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. }9 Z" I; T, Y) t; b: Q
of a sample, especially one which requires signal averaging or phase8 {& v' v- t. i
cycling, you need to have a temporally constant and spatially
/ K' H9 E3 `/ J' xhomogeneous magnetic field. Consistency of the Bo& C) p; s$ B( L7 G) T0 X0 s$ a
field over time will be discussed here; homogeneity will be discussed( g; q3 J- K3 @$ p& ~1 Q4 b
in the next section of this chapter. The field strength might vary over$ T6 V; w6 R$ S ]0 ~4 s
time due to aging of the magnet, movement of metal objects near the
3 `2 a8 x' Z; v* x& u+ `$ Tmagnet, and temperature fluctuations. Here is an example of a one line* j7 ~, {) B3 f( a, d- ?
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
5 P9 M. B! }2 ?+ x% G; gThe field lock can compensate for these variations.
, q( p) z: M; o2 YThe field lock is a separate NMR spectrometer within your spectrometer.
) i0 q$ z; ~% Q: gThis spectrometer is typically tuned to the deuterium NMR resonance
* f3 I/ O) d, }$ s0 Ufrequency. It constantly monitors the resonance frequency of the* }, B5 q! @& c \4 B) i; m6 m
deuterium signal and makes minor changes in the Bo magnetic field to keep the+ g p+ l, }( q/ G3 c: ^
resonance frequency constant. The deuterium signal comes from the
) ]# j" _! `9 @, u5 r5 w# d( qdeuterium solvent used to prepare the sample. The animation window
& P& A# r, j5 ?1 `. Hcontains plots of the deuterium resonance lock frequency, the small
( h4 q1 d' q; N* Eadditional magnetic field used to correct the lock frequency, and the7 H1 m7 L, d& g& A+ q
resultant Bo/ A, s/ I/ Z6 k, I& M
field as a function of time while the magnetic field is drifting. The
( S3 g( }# [ s m% wlock frequency plot displays the frequency without correction. In7 N1 @; o q4 ?
reality, this frequency would be kept constant by the application of# m: N3 T6 {5 A! I# i+ K
the lock field which offsets the drift.
, s3 ^$ i" C# ~! e' N3 {: G$ C9 f" s
. \0 t2 A( d7 a2 D! E' l4 Q
% }8 l7 d. |7 {( U4 A, U( {
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =08 H. z( F9 f, @, D$ g
reference. The resonance frequency of the deuterium signal in many lock& X3 ^" ^" _3 n5 i( X
solvents is well known. Therefore the difference in resonance frequency
* f |8 B1 [8 r/ U: |of the lock solvent and TMS is also known. As a consequence, TMS does
, x$ V2 U9 U; B, Unot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
7 R5 i k5 G n/ }% \ z=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
