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
( J" n0 {; N: n7 k: o% hof a sample, especially one which requires signal averaging or phase" _4 L" ]2 i" ]: f" \
cycling, you need to have a temporally constant and spatially4 I3 E9 c# q# O) [! ?; E
homogeneous magnetic field. Consistency of the Bo" F9 R% v6 K& u) G; `1 |
field over time will be discussed here; homogeneity will be discussed
& V) s9 M2 O' N/ ]in the next section of this chapter. The field strength might vary over. D, t' M+ l4 k0 M4 L( {
time due to aging of the magnet, movement of metal objects near the
" ^* a. Z+ H5 b* ]6 ?1 j6 vmagnet, and temperature fluctuations. Here is an example of a one line$ N# U" z, ]% t5 ?
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
1 S5 E7 B9 k" h: P3 n- Q tThe field lock can compensate for these variations.
* Y" W; ^" X% J5 b$ R8 BThe field lock is a separate NMR spectrometer within your spectrometer.
, S8 e+ C8 e5 Z5 X# S; D8 p- w9 e/ b/ \This spectrometer is typically tuned to the deuterium NMR resonance
2 l7 u! a+ X% D! p7 ]) B! ^9 wfrequency. It constantly monitors the resonance frequency of the2 t/ R1 _& O, x
deuterium signal and makes minor changes in the Bo magnetic field to keep the9 x) Q5 z2 U8 B: P0 g$ V; P
resonance frequency constant. The deuterium signal comes from the
/ V- ]: a: l5 B' x j* vdeuterium solvent used to prepare the sample. The animation window
* s1 Z5 s2 @8 xcontains plots of the deuterium resonance lock frequency, the small
; L' B" Q' ]' ~: A9 badditional magnetic field used to correct the lock frequency, and the
! V! j- ]7 y0 ^' i/ y: s5 nresultant Bo- v/ C1 i" T. Y% `3 F4 L* x
field as a function of time while the magnetic field is drifting. The! r Z0 S' a8 k9 M
lock frequency plot displays the frequency without correction. In
5 u4 p$ s" `% k% F9 V6 Kreality, this frequency would be kept constant by the application of
$ W8 `+ u/ j' h: Z" ?% ithe lock field which offsets the drift.
! ~' a- V, {' m: U8 E7 U- D
4 N* `3 E4 `# V! P, x' D6 w [, A+ w% m6 }- E4 L
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
; F3 Y' L" X7 A+ d% X) ?reference. The resonance frequency of the deuterium signal in many lock+ O4 g( A/ K1 W1 L" P) n! {. t
solvents is well known. Therefore the difference in resonance frequency
1 l4 j$ M, {, d& x. i1 V5 @7 Zof the lock solvent and TMS is also known. As a consequence, TMS does9 ] {% B- c& J; g* {
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
) P% R/ y0 y* N$ V=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
