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
" M1 B2 c/ k3 K, d. Z; ~of a sample, especially one which requires signal averaging or phase
0 _" f5 u! C& Y! `* qcycling, you need to have a temporally constant and spatially
& K; z5 P, Z6 f! G2 @2 @homogeneous magnetic field. Consistency of the Bo
4 H+ a' {! {. @ A( u; D& Ffield over time will be discussed here; homogeneity will be discussed
1 N' ^! m$ |4 j( Gin the next section of this chapter. The field strength might vary over
% ^1 i* Y# G& j$ d% m/ V0 a# R9 \1 xtime due to aging of the magnet, movement of metal objects near the
% d; k& e/ J7 K' h- D8 U* gmagnet, and temperature fluctuations. Here is an example of a one line' I& e, b+ ]( A% O `5 x# n2 I
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
: @1 k) I) N$ K' ]- ?& ~: q% x" B: g, O% eThe field lock can compensate for these variations.
0 J- B; F5 O0 |; Q4 B3 k
The field lock is a separate NMR spectrometer within your spectrometer.; a5 t+ S# |" a7 F8 |
This spectrometer is typically tuned to the deuterium NMR resonance! d: ?) f6 S) L$ R7 e
frequency. It constantly monitors the resonance frequency of the. `1 y" r4 O/ i) q8 c1 @- s
deuterium signal and makes minor changes in the Bo magnetic field to keep the
( O. g+ s' _% O. zresonance frequency constant. The deuterium signal comes from the2 \" e- I" `. O& A- l0 @! ]8 g
deuterium solvent used to prepare the sample. The animation window
8 o8 B9 N2 \7 d: ~contains plots of the deuterium resonance lock frequency, the small S7 E6 p0 |( P5 V7 L
additional magnetic field used to correct the lock frequency, and the7 x1 n7 v2 ]: \1 c
resultant Bo
' C4 l% _+ C0 d1 Ffield as a function of time while the magnetic field is drifting. The4 Y: n: @2 R: E* `! u+ c! a5 U
lock frequency plot displays the frequency without correction. In
, [. f+ A" C. ?) f) c* Lreality, this frequency would be kept constant by the application of
% e0 c2 v5 N' _# ?the lock field which offsets the drift.
% T( {% l( |5 b; H6 `
$ x! W" O" Y/ ?2 V! {
9 C8 n* b' R' P% N# HOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
0 Z! Z- @: \/ e# Freference. The resonance frequency of the deuterium signal in many lock
" s* W9 h% c& g, G* hsolvents is well known. Therefore the difference in resonance frequency4 N) q) \8 y5 |7 d1 G
of the lock solvent and TMS is also known. As a consequence, TMS does( T/ C, B' o* @- d
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
4 `/ f9 g0 x* v& S: L7 e1 z$ K4 A=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
