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$ r1 F6 ]7 d+ n0 S
of a sample, especially one which requires signal averaging or phase
* k4 B5 M: X0 ]) `4 [) H' d: Ucycling, you need to have a temporally constant and spatially% v; Q& q" u! K5 t: f
homogeneous magnetic field. Consistency of the Bo
: h+ l7 I1 u2 }8 K1 Ffield over time will be discussed here; homogeneity will be discussed
( S! d% S3 M4 l( Q; E; Pin the next section of this chapter. The field strength might vary over
+ E1 B( c8 |5 x) h. x# ptime due to aging of the magnet, movement of metal objects near the* R$ S+ N1 L* K" m* n/ \
magnet, and temperature fluctuations. Here is an example of a one line L7 i) }% D% s2 ~6 i( k* Y' |! Y! Z6 F
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
- a- W9 b0 `/ E. ?% X7 GThe field lock can compensate for these variations.
& D- N2 N' _) G0 U7 Q5 l _6 bThe field lock is a separate NMR spectrometer within your spectrometer./ v- Q- I" f0 w# E' |
This spectrometer is typically tuned to the deuterium NMR resonance
( j0 W# |6 g- F7 j9 B/ qfrequency. It constantly monitors the resonance frequency of the
[: i- _. Y+ {+ q6 u& cdeuterium signal and makes minor changes in the Bo magnetic field to keep the
X' q3 c- K- o, }resonance frequency constant. The deuterium signal comes from the# P9 u4 _ F7 P( K. l
deuterium solvent used to prepare the sample. The animation window
1 g9 z' U" i- z% y/ A1 g5 Y, zcontains plots of the deuterium resonance lock frequency, the small! B G6 @' k/ S7 R2 @
additional magnetic field used to correct the lock frequency, and the# j1 s" s. K5 h) m
resultant Bo
$ L( ~ i1 ]0 C7 @' o, efield as a function of time while the magnetic field is drifting. The/ U* P% ~2 M% Z* T9 u4 N
lock frequency plot displays the frequency without correction. In
" U9 b2 t/ N, {" M; M2 ]9 sreality, this frequency would be kept constant by the application of( K# S/ ?, S8 w$ x5 D
the lock field which offsets the drift.
; H: b1 C" M9 _, B z
" I* w. R# _+ j4 C. r9 [
0 p$ z0 m! z) g2 u
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
% l& V' u$ A+ S2 y, B4 z9 Jreference. The resonance frequency of the deuterium signal in many lock8 D0 O( _: m" q+ U# f4 Y5 J( e6 }
solvents is well known. Therefore the difference in resonance frequency
$ \ B8 X* C( i0 N% Xof the lock solvent and TMS is also known. As a consequence, TMS does* `# w' y+ l* p( ?* W+ C
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate ; [. t6 P6 }* {' w
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
