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
3 M2 ], `+ i$ A( gof a sample, especially one which requires signal averaging or phase
7 j- @( ^ m, J4 Dcycling, you need to have a temporally constant and spatially
! U5 X9 Q- p. w5 S" phomogeneous magnetic field. Consistency of the Bo* ]. o, O7 r: Y2 `
field over time will be discussed here; homogeneity will be discussed+ a ~; G4 R4 _9 K e( N
in the next section of this chapter. The field strength might vary over# E( Q! l& v5 r: n' Y4 x
time due to aging of the magnet, movement of metal objects near the$ t8 s: J8 U& [! p. b \
magnet, and temperature fluctuations. Here is an example of a one line7 N6 W9 [0 V/ q* t7 R/ }
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. % g" O. X6 g" U( W0 b
The field lock can compensate for these variations.
4 |+ n# z7 |3 Y. iThe field lock is a separate NMR spectrometer within your spectrometer.
- A, i; Y \$ Y" O9 DThis spectrometer is typically tuned to the deuterium NMR resonance
) Q$ H5 i1 i+ c, S4 h- Hfrequency. It constantly monitors the resonance frequency of the
5 e9 Z+ A4 [6 Tdeuterium signal and makes minor changes in the Bo magnetic field to keep the/ j: R9 @. x+ o8 e" }+ `2 J; @
resonance frequency constant. The deuterium signal comes from the% M2 c: C& q {' o
deuterium solvent used to prepare the sample. The animation window
7 D( [' q3 G, o' jcontains plots of the deuterium resonance lock frequency, the small
z' L8 r/ x$ ]+ l9 K& w& \additional magnetic field used to correct the lock frequency, and the5 t! i) A, [6 `& F+ U$ p
resultant Bo
0 s, |8 _! q6 |( |, Zfield as a function of time while the magnetic field is drifting. The. Q- c V' ?7 C- t
lock frequency plot displays the frequency without correction. In
' H; D) O1 b% q8 \reality, this frequency would be kept constant by the application of& ^( E# D2 f8 {9 z& c7 u9 n
the lock field which offsets the drift.
& L' ]( t2 i* k2 H) O$ A t, j3 m Z& `; ~1 l
' U, t; g- b/ G# ~& G, ^
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0, x6 t k5 P, }( J
reference. The resonance frequency of the deuterium signal in many lock
x9 k D) _. p0 Q3 z2 ysolvents is well known. Therefore the difference in resonance frequency
9 I8 E% S1 n# x5 ?" F: oof the lock solvent and TMS is also known. As a consequence, TMS does
. g- B) X: v, lnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
9 S/ v5 Q8 ~1 F$ G* j& ~- U) u=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
