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 spectrum3 x& k; E" U* n
of a sample, especially one which requires signal averaging or phase+ F# |+ ?* i- m* t% d1 g9 k
cycling, you need to have a temporally constant and spatially4 ^* v& g5 \# M/ d6 b, K, Z4 O
homogeneous magnetic field. Consistency of the Bo
4 X( q) v1 N& _8 dfield over time will be discussed here; homogeneity will be discussed
, n$ U! \% h5 ein the next section of this chapter. The field strength might vary over
, i+ Y8 `3 q# {: O+ }3 P# F+ Htime due to aging of the magnet, movement of metal objects near the1 Y$ b+ }9 P* b# A- [& L4 h- Y1 F; s! L. S
magnet, and temperature fluctuations. Here is an example of a one line
5 F2 i6 q$ q$ p/ Y' yNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
; s5 S+ K* v1 G& \4 j! IThe field lock can compensate for these variations.
& Z- L5 P+ R1 GThe field lock is a separate NMR spectrometer within your spectrometer.
6 v' {# u: G5 b" x! VThis spectrometer is typically tuned to the deuterium NMR resonance
5 N# R! N- `% F, i* \frequency. It constantly monitors the resonance frequency of the/ y' y/ _1 X8 N- q
deuterium signal and makes minor changes in the Bo magnetic field to keep the6 ~" }" Z! L' @# S' \! k4 _5 o
resonance frequency constant. The deuterium signal comes from the
: F4 \) B1 c, Xdeuterium solvent used to prepare the sample. The animation window
$ q/ z9 l9 t* i) `' Rcontains plots of the deuterium resonance lock frequency, the small- \3 T. v6 \" }$ ?/ v% R; B" o
additional magnetic field used to correct the lock frequency, and the
; d& ~" ~- r. K. d6 T3 Lresultant Bo9 n' d! m" W( W& b8 B
field as a function of time while the magnetic field is drifting. The6 l% D c* t( w+ F6 Z
lock frequency plot displays the frequency without correction. In
) N8 l, F0 G2 ?( @1 yreality, this frequency would be kept constant by the application of
8 I \ a; ~+ K2 ^- z+ A" r! ethe lock field which offsets the drift.
8 Q& w0 A! s- o0 H N w! r
~* R6 K& i( m2 ~1 u, |8 b; x2 x8 W! T" U
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
& ^* `( [9 j3 f! R! i( {2 {reference. The resonance frequency of the deuterium signal in many lock
$ I1 A# T) w) j( [$ s1 Gsolvents is well known. Therefore the difference in resonance frequency
0 ]9 ?" y0 n1 ~; j; c2 P$ f& y& e! oof the lock solvent and TMS is also known. As a consequence, TMS does
8 h) d6 m, w3 B% Cnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
" h! b% N* Z" f1 a; ?( o+ L# }=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
