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# l2 m2 m8 P; B9 u
of a sample, especially one which requires signal averaging or phase
! _4 y5 p7 R! c1 O- m$ i( d5 R: [/ |cycling, you need to have a temporally constant and spatially
1 U& l# R o* g1 H( Zhomogeneous magnetic field. Consistency of the Bo- o* ~% j7 J/ G9 P1 v0 B5 n- B
field over time will be discussed here; homogeneity will be discussed
7 J! p$ p: @& N. qin the next section of this chapter. The field strength might vary over( V; k, S! w, e) U9 w& r
time due to aging of the magnet, movement of metal objects near the& i/ X1 k" B( P! f7 l3 M
magnet, and temperature fluctuations. Here is an example of a one line% {. V# z/ \+ c0 `
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. 6 r: ]) X, X, U$ {3 b
The field lock can compensate for these variations.
$ J- x* x, Z9 H% ?+ p
The field lock is a separate NMR spectrometer within your spectrometer., l( U6 N$ f; u
This spectrometer is typically tuned to the deuterium NMR resonance" ^+ v0 g2 u( T7 e
frequency. It constantly monitors the resonance frequency of the% t( ?( R1 E1 V7 T9 U- M- ^
deuterium signal and makes minor changes in the Bo magnetic field to keep the
4 w2 E9 L0 d* Jresonance frequency constant. The deuterium signal comes from the
n( o+ ^" [" \" c' ?deuterium solvent used to prepare the sample. The animation window 6 G1 X a# M6 p
contains plots of the deuterium resonance lock frequency, the small2 ?" S8 y4 [" g$ K" c5 P$ ^
additional magnetic field used to correct the lock frequency, and the0 H+ v0 T( C9 G" j3 x% w, ?
resultant Bo% P5 m$ m8 {* |! t9 m, g1 P
field as a function of time while the magnetic field is drifting. The
, @! X1 u: H" N; Qlock frequency plot displays the frequency without correction. In) l0 o9 z: q9 V1 q4 J! r4 m
reality, this frequency would be kept constant by the application of3 D( \/ p, `7 z4 a# o
the lock field which offsets the drift.
4 u- A9 E. P1 J% Q0 l4 K
* z; M+ ^9 D+ O+ V* \6 G/ R7 u+ _- `9 x8 j+ g2 x
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
8 E2 `- ]" v8 J: f; \0 vreference. The resonance frequency of the deuterium signal in many lock4 N& o+ D8 k) x1 w7 C7 T
solvents is well known. Therefore the difference in resonance frequency- S. f( O" B2 v0 I }
of the lock solvent and TMS is also known. As a consequence, TMS does% b0 { g3 r2 P8 q+ {% J* l% o
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate ) ?& l6 w0 A4 c0 v: R
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
