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 s% M1 f- W, A$ ?! S0 M0 s) Sof a sample, especially one which requires signal averaging or phase* [- e; P4 [: t8 l- s4 P; ?
cycling, you need to have a temporally constant and spatially
# ]. E: L/ k1 `! {+ D3 t. ihomogeneous magnetic field. Consistency of the Bo" b% o% c. p% J$ \
field over time will be discussed here; homogeneity will be discussed
}" C; u, t; o3 x0 E! n- ^in the next section of this chapter. The field strength might vary over' X- i/ Q5 x8 m
time due to aging of the magnet, movement of metal objects near the
5 e+ o8 ^+ |6 e c8 imagnet, and temperature fluctuations. Here is an example of a one line
+ G$ `" P: [ s# }3 YNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
8 f" W3 `6 Z. M7 s/ @5 M$ K* n! dThe field lock can compensate for these variations.
4 z/ e6 w; X$ Q
The field lock is a separate NMR spectrometer within your spectrometer.
3 E9 | W/ k) E E4 b0 C* b QThis spectrometer is typically tuned to the deuterium NMR resonance
$ r1 Y* R$ A `6 ^" y- Ofrequency. It constantly monitors the resonance frequency of the
# L5 R" ?, p: o; n- b& tdeuterium signal and makes minor changes in the Bo magnetic field to keep the
2 t' ]- p) d5 v& [( @resonance frequency constant. The deuterium signal comes from the0 \0 D v: f G U4 v
deuterium solvent used to prepare the sample. The animation window
' t c3 r" t6 O' zcontains plots of the deuterium resonance lock frequency, the small5 W6 i W8 {( |2 o @- I% u
additional magnetic field used to correct the lock frequency, and the
; g# M: p: D8 Q$ h9 O; g. w$ wresultant Bo" @/ T \0 A& ]6 r2 y
field as a function of time while the magnetic field is drifting. The
$ q$ o( a( T8 n: Mlock frequency plot displays the frequency without correction. In! w) e/ o# D4 \/ s y
reality, this frequency would be kept constant by the application of
0 Q( e7 k# J% o _8 l7 N. ]3 o, athe lock field which offsets the drift.
9 O9 b7 e4 j; r* T. k% w) V2 f$ X$ B4 D( Y
7 W/ ~* r( o: g9 S8 r
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
$ z6 `8 z( s3 W% ]0 `) U2 preference. The resonance frequency of the deuterium signal in many lock5 v: Y3 G* U" C5 }1 Q$ y
solvents is well known. Therefore the difference in resonance frequency
$ U- r; N' `+ a; b7 I4 c" D# rof the lock solvent and TMS is also known. As a consequence, TMS does
% _ P7 n, g, D/ f, W* vnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
# `% [/ ~- w4 k: L1 ^; @$ e* J=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
