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 spectrum7 H, d& t8 U0 ?' [
of a sample, especially one which requires signal averaging or phase
9 x( A6 v3 q% `; ^; Hcycling, you need to have a temporally constant and spatially
2 E, d2 i. t$ r Z# i: Ghomogeneous magnetic field. Consistency of the Bo) `" p v7 B% ^& h: I
field over time will be discussed here; homogeneity will be discussed
8 |- \" v% k( O4 rin the next section of this chapter. The field strength might vary over; N9 |- N- L. T9 A) f0 U
time due to aging of the magnet, movement of metal objects near the! ]: i9 C' G( Q: N! a
magnet, and temperature fluctuations. Here is an example of a one line
2 e6 m. h/ l8 f- c4 p' O' [' ONMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. . a+ {, O- `$ d, P
The field lock can compensate for these variations.
4 M( I. p% @* T- A R9 S" EThe field lock is a separate NMR spectrometer within your spectrometer.
! F( k3 @0 h. _- B3 a; AThis spectrometer is typically tuned to the deuterium NMR resonance2 S. U" F* v: j! a* |& i! t. v
frequency. It constantly monitors the resonance frequency of the2 {" a' y; P& b3 b% C0 ?
deuterium signal and makes minor changes in the Bo magnetic field to keep the
" G( v( N3 ~9 u: I! j; Rresonance frequency constant. The deuterium signal comes from the3 ^% m3 _: N7 S- l* Q4 G
deuterium solvent used to prepare the sample. The animation window
! `3 z q6 {+ V% ycontains plots of the deuterium resonance lock frequency, the small
* Y2 m$ V: A6 W8 r- b6 @' Jadditional magnetic field used to correct the lock frequency, and the
/ c; u% i/ t: z2 l2 u! b/ O0 u( e( Dresultant Bo
( M1 _7 Y$ ?0 d& O2 F* S5 hfield as a function of time while the magnetic field is drifting. The
! o' k. i- f1 r7 a3 y d* r+ nlock frequency plot displays the frequency without correction. In
- W! h" u9 S, O7 u6 V: y0 a: oreality, this frequency would be kept constant by the application of7 M9 U% w ?) o4 o$ |, Z5 n) H
the lock field which offsets the drift.
9 A; X" Z$ Z M) h
8 z: B# M- X0 m5 W
1 H7 i* O& g1 m9 M9 c7 rOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =09 q7 x2 W% g V5 k2 c" \* d5 x. `
reference. The resonance frequency of the deuterium signal in many lock
6 j( C# K3 g* A7 n/ [1 m* K4 P+ T O& psolvents is well known. Therefore the difference in resonance frequency, i- g6 ?, v8 o+ @
of the lock solvent and TMS is also known. As a consequence, TMS does; r+ ^# V/ h. D# J% | {
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
: A5 e& E0 }7 V0 ^6 H. u=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
