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 O& h4 U" F' ^/ q, }- U6 H) E% |of a sample, especially one which requires signal averaging or phase% f& `% S3 m1 e z5 O$ C; x
cycling, you need to have a temporally constant and spatially
% T+ y) J% N1 J. L8 `" e8 O" Vhomogeneous magnetic field. Consistency of the Bo0 U# W% f, G5 m$ `
field over time will be discussed here; homogeneity will be discussed& Q5 G% b+ k1 c+ V# ~! u$ x% ^
in the next section of this chapter. The field strength might vary over9 g5 z/ ?6 I& Z# f: `! r
time due to aging of the magnet, movement of metal objects near the# C- x4 J- d% W; p- ?
magnet, and temperature fluctuations. Here is an example of a one line
1 h6 [9 P7 m6 r2 m7 ]7 pNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. ' c! r+ v* _: |3 ]+ w, {- R
The field lock can compensate for these variations.
8 a/ \( W6 A2 ]+ f
The field lock is a separate NMR spectrometer within your spectrometer. _% {. d- c/ a2 y \- L
This spectrometer is typically tuned to the deuterium NMR resonance+ @' u, @5 X+ m+ ]. \
frequency. It constantly monitors the resonance frequency of the/ }) e6 f" k5 T1 @6 [8 O
deuterium signal and makes minor changes in the Bo magnetic field to keep the
, c8 v" b% \$ Eresonance frequency constant. The deuterium signal comes from the
4 r. {4 o6 a& n& }4 g' Ideuterium solvent used to prepare the sample. The animation window ) b6 z \6 w9 h, z5 V! `8 A
contains plots of the deuterium resonance lock frequency, the small. S& O3 c3 ?6 n' H1 d
additional magnetic field used to correct the lock frequency, and the! z8 b0 p+ C* _3 U
resultant Bo
! n3 Y+ F( Q& q5 T2 }! m# Qfield as a function of time while the magnetic field is drifting. The& F1 a' c' m) u, S
lock frequency plot displays the frequency without correction. In
7 |, e8 Y Z& \# Areality, this frequency would be kept constant by the application of2 m; o6 `2 P! P4 Y
the lock field which offsets the drift.
0 k' t2 P$ s0 T }, v
w0 j' U- d/ {( _6 `9 }/ E( H3 W) G2 W7 T V& V% p0 S% `
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
$ ?5 |4 p# k- B1 {* P% ?4 Creference. The resonance frequency of the deuterium signal in many lock2 h2 X: Q! n2 w' n! Q/ k$ T8 N
solvents is well known. Therefore the difference in resonance frequency
- f( U) G ~) c& D: Yof the lock solvent and TMS is also known. As a consequence, TMS does
. i8 M/ Z" |, pnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
' M% o* m( L7 }* t8 y=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
