser effect (NOE), which arises from dipolar cross-relaxation.

One method may be through NOE; for example, for 13C signal, the signal-to-noise ratio can be improved three-fold when the attached protons are saturated.

[2] The pulse sequence of INEPT, as represented in the diagram, can be read as a combination of a spin echo and selective population inversion (SPI).

The refocusing allows all the proton chemical shifts to undergo population inversion in the SPI step without its undesirable selectivity.

This is the population inversion part of the scheme, where a further 90° pulse after a time period on both the sensitive and insensitive nuclei rotate the magnetization onto the z-axis.

This has the effect of producing an antiphase alignment of magnetization on the z axis, an important step during which the polarization is transferred from the sensitive nucleus to the insensitive one.

Diagrammatic representation of the INEPT NMR pulse sequence. The thin bar denotes a 90° pulse, while the thick bar denotes a 180° pulse. INEPT is utilized often to improve 15 N resolution because it can accommodate negative gyromagnetic ratios, increases Boltzmann population, and decreases T 1 relaxation. [ 3 ]