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
* l# A; a+ d. jof a sample, especially one which requires signal averaging or phase: l. O# }$ g4 W; K V, z
cycling, you need to have a temporally constant and spatially
) C9 O& V8 i- }: Shomogeneous magnetic field. Consistency of the Bo
: l4 O, m0 z* Y* X/ R- i& ~field over time will be discussed here; homogeneity will be discussed* e4 b/ h! t2 A2 V$ e' R) _8 h% ^
in the next section of this chapter. The field strength might vary over
6 P5 B6 z' K1 Q) |5 M6 dtime due to aging of the magnet, movement of metal objects near the
8 m& Q* ]; J+ a; a& Q0 N) {magnet, and temperature fluctuations. Here is an example of a one line+ L- c$ U( V. E2 ?+ a
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. / H" ]& }! J+ h1 D: G0 \8 e( o2 W7 ?0 R6 j$ j
The field lock can compensate for these variations.
! ?" P5 B$ h0 D
The field lock is a separate NMR spectrometer within your spectrometer.
% }6 g6 D8 q. L3 dThis spectrometer is typically tuned to the deuterium NMR resonance
- R1 |3 O. k7 G h* Qfrequency. It constantly monitors the resonance frequency of the
/ d5 M5 d( A3 z) H/ z) tdeuterium signal and makes minor changes in the Bo magnetic field to keep the
2 O {# M- C+ R5 {- T2 eresonance frequency constant. The deuterium signal comes from the( J* }; l( f7 N0 l3 Y! T4 @5 r0 Y
deuterium solvent used to prepare the sample. The animation window
* ~, p' A6 k1 e4 k/ ccontains plots of the deuterium resonance lock frequency, the small' J i* K) G) }* u3 v6 w) U
additional magnetic field used to correct the lock frequency, and the
( c/ o) l# i+ q* jresultant Bo3 E1 n8 H: j3 C( ^8 a, j# F: E
field as a function of time while the magnetic field is drifting. The, Z0 R2 S, e7 z6 k. C' m
lock frequency plot displays the frequency without correction. In6 A" l6 `& z7 b- D! p
reality, this frequency would be kept constant by the application of
* |/ i& q/ |3 o% z5 j9 _the lock field which offsets the drift.
, G; Q! ^2 r0 P) C, f+ D, e0 ?: _; B2 V# z. y8 n
) G8 p2 C7 y/ x) VOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
; }% @( s7 c% |* ~7 ereference. The resonance frequency of the deuterium signal in many lock5 ?6 A9 I1 n9 R/ O8 P$ g# F
solvents is well known. Therefore the difference in resonance frequency, [5 R+ t/ C3 q) @( K# h% ?
of the lock solvent and TMS is also known. As a consequence, TMS does
6 V0 V: j3 ~7 ~# [% Jnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate 1 z8 L+ @( k2 S
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
