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 spectrum7 _, ^, X2 v5 ?) e% m' S" R! m
of a sample, especially one which requires signal averaging or phase; p% i7 }/ H8 i6 D: g% W( U
cycling, you need to have a temporally constant and spatially
1 H$ k$ I1 l9 @- J5 Chomogeneous magnetic field. Consistency of the Bo5 \" A, J$ a3 t1 P/ g
field over time will be discussed here; homogeneity will be discussed
) R, U! k, w5 s4 f% v+ Yin the next section of this chapter. The field strength might vary over' x% N1 K8 Z; e$ o! D/ t) Z4 P$ j
time due to aging of the magnet, movement of metal objects near the Y' z- P9 C& ]* @6 p |
magnet, and temperature fluctuations. Here is an example of a one line1 T' K* w4 @; L( x. @0 ]0 T
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
6 [% d$ j1 n% O; I+ AThe field lock can compensate for these variations.
- a/ [2 w! X' n9 N: i$ g, ] @$ j: g
The field lock is a separate NMR spectrometer within your spectrometer.( N% Y# r; h$ z& y
This spectrometer is typically tuned to the deuterium NMR resonance* V1 G$ z2 J7 X/ D, r6 c' v
frequency. It constantly monitors the resonance frequency of the% ^1 i7 x8 U5 M* T: B3 {( d
deuterium signal and makes minor changes in the Bo magnetic field to keep the/ j4 t3 ?2 {0 `; l+ [$ @
resonance frequency constant. The deuterium signal comes from the
# y* p% Q$ K# rdeuterium solvent used to prepare the sample. The animation window & c3 ?' V I, ~, B* u) \1 y
contains plots of the deuterium resonance lock frequency, the small
& r% r( ^* q5 kadditional magnetic field used to correct the lock frequency, and the
) U7 H0 ^- [3 L* Z: @# G# Q8 \/ ~5 u& lresultant Bo
, w1 a' a( n" B6 O7 B3 K9 I, R" @field as a function of time while the magnetic field is drifting. The7 i+ v: u+ ~3 j8 H9 E: _) O
lock frequency plot displays the frequency without correction. In$ K! ]! A7 B0 K4 ~+ M
reality, this frequency would be kept constant by the application of* ~! U# R% J, ^; P. t5 g7 J
the lock field which offsets the drift.
$ p0 \+ ]( A( n) m' p2 G# }4 P
! V* w3 Y1 y! T+ n, N& R7 I
5 ~* j Q4 R9 yOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
/ X# s0 _( h! U3 c. A# }reference. The resonance frequency of the deuterium signal in many lock+ x0 X& j5 x9 d! d' u
solvents is well known. Therefore the difference in resonance frequency
0 `' T5 i* M: i" hof the lock solvent and TMS is also known. As a consequence, TMS does
& T- w8 g( Z) M* z7 pnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
% u8 D( C- E! o3 N% k. `, n=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
