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& a* Q6 W! {% k+ C" w; {! tof a sample, especially one which requires signal averaging or phase
+ | w! S7 T4 B3 g, Ncycling, you need to have a temporally constant and spatially$ p1 d9 ` T+ [9 g, j/ B3 B
homogeneous magnetic field. Consistency of the Bo4 a# h5 W8 x! D7 ^2 O8 L
field over time will be discussed here; homogeneity will be discussed
% b0 X" Z- E& G/ Y/ e" min the next section of this chapter. The field strength might vary over, ~, _5 c2 e8 R, u' {# N+ a
time due to aging of the magnet, movement of metal objects near the
$ H7 |, F+ X5 `magnet, and temperature fluctuations. Here is an example of a one line# l( ^$ Q. D/ U, f0 c
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
' f" _, O& J; ]. ^' w6 n5 `The field lock can compensate for these variations.
* N; R- J; P/ O; U- _$ Q
The field lock is a separate NMR spectrometer within your spectrometer.
& B L0 J" y& P- |1 QThis spectrometer is typically tuned to the deuterium NMR resonance$ n- l! T8 l) K) i! B) ~
frequency. It constantly monitors the resonance frequency of the
9 F4 @# I" R8 Rdeuterium signal and makes minor changes in the Bo magnetic field to keep the
1 j+ s9 l) y+ j( e2 C% v% I" xresonance frequency constant. The deuterium signal comes from the3 m/ [% t5 v! F
deuterium solvent used to prepare the sample. The animation window
1 |. F! ^! Z; `0 z! Scontains plots of the deuterium resonance lock frequency, the small
# X5 J& F* H1 |5 k" Q& sadditional magnetic field used to correct the lock frequency, and the
; h- ~" R d, t& F, presultant Bo. k1 S) h( T9 S% p- Q( W3 [& F; L
field as a function of time while the magnetic field is drifting. The
9 }9 [; b/ r$ p& n" ^1 B+ dlock frequency plot displays the frequency without correction. In
3 w9 T. e( ]4 N- b: }5 zreality, this frequency would be kept constant by the application of; X$ @* V4 i2 r% c
the lock field which offsets the drift.
9 }5 Q* ~+ C) A# g" W. g7 M
2 X$ d8 C5 T2 \$ r+ K- q+ r' g- }+ v" K* Z& @7 L8 @
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =07 m4 V; t, K, s; x& a) u
reference. The resonance frequency of the deuterium signal in many lock7 ` c, J2 t2 L4 U4 d2 W( b
solvents is well known. Therefore the difference in resonance frequency
3 b' Z( z. s' G3 P# {6 Oof the lock solvent and TMS is also known. As a consequence, TMS does2 ~# F( _* c2 f$ x' Y, u
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate ! U1 Q2 Q( l* V- |4 z- b
=0.
| 
发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
