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
( o4 C" k& ^5 y D& I1 U s D, ~* Y4 wof a sample, especially one which requires signal averaging or phase
9 `8 _" v: K" |7 w% `( lcycling, you need to have a temporally constant and spatially
( U: \' t; f9 c+ uhomogeneous magnetic field. Consistency of the Bo0 ]% S4 e( S9 u# x
field over time will be discussed here; homogeneity will be discussed* ^! }9 K+ n9 N8 W
in the next section of this chapter. The field strength might vary over" ~8 P' q: e/ _( y M5 f. s' c
time due to aging of the magnet, movement of metal objects near the" p) p1 e+ P6 p9 X* m0 b' _4 M
magnet, and temperature fluctuations. Here is an example of a one line6 `8 @6 L! h2 o) }. @
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
- v/ c3 t: i0 _The field lock can compensate for these variations.
' N; ~ v7 o3 [6 I, |$ \
The field lock is a separate NMR spectrometer within your spectrometer.# F/ v% o8 |- `1 U8 W: V
This spectrometer is typically tuned to the deuterium NMR resonance% n }! \1 }/ x* z) w+ [+ K
frequency. It constantly monitors the resonance frequency of the
6 M1 \% M$ J# J% z' fdeuterium signal and makes minor changes in the Bo magnetic field to keep the
5 {1 r) j: S; r' n5 u) |resonance frequency constant. The deuterium signal comes from the
# _( N( B9 N: X3 \deuterium solvent used to prepare the sample. The animation window ( M: r7 a' r- R
contains plots of the deuterium resonance lock frequency, the small
& Q2 r' G# b$ ladditional magnetic field used to correct the lock frequency, and the
8 P$ [8 a3 D9 @' Iresultant Bo
- {% j$ ~' `$ A$ K. i$ afield as a function of time while the magnetic field is drifting. The
1 |4 B+ {- |# H/ N# U1 I5 \7 xlock frequency plot displays the frequency without correction. In0 ]+ A& \1 J& {, R& a4 x
reality, this frequency would be kept constant by the application of
5 a' T% k& x3 c# k# |4 xthe lock field which offsets the drift.
+ t6 T# s: x4 l3 a- ?: t
; E; c' D) {4 f, W7 d, w/ N# y$ U% c4 e# o, |6 n$ d
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0* {' j' T1 s/ Z
reference. The resonance frequency of the deuterium signal in many lock, \* V1 T W1 E* d- m2 @" r( q- I/ @. d
solvents is well known. Therefore the difference in resonance frequency, Y' v6 U5 ~" f; y( s1 K/ F
of the lock solvent and TMS is also known. As a consequence, TMS does! f0 N4 g% r0 T! ?
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
: D& U+ A6 Q1 u0 c( \ q: C=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
