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
8 U/ z- M% k& ~% X" {1 yof a sample, especially one which requires signal averaging or phase
4 U. o7 ^' R' m; T9 M7 ? ?) `cycling, you need to have a temporally constant and spatially" s9 t1 u; U( `! v9 ^/ B& }
homogeneous magnetic field. Consistency of the Bo
! \0 `- Y4 e) L, t5 Q' ffield over time will be discussed here; homogeneity will be discussed6 p) R( f1 V5 m
in the next section of this chapter. The field strength might vary over$ s2 o3 ^0 B5 z
time due to aging of the magnet, movement of metal objects near the
* p; v* c; V% V; H% @* O( e' `9 r. emagnet, and temperature fluctuations. Here is an example of a one line
# r# B5 A- Z9 l9 [NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. $ G% i2 c! P( @- L! R8 r4 r
The field lock can compensate for these variations.
" q! g( @: F. z6 y
The field lock is a separate NMR spectrometer within your spectrometer.
% g3 f$ l, U8 u- ` h4 F% Q+ PThis spectrometer is typically tuned to the deuterium NMR resonance
* T+ @% ?' j4 {. rfrequency. It constantly monitors the resonance frequency of the
! j/ ^5 X8 {8 G" K H1 s+ fdeuterium signal and makes minor changes in the Bo magnetic field to keep the
) F3 z& g/ E; v3 G3 T2 @: s: Fresonance frequency constant. The deuterium signal comes from the. a1 l! q* @1 s3 r
deuterium solvent used to prepare the sample. The animation window : ]; U6 `8 G# K4 i1 G
contains plots of the deuterium resonance lock frequency, the small; n, s# s8 O& K3 }$ F
additional magnetic field used to correct the lock frequency, and the* G/ W0 H2 r; h( e/ ]
resultant Bo3 c4 Z1 A" b, K: w& A9 Z; `7 `
field as a function of time while the magnetic field is drifting. The
# b7 T2 ~; Q4 M% [8 S) X& V) S6 T2 llock frequency plot displays the frequency without correction. In
) ?0 P2 d% |$ n2 W" Q+ `. r9 Jreality, this frequency would be kept constant by the application of
7 q' @, a; P+ Q" f1 g" s6 i0 }the lock field which offsets the drift.
+ j8 C/ J( ~6 a% q5 E
, z. p5 Z" f) a( Z" G K
' m$ ]+ R6 W. uOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0 ~; r" E) n4 A# W/ y* a
reference. The resonance frequency of the deuterium signal in many lock
. |+ ?/ G' K6 n; m/ h7 Q, c2 gsolvents is well known. Therefore the difference in resonance frequency
, s$ Y( x) S9 R% }7 Rof the lock solvent and TMS is also known. As a consequence, TMS does
2 g# c) @7 M# i' ynot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
' G! i8 W7 j8 _" t' E=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
