NMR Sensitivity
DE = g h Bo / 2p
NMR signal depends on:
1)Number
of Nuclei (N) (limited to field homogeneity and filling factor)
2)Gyromagnetic
ratio (in practice g3)
3)Inversely
to temperature (T)
4)External
magnetic field (Bo2/3,
in practice, homogeneity)
5)B12 exciting field strength
Na / Nb = e DE / kT
Increase energy gap
-> Increase population difference -> Increase NMR signal
DE
≡
Bo
≡
g
g
- Intrinsic
property of nucleus can not be changed.
(gH/gC)3 for
13C is 64x
(gH/gN)3 for
15N is 1000x
1H
is ~ 64x as sensitive as 13C and 1000x as sensitive as 15N
!
Consider that the natural abundance
of 13C is 1.1% and 15N is 0.37%
relative sensitivity increases to
~6,400x and ~2.7x105x !!
signal (s) % g4Bo2NB1g(u)/T