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
9 ^' W3 y8 J9 M2 P. R9 u3 Nof a sample, especially one which requires signal averaging or phase9 Y$ l; _5 V2 i( W; @; y* Y
cycling, you need to have a temporally constant and spatially5 x3 b! r9 C) q4 Q" C0 I
homogeneous magnetic field. Consistency of the Bo, W$ _7 ]; i3 w9 w1 y+ A. ]
field over time will be discussed here; homogeneity will be discussed3 }% o5 t- _/ e
in the next section of this chapter. The field strength might vary over* V5 U& v: w% ^) @# _4 O. H
time due to aging of the magnet, movement of metal objects near the2 A/ K+ f4 W$ [) }5 c. m0 s! z" \
magnet, and temperature fluctuations. Here is an example of a one line
H3 y8 S6 R; e( ^2 U; }5 pNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
* A8 Q0 g+ o' h$ z& A, OThe field lock can compensate for these variations.
5 ^0 z- A# T, s4 A6 jThe field lock is a separate NMR spectrometer within your spectrometer.
/ ]3 {- X) u; i/ x" S, e; ]This spectrometer is typically tuned to the deuterium NMR resonance
1 T2 _3 N# Z1 V' z' N" Efrequency. It constantly monitors the resonance frequency of the1 F5 {7 Y2 F ?% k* c, ^# J
deuterium signal and makes minor changes in the Bo magnetic field to keep the
, ]1 z- j! k. ~/ z$ o- S A( dresonance frequency constant. The deuterium signal comes from the
) }: a9 ^) a! Ideuterium solvent used to prepare the sample. The animation window 2 g6 U& m5 i0 A9 X+ V
contains plots of the deuterium resonance lock frequency, the small
! i: R- e0 x5 [2 ~3 `: R/ dadditional magnetic field used to correct the lock frequency, and the
8 M" d* y) i. Y$ C" p5 Cresultant Bo
H7 H* E$ M8 U" |. P& k2 ?field as a function of time while the magnetic field is drifting. The
9 U9 i/ K: U. @6 S* X0 ?; |3 Flock frequency plot displays the frequency without correction. In
# K' Y: D7 S4 P% v- [- T- ?reality, this frequency would be kept constant by the application of: }+ r& S% P9 ?2 `( M$ t
the lock field which offsets the drift.
: `( { ?8 \# y/ \- c" q5 K" t
& E2 w I5 F) y! ?" A8 I
& W1 A) p& p- w. l7 h. VOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
8 j9 K! `; R: W: P$ t1 v1 [ Lreference. The resonance frequency of the deuterium signal in many lock
7 v( x; E' z1 E# u2 p0 osolvents is well known. Therefore the difference in resonance frequency* n* N P( ~+ y0 r) A D; g" ]7 H2 b
of the lock solvent and TMS is also known. As a consequence, TMS does$ z" R; C6 n* R9 L
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
. w5 \+ }2 b3 T( o. w" r" Y=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
