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
5 T% n# \- s0 @4 K9 Kof a sample, especially one which requires signal averaging or phase4 r# p. v A" z4 a( N: h( I+ E$ J" ~
cycling, you need to have a temporally constant and spatially: S" T9 i+ l# w1 m" m" W' x
homogeneous magnetic field. Consistency of the Bo; e- |# D2 w, Q, ?0 N! w% Q5 E
field over time will be discussed here; homogeneity will be discussed
1 i# q& f( s( n4 D$ b( y: Ain the next section of this chapter. The field strength might vary over
- G5 J% c. O4 Y# E. y7 K6 \, e% P) otime due to aging of the magnet, movement of metal objects near the3 y6 f/ w0 S) b m; W5 ^5 {5 i
magnet, and temperature fluctuations. Here is an example of a one line, ^" C# \4 C3 J: n% H5 y& W
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. 1 K0 B) N% s% e. k8 x
The field lock can compensate for these variations.
% N, G. `# }9 d, S; T: i' d6 S$ R
The field lock is a separate NMR spectrometer within your spectrometer.8 M# T9 v! G3 ~5 @* d' E5 R
This spectrometer is typically tuned to the deuterium NMR resonance
& G/ C* ^8 M9 |" D1 vfrequency. It constantly monitors the resonance frequency of the
1 C/ x. ^, q6 n4 `+ K8 bdeuterium signal and makes minor changes in the Bo magnetic field to keep the
h' S0 W9 ^( y& L2 N- aresonance frequency constant. The deuterium signal comes from the1 A/ C4 Y7 ? _8 L/ W9 z
deuterium solvent used to prepare the sample. The animation window
+ o% z& i7 S( S+ ^* Pcontains plots of the deuterium resonance lock frequency, the small
5 Z$ e: R0 z/ d2 u3 `3 vadditional magnetic field used to correct the lock frequency, and the
9 _, N3 Y& x0 }( ~resultant Bo+ q4 T- g' G! ?' ?) N/ Z
field as a function of time while the magnetic field is drifting. The/ g" T; N0 ~$ l7 b3 t3 [
lock frequency plot displays the frequency without correction. In
5 J t; z$ [# v3 A4 f( ]reality, this frequency would be kept constant by the application of R) V* t6 G- g2 a9 F; N
the lock field which offsets the drift.
" o0 K: i4 E& q8 X/ l8 W
* d% k- G4 | R7 Q/ d
% ?' j: B. F; x6 ^On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
2 B$ [2 O8 w" T( n; ~7 q3 sreference. The resonance frequency of the deuterium signal in many lock7 V6 _* Q4 \* P1 ^4 A% `4 t: E- z3 T! R; _
solvents is well known. Therefore the difference in resonance frequency( Q4 V- o7 O4 Y! A
of the lock solvent and TMS is also known. As a consequence, TMS does7 ~4 I7 j2 |- B# a( r
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
$ q4 ^# P! v- d5 x1 P% L=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
