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
/ I5 [& F# V% i2 X9 F+ K3 gof a sample, especially one which requires signal averaging or phase
; u' u7 Q; v+ N; I: Lcycling, you need to have a temporally constant and spatially; t$ Q7 \0 ~" c0 l
homogeneous magnetic field. Consistency of the Bo
; n' R% h5 t k* S# v; C% g% efield over time will be discussed here; homogeneity will be discussed
* }& D* r; o, y1 y. {in the next section of this chapter. The field strength might vary over
7 V, a1 o* ~ S- u; Etime due to aging of the magnet, movement of metal objects near the7 h2 T; _9 v/ U. U) C
magnet, and temperature fluctuations. Here is an example of a one line
# [3 {* j% y N+ B# ~4 C# ~; WNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
; I8 Q1 U- w. S: K9 N5 i. f( C' kThe field lock can compensate for these variations.
, R, B; w" A. Z( e$ S$ h
The field lock is a separate NMR spectrometer within your spectrometer.
: v; {5 b% Y0 A) z; w5 OThis spectrometer is typically tuned to the deuterium NMR resonance% L+ N' i( y/ u' t: p
frequency. It constantly monitors the resonance frequency of the
! W; C6 Q; Q$ N4 n6 N: cdeuterium signal and makes minor changes in the Bo magnetic field to keep the( H, j8 |1 G7 K, M
resonance frequency constant. The deuterium signal comes from the
% e+ V S$ h& d7 T3 edeuterium solvent used to prepare the sample. The animation window
0 Z9 Z& x5 B, ]* K8 _0 jcontains plots of the deuterium resonance lock frequency, the small, @9 Y5 c: {5 k
additional magnetic field used to correct the lock frequency, and the4 Y& b; v. k, i5 q; P: E! G: Y0 u$ G: L
resultant Bo( @7 `! e) Z+ L& P3 W
field as a function of time while the magnetic field is drifting. The1 W s0 {1 |7 L- K- f1 ^0 d7 L
lock frequency plot displays the frequency without correction. In9 A) J. R5 f' ^) B- n* N
reality, this frequency would be kept constant by the application of
0 M! g% u3 m: U/ V: [; Vthe lock field which offsets the drift.
6 s. H5 Q1 ]! F+ w1 o7 l. L
; `0 ]8 _; \/ C( s( v7 [$ x
& \2 b, H2 C" G8 v1 V- bOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =00 ]6 `# T' L( V9 q8 \8 A
reference. The resonance frequency of the deuterium signal in many lock
! \; m/ T* V3 m |# ]solvents is well known. Therefore the difference in resonance frequency
. ^ {& e; Z# _8 s8 N2 E- \of the lock solvent and TMS is also known. As a consequence, TMS does
4 C% f- X7 @$ rnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate 8 V8 o" u8 K/ P+ u; h
=0.
| 
发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
