Insensitive Nuclei Enhanced By Polarization Transfer - Background

Background

The sensitivity of NMR signal detection depends on the gyromagnetic ratio (γ) of the nucleus. In general, the signal intensity produced from a nucleus with a gyromagnetic ratio of γ is proportional to γ3 because the magnetic moment, the Boltzmann populations, and the nuclear precession all increase in proportion to the gyromagnetic ratio γ. For example, the gyromagnetic ratio of 13C is 4 times lower than the proton, so the signal intensity it produced will be 64 times lower than that of a proton. However, noise also increases as the square root of the frequency, the sensitivity therefore becomes roughly proportional to γ5/2. A 13C nucleus would be 32 times less sensitive than a proton, and 15N around 300 times less sensitive. Sensitivity enhancement techniques are therefore desirable when recording an NMR signal from an insensitive nucleus.

The sensitivity can be enhanced by artificially by increasing the Boltzmann factors. One method may be through nuclear Overhauser effect (NOE), for example, for 13C signal, the signal-to-noise ratio can be improved three-fold when the attached protons are saturated. However, for NOE, a negative value of K may result in a reduction in signal intensity; for 15N which has a negative gyromagnetic ratio, if the dipolar relaxation has to compete with other mechanisms, the observed 15N signal can be near zero. Alternative methods are therefore necessary for nuclei with a negative gyromagnetic ratio, and one such method using the INEPT pulse sequence was proposed by Ray Freeman in 1979.