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& |- x! u) w$ R3 ~
of a sample, especially one which requires signal averaging or phase1 n R0 C/ I. l2 y! {
cycling, you need to have a temporally constant and spatially
" r: D5 w% f. j- Thomogeneous magnetic field. Consistency of the Bo1 }3 m8 P1 X7 G0 A) B& f/ O
field over time will be discussed here; homogeneity will be discussed
* h2 L4 {( E. zin the next section of this chapter. The field strength might vary over
# R# d1 v8 n0 p7 h% G# y8 ~time due to aging of the magnet, movement of metal objects near the# a% Y9 }3 c+ [: c! J
magnet, and temperature fluctuations. Here is an example of a one line
# ~* t2 r6 M9 bNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. " [, j q9 H3 |0 S5 _/ v
The field lock can compensate for these variations.
, N9 F6 K7 ^* `2 h; yThe field lock is a separate NMR spectrometer within your spectrometer./ z) H5 L* \* u/ v
This spectrometer is typically tuned to the deuterium NMR resonance
$ k e s% }( I% V: O' W zfrequency. It constantly monitors the resonance frequency of the5 S1 k7 D+ R0 g8 ]
deuterium signal and makes minor changes in the Bo magnetic field to keep the/ B9 `) L# }6 N! p
resonance frequency constant. The deuterium signal comes from the
2 ]3 H& L# F" A' Jdeuterium solvent used to prepare the sample. The animation window
! j$ {1 Q, `3 W6 acontains plots of the deuterium resonance lock frequency, the small0 f" x1 w) a) S1 R- G+ r
additional magnetic field used to correct the lock frequency, and the
. d- B: j z' Iresultant Bo- k) [( i# P- ]0 t9 [* v- |, S5 K
field as a function of time while the magnetic field is drifting. The
1 r: i" ^9 b, B9 S0 h/ h% elock frequency plot displays the frequency without correction. In+ W$ G$ X6 ?4 D& Y7 @. P$ b- N
reality, this frequency would be kept constant by the application of
4 G8 W# ~9 M7 ~% Q5 Ithe lock field which offsets the drift.
6 j4 |" U4 i" h2 i2 q$ c/ h6 Q
5 {$ F* D/ `$ p1 ] b& o
0 j# k9 {/ _$ k; q$ @$ f0 U
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
3 ^" B8 S1 I* I: U! Sreference. The resonance frequency of the deuterium signal in many lock
i- n$ o) e/ I. wsolvents is well known. Therefore the difference in resonance frequency
1 L% V2 A6 v: x" u* ~- Cof the lock solvent and TMS is also known. As a consequence, TMS does
: M4 f. N) m* X/ ~& b# qnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
% O8 F5 Z( {) Z& h4 u=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
