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 spectrum2 @" L* r+ [6 p3 }- t# g! \& Q
of a sample, especially one which requires signal averaging or phase# L! p; C: U" W
cycling, you need to have a temporally constant and spatially$ d! W, R' g1 \9 ]
homogeneous magnetic field. Consistency of the Bo
! B1 ^" A. c" _5 zfield over time will be discussed here; homogeneity will be discussed
- X7 Y2 `8 v! }5 ~9 Qin the next section of this chapter. The field strength might vary over
& t5 F; l& V+ J+ u7 K- X ~time due to aging of the magnet, movement of metal objects near the! N: v2 E) u O1 S) C
magnet, and temperature fluctuations. Here is an example of a one line
+ G9 `$ ]$ N4 I& t) I; ]! S( yNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. % r3 i, t8 g* i }
The field lock can compensate for these variations.
/ e2 D% G7 M- T8 Y: ^0 K. }* RThe field lock is a separate NMR spectrometer within your spectrometer.2 b+ K$ l9 x9 h% T6 J
This spectrometer is typically tuned to the deuterium NMR resonance
) B" G: {" c" \7 m5 Kfrequency. It constantly monitors the resonance frequency of the
5 U5 F6 [) Y2 K: F" d2 ?4 Mdeuterium signal and makes minor changes in the Bo magnetic field to keep the( }! w' u+ x) A- T6 O1 P
resonance frequency constant. The deuterium signal comes from the$ r! c, s1 k; E8 O8 U5 S8 K
deuterium solvent used to prepare the sample. The animation window
4 }- t$ _* u% Z! D% M5 J, \7 t! mcontains plots of the deuterium resonance lock frequency, the small
# F; D3 K; c) A8 d5 C/ jadditional magnetic field used to correct the lock frequency, and the
3 a2 @1 H# ?4 i; _- F3 w: ~resultant Bo) o W8 Y. H/ Z. o
field as a function of time while the magnetic field is drifting. The
7 x0 I8 @( |! m9 Q7 tlock frequency plot displays the frequency without correction. In
# e0 e( d' p a3 J9 q$ H; u' V0 Areality, this frequency would be kept constant by the application of
* c3 c4 S3 Y' G8 p( c" ~the lock field which offsets the drift.
" L9 G/ }5 |( Q, `: `
- Q7 I+ s N2 s3 p' y. C* o0 x# D O" u# k& `
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
* x: v4 `8 r- X( E. vreference. The resonance frequency of the deuterium signal in many lock$ k$ c) G) }! S0 u7 T8 Q
solvents is well known. Therefore the difference in resonance frequency8 I9 j3 _% e- i$ P+ w+ ^- U6 V: O$ ]
of the lock solvent and TMS is also known. As a consequence, TMS does
. ~: i: o9 O {7 T& C& `$ cnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate ' {2 O9 N p- Z" @
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
