Pulsed Field Gradient

Pulsed field gradient (PFG) in NMR, as the name suggests, is a spatially dependent magnetic field (B_g (r)) applied across the NMR sample volume, for a short duration (typically <10 msec), during which time the static magnetic field (B₀) becomes spatially inhomogeneous. Application of PFG for a short duration (δ) introduces a spatially dependent phase (Ф (δ, r)) in the transverse magnetization, which depends on B_g(r) as well as on the gyromagnetic ratio (γ) and the coherence order (p) of the individual nucleus (j) involved and is given by (Keeler et al. 1994)

$$ \Phi \left(\updelta, \mathrm{r}\right)=s\ {\mathrm{B}}_{\mathrm{g}}\left(\mathrm{r}\right)\ \updelta\ \sum \limits_j{\mathrm{p}}_j{\upgamma}_j $$

The γ dependence of the phase factor makes the gradient less efficient for nuclei with smaller γ. This indicates that the spread of resonance frequencies across a sample on application of the same gradient will be four times less for ¹³C and ten times less for ¹⁵N as compared to ¹H...