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
. r) E, ]. `5 e3 X hof a sample, especially one which requires signal averaging or phase
, n* C6 m2 `. |cycling, you need to have a temporally constant and spatially. P! v0 i9 [7 y6 s) O% P
homogeneous magnetic field. Consistency of the Bo
, [' \( w! a- ]field over time will be discussed here; homogeneity will be discussed. h. i" Q; y, F6 E# [7 b
in the next section of this chapter. The field strength might vary over
3 f* K! p2 V) H, Htime due to aging of the magnet, movement of metal objects near the
% N5 U! M, C% _' Z! U& b1 N; M7 | [magnet, and temperature fluctuations. Here is an example of a one line
$ [) v! F/ j/ k9 `2 |+ [NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. ; _6 M1 B9 \/ i
The field lock can compensate for these variations.
$ B" z' V& E ]: D4 ]) B
The field lock is a separate NMR spectrometer within your spectrometer.
3 A. K2 f6 s' e% Q+ [& uThis spectrometer is typically tuned to the deuterium NMR resonance$ u) @1 U. s8 |: g9 `) t
frequency. It constantly monitors the resonance frequency of the
N( j/ p M4 B+ ~deuterium signal and makes minor changes in the Bo magnetic field to keep the
U( [. f! c1 vresonance frequency constant. The deuterium signal comes from the1 q4 l6 {+ \. B
deuterium solvent used to prepare the sample. The animation window
D4 W( n1 K2 t# e! tcontains plots of the deuterium resonance lock frequency, the small3 z* {. @8 j' z' Q1 u, z
additional magnetic field used to correct the lock frequency, and the
0 Q0 ^& | C2 d4 Q- O+ Nresultant Bo
/ t6 {( A+ V+ L) C2 \3 C$ m$ Vfield as a function of time while the magnetic field is drifting. The
6 D! k1 X/ Y# {8 f, Klock frequency plot displays the frequency without correction. In
4 c' s& j3 n2 ~6 x# d rreality, this frequency would be kept constant by the application of( \! L( z+ V' S! a5 L2 E. Y3 v7 I
the lock field which offsets the drift.
% ~4 d# |5 ~ b j& U
2 o9 w. ]6 H F! f0 u: o+ _
) `0 y `0 p- a( k% [On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0* v0 K) F, F0 G; m8 G0 c* u. \" D- t* w
reference. The resonance frequency of the deuterium signal in many lock, X9 Y* M7 I N: J5 T$ C
solvents is well known. Therefore the difference in resonance frequency/ ]# c' S+ K# g
of the lock solvent and TMS is also known. As a consequence, TMS does. Z! o6 [7 Y; U7 m6 ^( T1 z6 S# q
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
8 X9 w: ^1 _# t. K0 W=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
