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
# O! h3 q; Z+ n; y7 Y% [2 U5 Bof a sample, especially one which requires signal averaging or phase7 Y4 N( ^7 ^3 E0 g; ^6 u2 W
cycling, you need to have a temporally constant and spatially
( E6 p. r( t+ {3 [7 Zhomogeneous magnetic field. Consistency of the Bo) e6 ^ r- o2 ]/ V' p
field over time will be discussed here; homogeneity will be discussed
8 O3 k W; m" v# c2 M% Bin the next section of this chapter. The field strength might vary over( m3 p+ p' K/ i1 M
time due to aging of the magnet, movement of metal objects near the
3 W: S$ R! m: g* N/ b; t emagnet, and temperature fluctuations. Here is an example of a one line9 U! m1 L( T: Y9 W$ }( y
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
/ I5 B' l3 ]9 @7 ^The field lock can compensate for these variations.
3 o1 L, ], ?2 ]( V6 j
The field lock is a separate NMR spectrometer within your spectrometer.' H: O6 J, N- E) ^: J' z$ P
This spectrometer is typically tuned to the deuterium NMR resonance3 b- u% \/ Y$ P! G; [) h- g
frequency. It constantly monitors the resonance frequency of the5 S6 }. k, ]' L3 O! y) ~
deuterium signal and makes minor changes in the Bo magnetic field to keep the9 H' ?! p1 m3 S1 t* P! s
resonance frequency constant. The deuterium signal comes from the) P& ?; m$ o0 K$ e" J
deuterium solvent used to prepare the sample. The animation window
V& P, M. n6 Z. U: Xcontains plots of the deuterium resonance lock frequency, the small0 _4 O6 p* `. E2 J) t0 X8 X
additional magnetic field used to correct the lock frequency, and the
6 F/ E5 Z% g Z3 K2 @0 G; aresultant Bo
3 C% T F$ l h: X! dfield as a function of time while the magnetic field is drifting. The r4 n+ C& R& k& ~8 l& G+ O3 X$ ?% y
lock frequency plot displays the frequency without correction. In
' x1 S) E. R/ z9 breality, this frequency would be kept constant by the application of
/ |) j' l& N3 s8 r% [0 Ythe lock field which offsets the drift.
. ^, d( ?- j, E8 `" ^" ?
" s8 W9 p* ?# B7 `
- O' I0 `- {# X2 b3 eOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0$ @0 ]3 @9 }% U& L4 h1 e
reference. The resonance frequency of the deuterium signal in many lock& Y U9 z. D" Z
solvents is well known. Therefore the difference in resonance frequency2 x4 ^/ G) C2 ^% D
of the lock solvent and TMS is also known. As a consequence, TMS does" z9 ]4 v) d @2 S, r0 B" j
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
! _$ t, ] Z$ h4 Y! A* @+ C=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
