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
$ f2 o: k; E( Zof a sample, especially one which requires signal averaging or phase, K) b1 {3 U/ r2 q
cycling, you need to have a temporally constant and spatially
- U v) v7 M+ yhomogeneous magnetic field. Consistency of the Bo, X& Y/ S' x% p$ i
field over time will be discussed here; homogeneity will be discussed8 e. v# R5 X& p, r
in the next section of this chapter. The field strength might vary over
) Z: k# ?5 ^3 F: Ntime due to aging of the magnet, movement of metal objects near the2 h! P u: V( P; v
magnet, and temperature fluctuations. Here is an example of a one line
; O# }' S% f% S/ ~& u' e- ANMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. 4 O5 f5 `2 }; Z9 s8 t
The field lock can compensate for these variations.
5 w% w3 W% [5 Z0 I
The field lock is a separate NMR spectrometer within your spectrometer.% p& f! T6 X6 T. ?! P
This spectrometer is typically tuned to the deuterium NMR resonance( u4 G9 W* ~+ |& H6 ^
frequency. It constantly monitors the resonance frequency of the
. s( L8 L1 g6 ydeuterium signal and makes minor changes in the Bo magnetic field to keep the
5 b& o3 y: ]5 l. m0 Gresonance frequency constant. The deuterium signal comes from the
, y4 i# Q& S( s8 o# S- Bdeuterium solvent used to prepare the sample. The animation window
7 V# h2 c' {# Q' V0 e% ^8 zcontains plots of the deuterium resonance lock frequency, the small
w J! M. a) y2 T$ y! Vadditional magnetic field used to correct the lock frequency, and the
+ R+ E3 \: Z+ _ P/ G2 P7 Qresultant Bo
2 g" a7 D K% N" C7 Q: e+ N; ofield as a function of time while the magnetic field is drifting. The
, g/ l9 h e8 H( ~lock frequency plot displays the frequency without correction. In
; D- a/ U- w3 g" {) B" T. p! y1 [3 n) Ureality, this frequency would be kept constant by the application of
$ }# Z' B' A% bthe lock field which offsets the drift.
% v m. |0 Z0 r5 s: u- y3 E6 @
% R6 A: L, d+ Z/ G9 J1 N) Q
* ^: e1 |& f; P* iOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0- M1 N% ]# A# Z
reference. The resonance frequency of the deuterium signal in many lock
8 @ x8 m6 X* \+ h# k9 j! e8 ]solvents is well known. Therefore the difference in resonance frequency8 P% \* I7 }7 C! {8 e& E
of the lock solvent and TMS is also known. As a consequence, TMS does& t; [; K$ h. G7 g& y
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
8 b4 C& Q) R/ M* [=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
