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
: A; o8 w: f; f6 d/ qof a sample, especially one which requires signal averaging or phase
1 n [* g! E# D) n! n7 F3 _cycling, you need to have a temporally constant and spatially
' ]0 z3 o9 i+ b- ~! |9 n0 |$ Ohomogeneous magnetic field. Consistency of the Bo
) I3 H7 W' `4 ~& D0 I5 w! ]$ xfield over time will be discussed here; homogeneity will be discussed
1 ^4 S% {3 a# l2 D4 Gin the next section of this chapter. The field strength might vary over$ `0 m$ v3 b/ G8 W& j# k! j
time due to aging of the magnet, movement of metal objects near the& ~# @" |9 y \/ H0 `
magnet, and temperature fluctuations. Here is an example of a one line+ x" `5 g& s7 b' w4 F
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. . a# p4 G+ `! g( H
The field lock can compensate for these variations.
: F4 @& e+ A7 f; Y+ h- z9 m; ZThe field lock is a separate NMR spectrometer within your spectrometer.! p2 o$ H( A' Q+ `( G
This spectrometer is typically tuned to the deuterium NMR resonance
3 K4 i+ C2 s' \frequency. It constantly monitors the resonance frequency of the+ A( H; I8 \" C7 X
deuterium signal and makes minor changes in the Bo magnetic field to keep the" T( U; d7 o% q% f1 G& l% w
resonance frequency constant. The deuterium signal comes from the
6 r+ h9 {/ ?# G$ |deuterium solvent used to prepare the sample. The animation window
( Q( I( y7 y5 w+ ^contains plots of the deuterium resonance lock frequency, the small
) G4 h2 Y$ l& U" R* d. M' _additional magnetic field used to correct the lock frequency, and the
: O$ H9 K$ I; iresultant Bo' ^) a2 Y8 C: c% g; w
field as a function of time while the magnetic field is drifting. The1 S2 B, y7 ] b S3 ~' \
lock frequency plot displays the frequency without correction. In' C+ Z; a2 c( u4 v7 X6 t
reality, this frequency would be kept constant by the application of
" Z5 T4 U! ]# t8 N8 ^5 k: `( _the lock field which offsets the drift.
% F1 `& i- s; R% ~
" i' v2 r4 j8 t' R% p' c& X) q
& F6 f8 p& K( B; b
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
- w/ f5 ~+ L, m% F; Yreference. The resonance frequency of the deuterium signal in many lock: L7 P% d' f) P: W q
solvents is well known. Therefore the difference in resonance frequency
3 I7 F% Z; i |of the lock solvent and TMS is also known. As a consequence, TMS does
% Y/ P; ?2 e/ z1 C! f3 q0 @! Jnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
# g' L% M9 W, V7 G0 p=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
