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
' E: @; C( e; O. h3 |4 rof a sample, especially one which requires signal averaging or phase4 p# W2 d4 |+ I4 F
cycling, you need to have a temporally constant and spatially
t$ G. z3 b, V- g( P5 R1 ]# }homogeneous magnetic field. Consistency of the Bo
6 |! l: v6 @) i" O0 Kfield over time will be discussed here; homogeneity will be discussed
: i8 x7 ~$ q' R9 y& L; oin the next section of this chapter. The field strength might vary over, t( Y- K: ?8 t% c
time due to aging of the magnet, movement of metal objects near the
/ a9 P& H/ G3 m4 hmagnet, and temperature fluctuations. Here is an example of a one line- L T' N9 q4 n+ O0 t" ^
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. $ _/ ?- W0 h! R- P. b
The field lock can compensate for these variations.
# n1 @* [. O1 P/ R5 G" p
The field lock is a separate NMR spectrometer within your spectrometer.7 R. @% `$ {, h4 s
This spectrometer is typically tuned to the deuterium NMR resonance; Y% Q, _6 _1 s# [8 J$ K* x
frequency. It constantly monitors the resonance frequency of the3 u7 d/ S. S% ^/ h8 x
deuterium signal and makes minor changes in the Bo magnetic field to keep the8 q1 p+ C6 \9 n' s
resonance frequency constant. The deuterium signal comes from the
C5 E1 U1 y. z/ i/ n4 W2 qdeuterium solvent used to prepare the sample. The animation window # c( I* j6 |3 x; r# N: W9 `
contains plots of the deuterium resonance lock frequency, the small8 V+ @1 P0 Y6 T2 z& ~5 K y
additional magnetic field used to correct the lock frequency, and the; E# O9 N( D! V7 z4 S9 F& ^( J
resultant Bo" Q: Z: i# p) r* k* _" q8 g8 q
field as a function of time while the magnetic field is drifting. The
+ ~/ E( K, R @* S {" N9 D0 T3 R3 xlock frequency plot displays the frequency without correction. In- M0 P' h" d' b# E
reality, this frequency would be kept constant by the application of
& N/ l n) Y5 B) Hthe lock field which offsets the drift.
* L4 ^7 |9 ^- ?8 r- V
* f8 V( g& r8 x: C' c
0 w0 a8 _( f" q8 k! k/ o0 P8 LOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
9 x# x+ t( i- ireference. The resonance frequency of the deuterium signal in many lock$ k5 y- o6 T( M1 @6 |' q/ V% T8 F
solvents is well known. Therefore the difference in resonance frequency
, T I+ v/ n* z# {3 u9 ^of the lock solvent and TMS is also known. As a consequence, TMS does/ E: o) c c6 [" O+ r6 d% S
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate / m$ r+ D# P8 U5 e
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
