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
5 ~5 S+ c/ a5 Vof a sample, especially one which requires signal averaging or phase0 @ R! D1 ]! C" Q' q4 O' H
cycling, you need to have a temporally constant and spatially
) ~: \; |# F4 Q$ O8 e9 W2 I' \homogeneous magnetic field. Consistency of the Bo
& \6 ~% _; y; v% v. x1 T4 P$ bfield over time will be discussed here; homogeneity will be discussed7 z1 G2 `0 t' o' i+ B8 R( C3 r
in the next section of this chapter. The field strength might vary over3 f) @* q L8 ?# B" M
time due to aging of the magnet, movement of metal objects near the5 \0 c+ |* c; _: r- D
magnet, and temperature fluctuations. Here is an example of a one line
! r: h- w- \& T$ G3 Q0 |NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. 7 I+ ^, K8 }# p3 l5 @0 k
The field lock can compensate for these variations.
2 U! J ] Q8 G1 X N9 j* tThe field lock is a separate NMR spectrometer within your spectrometer.( x/ _; {2 T: O/ E# [
This spectrometer is typically tuned to the deuterium NMR resonance+ V0 z# X7 n6 R! k
frequency. It constantly monitors the resonance frequency of the) j- D( m$ S4 N1 o
deuterium signal and makes minor changes in the Bo magnetic field to keep the
c3 k" U$ q' }; B4 ]resonance frequency constant. The deuterium signal comes from the' }5 n v: j: W# C) s
deuterium solvent used to prepare the sample. The animation window $ z# `% h# W# W+ S/ A
contains plots of the deuterium resonance lock frequency, the small
. h& Y7 l& L, Cadditional magnetic field used to correct the lock frequency, and the
; q7 P! e7 [4 mresultant Bo# C/ A1 l' T( R) M9 u! j
field as a function of time while the magnetic field is drifting. The
0 D) E* Q! U n/ hlock frequency plot displays the frequency without correction. In/ W( Q @& D4 b
reality, this frequency would be kept constant by the application of4 y- h$ A9 @) q" G
the lock field which offsets the drift.
2 G5 |) _$ i+ \: e: h3 k5 i) e D/ k. n3 Z& P9 p6 m7 X; P5 `
9 Y0 X$ ~: t" H7 ~
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
& x2 A3 J: a- k- [+ D! \ ireference. The resonance frequency of the deuterium signal in many lock
3 b3 Z' O- c4 i- lsolvents is well known. Therefore the difference in resonance frequency9 s: p* g( y* U" H
of the lock solvent and TMS is also known. As a consequence, TMS does! Z0 y; | L. m% y2 m' M1 ?
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate y4 b9 f: o- F# Y8 O* A
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
