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 spectrum6 b6 Z4 Y Q' C3 t& o4 |
of a sample, especially one which requires signal averaging or phase$ {4 D. h R( [: [1 R
cycling, you need to have a temporally constant and spatially
: o& ^# E+ s l' [' {) D7 \3 bhomogeneous magnetic field. Consistency of the Bo
& M3 R. h$ U2 m7 i/ _# efield over time will be discussed here; homogeneity will be discussed9 N+ J5 I0 C% ^6 A" q: F( t( q9 A
in the next section of this chapter. The field strength might vary over
4 c* C' Y$ N' o% stime due to aging of the magnet, movement of metal objects near the
9 Z+ s3 y0 z0 `5 o4 q# vmagnet, and temperature fluctuations. Here is an example of a one line1 x _' v7 X# v, Q9 s9 _$ I
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
0 P5 S9 t! V0 o5 ?# B& l; AThe field lock can compensate for these variations.
8 x. a0 v: Q3 M$ s D% x+ P/ T
The field lock is a separate NMR spectrometer within your spectrometer.
7 }& F8 j* F9 J" g; Q h- \! ^, PThis spectrometer is typically tuned to the deuterium NMR resonance3 b1 Q2 f+ F! R, G: e g/ e$ l
frequency. It constantly monitors the resonance frequency of the
5 Q) E' j g$ x. fdeuterium signal and makes minor changes in the Bo magnetic field to keep the
p2 p( `1 `* V) _resonance frequency constant. The deuterium signal comes from the# {2 J9 M) h$ I/ P; v3 G t
deuterium solvent used to prepare the sample. The animation window v/ V! b0 Q% H$ z3 Z! T
contains plots of the deuterium resonance lock frequency, the small# _, i# f7 d. S, B9 K$ z7 f
additional magnetic field used to correct the lock frequency, and the
. v, ]: D1 R: V b9 zresultant Bo
2 Z# l0 `7 e$ O: V) m* o& B1 gfield as a function of time while the magnetic field is drifting. The
" b9 d. a% ]1 E8 rlock frequency plot displays the frequency without correction. In9 D5 l4 b0 I! }$ i- Z' x9 s! w
reality, this frequency would be kept constant by the application of
3 w: x9 r7 j1 I! j* {" Tthe lock field which offsets the drift.
5 m X7 e% l& _) }0 I. x
& ~. [7 t# c- [
' T, {$ [2 w" i5 P& b) S1 P1 F
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
5 V( a# B7 t5 x1 u0 s" x$ @6 Preference. The resonance frequency of the deuterium signal in many lock! T9 P; Y/ T* C& g" Z
solvents is well known. Therefore the difference in resonance frequency
! b6 m& S0 T2 c" \of the lock solvent and TMS is also known. As a consequence, TMS does1 d; X* ?' B4 y
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate 9 t Q$ r0 x7 v; I+ A
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
