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
% p0 X2 X0 ]3 c' J4 {; D, lof a sample, especially one which requires signal averaging or phase% N5 ~$ {. a8 v1 ^
cycling, you need to have a temporally constant and spatially$ K X6 J0 y( m& r
homogeneous magnetic field. Consistency of the Bo! c2 G+ x+ B$ Q- d. K, H
field over time will be discussed here; homogeneity will be discussed) D, c5 r1 ^& _- }' f3 [
in the next section of this chapter. The field strength might vary over
0 s& i4 j A4 Q# L( L& P, H$ \time due to aging of the magnet, movement of metal objects near the! ] P- K6 w* d" Q5 D. ]
magnet, and temperature fluctuations. Here is an example of a one line+ c* C9 W. l2 ~$ P
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. 2 b& i+ U! A6 A, z5 ~4 Y
The field lock can compensate for these variations.
1 t# }$ n3 b5 i1 cThe field lock is a separate NMR spectrometer within your spectrometer.. M9 x# c; H0 ~0 F
This spectrometer is typically tuned to the deuterium NMR resonance: V5 f: q# v' m
frequency. It constantly monitors the resonance frequency of the/ x4 b/ c: C2 ?. D# h
deuterium signal and makes minor changes in the Bo magnetic field to keep the# {( {0 q$ D7 F$ Q) @4 c! b
resonance frequency constant. The deuterium signal comes from the; f. |3 p2 p7 O! e |% }
deuterium solvent used to prepare the sample. The animation window / ?, _$ R# Q: c, _$ z) x" n
contains plots of the deuterium resonance lock frequency, the small0 q4 H! R$ \( J. V k6 Y+ K
additional magnetic field used to correct the lock frequency, and the
% G6 {# t1 g* R& K Sresultant Bo' V+ l/ A. I. |" Z: [" z' ~9 Z+ ?
field as a function of time while the magnetic field is drifting. The r2 _4 w P7 Q7 b V2 p5 W: S
lock frequency plot displays the frequency without correction. In3 O. @1 p4 Y+ W
reality, this frequency would be kept constant by the application of
; v/ f# B. U6 _3 Hthe lock field which offsets the drift.
, H9 a$ L2 G2 e0 p; C5 n" ^& A/ D) y% S
+ e; y# a" N5 T6 U$ y% UOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
" \; o" ?( d- ]/ k8 b8 ]- zreference. The resonance frequency of the deuterium signal in many lock
% X" q" U& S: u- s- msolvents is well known. Therefore the difference in resonance frequency
8 K6 u0 ^& a& Y1 C! V2 Pof the lock solvent and TMS is also known. As a consequence, TMS does1 J: b% P1 D$ f% F% s3 A2 z$ o
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate L1 ^* m3 [) a$ B
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
