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% v; P* B! H: k3 m3 K
of a sample, especially one which requires signal averaging or phase
( ]) U: J7 }, ^5 ~8 {( wcycling, you need to have a temporally constant and spatially2 L5 {( F5 I/ h; c; a3 b" X
homogeneous magnetic field. Consistency of the Bo0 I7 y# x0 y# v) m0 n+ L
field over time will be discussed here; homogeneity will be discussed
% X+ u/ {* ?! ^$ z/ M) r1 a7 Rin the next section of this chapter. The field strength might vary over: [6 _# `2 ]5 i4 K" x2 @0 |
time due to aging of the magnet, movement of metal objects near the
$ V% i, E( q0 q% J2 }magnet, and temperature fluctuations. Here is an example of a one line
7 w2 b2 X9 u8 a# b$ J) D* I) _NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
0 {& w- i/ r% S5 xThe field lock can compensate for these variations.
6 g6 R( l# ?% W3 L6 lThe field lock is a separate NMR spectrometer within your spectrometer.; |3 g) V9 {7 j( ~4 V0 a
This spectrometer is typically tuned to the deuterium NMR resonance
% Z9 h1 F# E( sfrequency. It constantly monitors the resonance frequency of the8 J& h- ]: A3 r6 P/ Z6 j
deuterium signal and makes minor changes in the Bo magnetic field to keep the* o; w# E/ x, Z% r U; x. ]. g
resonance frequency constant. The deuterium signal comes from the$ o9 ~+ S( ?; [, r$ w
deuterium solvent used to prepare the sample. The animation window 8 m; N+ u/ j3 F8 J1 q# ~: T
contains plots of the deuterium resonance lock frequency, the small
2 j. _+ C6 [& V; ^additional magnetic field used to correct the lock frequency, and the
$ ~) _- D2 d! c$ @6 vresultant Bo
# H. u+ ?/ H$ Q% W( S, _/ v1 [' Mfield as a function of time while the magnetic field is drifting. The
: {4 t& J( n. n4 T, mlock frequency plot displays the frequency without correction. In
6 s. }! {/ l V) \) ^6 Dreality, this frequency would be kept constant by the application of$ T+ n) m' N8 k& T* {2 J7 g6 I- p
the lock field which offsets the drift.
" l9 g% l1 {( A6 {- ^
( t5 s5 u8 Z7 ?* K7 ?* g# i/ U& l/ {4 \# K5 d# M8 C* C, q
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0+ O7 }, X5 u# I% K
reference. The resonance frequency of the deuterium signal in many lock
6 p/ {6 _6 ^2 K( `solvents is well known. Therefore the difference in resonance frequency
5 u: v4 t S/ Dof the lock solvent and TMS is also known. As a consequence, TMS does0 Y. W. y' @' H. ]
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
! @5 |6 \( i$ y3 @=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
