INEPT

The sensitivity of a low γ, spin I = ½ nucleus is determined by the difference in populations between the low energy and high energy states, governed by the Boltzmann distribution. If the low γ, spin I = ½ nucleus is coupled to a proton the energy level diagram is more complicated than simply two levels and is shown in the figure below where a ¹³C-¹H spin pair is used as an example.

The populations of the states involved in the ¹³C transitions and hence the sensitivity of the ¹³C signal can be altered by inverting the H1 or H2 ¹H transitions with 180° pulses. This is illustrated in the figure below.

In the left panel, the H1 transition of a ¹³C-¹H spin pair is inverted (i.e. the populations of the two energy levels of the H1 transition are swapped). This also affects the populations of the energy levels involved in the C1 and C2 ¹³C transitions. After inversion of the H1 ¹H transition, the intensities of the C1 and C2 ¹³C transitions have changed from their equilibrium value of 1:1 to an enhanced value of 5:-3. If the H2 transition is inverted (right-hand panel), the C1:C2 intensity ratio is -3:5. In both cases the sensitivity of the ¹³C doublet has been enhanced compared to its equilibrium value. This enhancement is called INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) and is one of the most common sensitivity enhancement techniques used in NMR pulse sequences. The simplest implementation of INEPT is shown in the figure below along with the vector diagrams.

Phase cycling can be employed to obtain a -4:4 anti-symmetric doublet, rather than doublets with components of unequal magnitude. This is represented in the figure below.

A refocusing element can be added to the end of the sequence to refocus the anti-symmetric doublets and data can be collected with proton decoupling.

The result is a singlet with 4 times  (i.e. γ_H/γ_C) the intensity of the singlet one would expect under equilibrium conditions without an NOE.  For ¹⁵N, one obtains a sensitivity gain of ~10. The results of these implementations of INEPT are compared to the equilibrium situation in the figure below.

INEPT has the additional advantage that its repetition rate is determined by the ¹H T₁ rather than the ¹³C T₁.  This is a tremendous additional sensitivity improvement when multiple scans are collected because the ¹H T₁ is often shorter than the ¹³C T₁ by an order of magnitude.  One can collect approximately ten times as many scans per unit time.  This advantage is even more significant for ¹⁵N.  Reverse INEPT is used in the collection of ¹H data for  carbon-proton pairs to suppress the protons bound to ¹²C.