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
& B) P; d7 V. n! b# R0 m& O" zof a sample, especially one which requires signal averaging or phase( n6 p% w, } d# i0 g# j, P9 B2 E
cycling, you need to have a temporally constant and spatially
! ]% g D3 R2 qhomogeneous magnetic field. Consistency of the Bo
& ?5 S$ k" @0 V8 U, p7 vfield over time will be discussed here; homogeneity will be discussed6 T! L- h% W& q! E# ~
in the next section of this chapter. The field strength might vary over% F$ I. l! ^3 v8 H& S# _& ?" ^
time due to aging of the magnet, movement of metal objects near the
- t: j: O5 n) B zmagnet, and temperature fluctuations. Here is an example of a one line
7 S; y! w1 V/ a3 p# QNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. 6 e$ ~5 `# `9 x4 S. x& [' G% P! T
The field lock can compensate for these variations.
6 @. v8 m. b3 Q3 Z
The field lock is a separate NMR spectrometer within your spectrometer., L/ R# D) { Z2 J8 P( {+ s: v
This spectrometer is typically tuned to the deuterium NMR resonance9 ?4 J4 r7 b2 F" [# C1 A, b7 Q7 F
frequency. It constantly monitors the resonance frequency of the6 K' a# v9 G9 J
deuterium signal and makes minor changes in the Bo magnetic field to keep the
p2 r" K9 A5 i8 r5 rresonance frequency constant. The deuterium signal comes from the
, ?0 x! v3 \/ Y U k! Mdeuterium solvent used to prepare the sample. The animation window $ a% q2 d' u3 T: d# @- J( L4 S) _
contains plots of the deuterium resonance lock frequency, the small8 @( r. S. G4 O1 P1 h! C
additional magnetic field used to correct the lock frequency, and the
: L, O0 V/ |! uresultant Bo
) g3 O- Q. U% D7 j7 w9 w9 ^field as a function of time while the magnetic field is drifting. The
4 y( Y) f, {3 T5 J0 U* Ilock frequency plot displays the frequency without correction. In
" X) Z8 [: c5 U! j3 Ereality, this frequency would be kept constant by the application of
' X5 |8 o1 T' T) u: zthe lock field which offsets the drift.
" x# n1 s7 n- X" S0 O0 H I; O% ^8 z1 s+ A
; o8 E$ I- d. y Q
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
' P5 C# R* U6 K5 T. E+ q! [: [; M2 ]reference. The resonance frequency of the deuterium signal in many lock
5 K( @1 x3 f/ Y$ ^4 ?" }1 ssolvents is well known. Therefore the difference in resonance frequency/ ~- b' U$ r/ B2 b* j! S
of the lock solvent and TMS is also known. As a consequence, TMS does
6 C0 i3 y3 v% T6 s" _% Cnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate 8 N6 @2 ]( a: U
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
