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" I4 l* E+ v2 \ A2 S: j
of a sample, especially one which requires signal averaging or phase
; O! X& Y5 G) [ R! l1 [5 r( r% Zcycling, you need to have a temporally constant and spatially
M5 S+ g4 ^5 Q7 shomogeneous magnetic field. Consistency of the Bo- A( a: X1 ~' }$ x( z
field over time will be discussed here; homogeneity will be discussed2 O/ M8 C& v' r/ F4 V, Q; m& x6 U
in the next section of this chapter. The field strength might vary over3 I# p6 V/ Q [ \6 v6 y
time due to aging of the magnet, movement of metal objects near the
1 p0 @8 X/ r6 M4 X7 xmagnet, and temperature fluctuations. Here is an example of a one line
4 ^% d- t1 B( C% |7 vNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
: a( _2 W7 Y) \; b* {: Q8 YThe field lock can compensate for these variations.
: Y& D' j8 _7 ]
The field lock is a separate NMR spectrometer within your spectrometer.& X |" T, K1 A
This spectrometer is typically tuned to the deuterium NMR resonance
3 x$ e& w2 Z8 W9 R2 ~$ k7 _frequency. It constantly monitors the resonance frequency of the
; ?6 ~' i% L; a! m% L: Gdeuterium signal and makes minor changes in the Bo magnetic field to keep the( Z" C6 w7 s3 h* w) s5 @& u2 f
resonance frequency constant. The deuterium signal comes from the
8 `0 m+ k/ ~" `0 O% vdeuterium solvent used to prepare the sample. The animation window
: n! c; a+ b+ ~6 { N' X% Ucontains plots of the deuterium resonance lock frequency, the small
) i1 W! ^4 q( \8 K+ v; Tadditional magnetic field used to correct the lock frequency, and the
2 j% H5 H* z/ V9 n7 c+ vresultant Bo( R6 u, z8 F: K9 y V! K
field as a function of time while the magnetic field is drifting. The, X/ C1 r" G6 u1 _0 s2 n
lock frequency plot displays the frequency without correction. In9 u% |5 a2 e* w5 {; I5 ~
reality, this frequency would be kept constant by the application of
! q/ L! B/ j* k+ f; b+ uthe lock field which offsets the drift.
9 g; ~1 m* q" \
# m5 J. D- j& o Q& m( [ K% z1 W/ H' _
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
" `9 K& U& B* e: \6 C; creference. The resonance frequency of the deuterium signal in many lock
5 N/ K0 k( n6 m/ P+ jsolvents is well known. Therefore the difference in resonance frequency7 ]9 S7 \6 W7 p5 K
of the lock solvent and TMS is also known. As a consequence, TMS does, S. a& e( [; u2 a, R) t
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
3 {/ D* f# E3 } o" h9 I0 O7 Z# Q=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
