The sensitivity of any NMR experiment is governed basically by the gyromagnetic ratios of the starting and the detected spins. Thus, in a IS spin system (I stands for the sensitive nucleus and S for the less sensitive nucleus), four different approaches can be designed:

• Direct observation for isotopes with short T₁ values.

• Direct observation for isotopes with long T₁ values and small natural abundance and/or small magnetogyric ratio by using effective transfer polarization from a high-abundant nucleus, as ¹H and ³¹P, using INEPT-type experiments.

• Indirect detection via a high-abundant nucleus (usually ¹H and ³¹P) using a reverse INEPT transfer.

• Indirect detection using inverse spectroscopy (HMQC or HSQC-type experiments).