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, v% `+ z0 b# E h; M( \
of a sample, especially one which requires signal averaging or phase8 A+ l. p2 J1 k1 g' @9 Z
cycling, you need to have a temporally constant and spatially
, }+ h7 B2 O1 ]homogeneous magnetic field. Consistency of the Bo6 k' h- H# |# x! s
field over time will be discussed here; homogeneity will be discussed5 o; E3 @& l$ z( v) r
in the next section of this chapter. The field strength might vary over. U, h7 D2 r: p
time due to aging of the magnet, movement of metal objects near the/ F4 r- O" L4 @ ?. H
magnet, and temperature fluctuations. Here is an example of a one line% A2 V$ v' F: R# a
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
# \+ u s; ]$ M" A) a# [The field lock can compensate for these variations.
6 O% y. j" u4 f, ~The field lock is a separate NMR spectrometer within your spectrometer.
& L7 ]! Q4 K/ [; jThis spectrometer is typically tuned to the deuterium NMR resonance$ P& a0 N/ f4 g/ H! s5 g. l; ~
frequency. It constantly monitors the resonance frequency of the8 ^% k5 m- o; O+ h4 ^
deuterium signal and makes minor changes in the Bo magnetic field to keep the
& M$ u+ ^0 Z, J* Nresonance frequency constant. The deuterium signal comes from the+ L& b; @7 E3 o
deuterium solvent used to prepare the sample. The animation window - r) f) Z% d( Q1 M3 R- n! [1 \8 r
contains plots of the deuterium resonance lock frequency, the small
+ b8 ?! U2 [% p- m4 H' s, W' Hadditional magnetic field used to correct the lock frequency, and the" x4 o6 L4 W ^" m
resultant Bo( v' l' s# B! v4 z. g" S/ T
field as a function of time while the magnetic field is drifting. The
# o5 I+ X, Z9 k3 d& ^1 p3 _lock frequency plot displays the frequency without correction. In* @1 O' X9 C/ _0 N) \1 x9 D' v" U
reality, this frequency would be kept constant by the application of& b. w8 A0 T* B6 {4 B
the lock field which offsets the drift.
0 U4 Q8 C1 Y, j7 [
3 e `8 }) w" `; I8 w; _/ Z5 V4 ]: I/ P& a1 M+ n
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
$ V3 s2 U3 i0 k) S& x6 mreference. The resonance frequency of the deuterium signal in many lock
8 n! e+ @' t8 B% ?0 B: Xsolvents is well known. Therefore the difference in resonance frequency
" T' M, N$ o3 K1 T' fof the lock solvent and TMS is also known. As a consequence, TMS does$ k! ^6 \, T. Z. ?2 d8 ]+ Z
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
7 x$ A* M2 |! y8 k1 G$ u$ Z: c' s) S=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
