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
5 S, N8 t) `! ?* W7 Nof a sample, especially one which requires signal averaging or phase
# U1 l! x, e4 Z. Vcycling, you need to have a temporally constant and spatially
( h8 i5 {; L, [+ i4 Xhomogeneous magnetic field. Consistency of the Bo1 U% t) o" a+ B6 E) G+ b: V
field over time will be discussed here; homogeneity will be discussed) V& V7 H. ~$ H1 p9 r7 h* ~
in the next section of this chapter. The field strength might vary over' J5 {" }8 z" ~# p2 l/ E
time due to aging of the magnet, movement of metal objects near the# [! C( O1 _# G; Q% C0 @
magnet, and temperature fluctuations. Here is an example of a one line
* q. [7 _2 t" pNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
" Q: C+ P q: TThe field lock can compensate for these variations.
7 f8 C0 g$ m5 ^" T6 n- `
The field lock is a separate NMR spectrometer within your spectrometer.
* B: c! X* D" WThis spectrometer is typically tuned to the deuterium NMR resonance K( U& w$ b+ g
frequency. It constantly monitors the resonance frequency of the
; D& Q9 x5 ~. M# W! sdeuterium signal and makes minor changes in the Bo magnetic field to keep the
5 e( T* @& t' ~$ E+ V9 Mresonance frequency constant. The deuterium signal comes from the
, H1 [ S' R7 t9 E8 c: _2 jdeuterium solvent used to prepare the sample. The animation window
3 o2 e/ R4 e$ L3 @* hcontains plots of the deuterium resonance lock frequency, the small
7 D: k% U1 D# Radditional magnetic field used to correct the lock frequency, and the
( m& _& B) e+ ~% h! Qresultant Bo& ]' r. f# t& v5 z0 X
field as a function of time while the magnetic field is drifting. The) y/ ~" a- C8 \3 x3 ~& u
lock frequency plot displays the frequency without correction. In
3 t$ Y" M- P/ C& R; E3 Z3 I( t e( Yreality, this frequency would be kept constant by the application of* f/ e: z9 B8 B7 }* Z D
the lock field which offsets the drift.
% n, _. o& ]& G: a7 `
4 p" l3 n& l1 n( C, J
4 [- `4 N# r8 g/ q% YOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =09 y7 B# ^' v: t9 B
reference. The resonance frequency of the deuterium signal in many lock% r& A, B- {" W4 x# X
solvents is well known. Therefore the difference in resonance frequency
2 Y+ s0 Q6 y* J: y# r+ \; t- |of the lock solvent and TMS is also known. As a consequence, TMS does0 l# m c/ k: ~/ ^
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate ; Y6 o4 T3 I$ T: |
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
