需要氘代溶剂来完成锁场(Field lock)和匀场(deuterium gradient shimming).
Field Lock In order to produce a high resolution NMR spectrum& O0 x; f* D5 V1 W6 O6 |9 Q
of a sample, especially one which requires signal averaging or phase
+ }- {0 j/ r; {+ h! Icycling, you need to have a temporally constant and spatially- B$ o4 ~+ z1 n# k0 |
homogeneous magnetic field. Consistency of the Bo
7 |/ t! P0 a& `* Ffield over time will be discussed here; homogeneity will be discussed
0 u# ]4 v3 S0 n- t/ `6 g8 nin the next section of this chapter. The field strength might vary over
3 u, o9 T) C3 X, X5 Otime due to aging of the magnet, movement of metal objects near the+ a* z, }9 N- D. L# x6 Y
magnet, and temperature fluctuations. Here is an example of a one line" G% \) @5 ?/ A- ]0 P2 |$ O/ L
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
1 a% D0 Y7 B( T$ @/ IThe field lock can compensate for these variations.
. x" p8 t6 i, _+ P \# o
The field lock is a separate NMR spectrometer within your spectrometer.- x5 x" Q! k6 h' i) A+ x
This spectrometer is typically tuned to the deuterium NMR resonance Q/ E* H% G* _, l: O7 X3 S/ b
frequency. It constantly monitors the resonance frequency of the$ T* _8 E' m+ k- Q4 x( ~
deuterium signal and makes minor changes in the Bo magnetic field to keep the2 O* H' d( Q% f6 }6 B. [. E8 e% r
resonance frequency constant. The deuterium signal comes from the3 u' M' G+ x4 g5 Z
deuterium solvent used to prepare the sample. The animation window
. K6 p4 u1 \8 L1 k0 R7 ^' Scontains plots of the deuterium resonance lock frequency, the small$ q w( T2 h# c! P
additional magnetic field used to correct the lock frequency, and the
) L# \% t& g) \4 L/ i/ {/ presultant Bo( A6 l4 f; O* M9 O+ H+ s
field as a function of time while the magnetic field is drifting. The
- k. |: q% G5 l) ?# }5 W, J5 S& vlock frequency plot displays the frequency without correction. In3 q4 N, g- ]4 u" }
reality, this frequency would be kept constant by the application of" i* A& y* k2 e
the lock field which offsets the drift.
' d4 M$ t+ \7 X+ |
$ p# E. a, K% Q2 _
6 k8 B* n6 n$ ^1 Y4 ~% G
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =07 y E# C) k) ]% A. t$ G/ W: _: d
reference. The resonance frequency of the deuterium signal in many lock. w4 {* r8 K% S/ y3 w
solvents is well known. Therefore the difference in resonance frequency
. @/ m) E& Z9 W& l8 I% x' [# oof the lock solvent and TMS is also known. As a consequence, TMS does; s1 ^- b6 \4 o5 @" C8 p) [' `
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
: z' Z# M6 U9 {8 q=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
