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
6 N! r2 s& a) M! _of a sample, especially one which requires signal averaging or phase
" K9 `8 F( k6 ~% I% M4 d0 _cycling, you need to have a temporally constant and spatially$ t# D7 _7 }% w; G. o+ A3 r0 e- i9 F
homogeneous magnetic field. Consistency of the Bo2 U: R" ~: H3 S: k7 z
field over time will be discussed here; homogeneity will be discussed
6 _/ K9 J+ g1 C/ ?in the next section of this chapter. The field strength might vary over
* x) I% f, g7 i0 {. i; [+ D! U" Ptime due to aging of the magnet, movement of metal objects near the
) r) e5 k- U0 K. ymagnet, and temperature fluctuations. Here is an example of a one line
4 W2 h7 f; Q- V$ d; c9 D0 QNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
8 e3 j& J8 K7 o2 \/ X( EThe field lock can compensate for these variations.
9 w+ n1 C% }4 q; R- R% R0 F
The field lock is a separate NMR spectrometer within your spectrometer.
) h" ?2 f) N5 d; e6 N& x tThis spectrometer is typically tuned to the deuterium NMR resonance
0 g9 c3 a5 e$ Jfrequency. It constantly monitors the resonance frequency of the5 ]4 @0 F9 i" l/ C3 M
deuterium signal and makes minor changes in the Bo magnetic field to keep the F. p: z' M( N% |
resonance frequency constant. The deuterium signal comes from the
* ~. P Q$ n B& j( ~" ^deuterium solvent used to prepare the sample. The animation window
! A9 ]9 S" a' E. o: Xcontains plots of the deuterium resonance lock frequency, the small
9 P t) [$ r0 u7 n& Tadditional magnetic field used to correct the lock frequency, and the+ [6 G4 C- `" o+ d! `0 U
resultant Bo( ?( [' Y: z2 V$ T) k8 a. o
field as a function of time while the magnetic field is drifting. The' e( T( J' b, X9 e( \
lock frequency plot displays the frequency without correction. In
4 V! J; |" D" ]* s2 greality, this frequency would be kept constant by the application of* h9 m' c1 Z+ l- h4 V& ?2 H
the lock field which offsets the drift.
% C9 o2 s( X5 n( q; d9 |5 V! p# [% u
& b0 n0 R* i$ R1 x" i: }* c5 ZOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =09 a4 b0 _/ c \& `% ?
reference. The resonance frequency of the deuterium signal in many lock
- q3 |( s" ?) r% V' osolvents is well known. Therefore the difference in resonance frequency
# k5 |' {% ^" n9 B$ u4 r' yof the lock solvent and TMS is also known. As a consequence, TMS does2 g: z4 h# ^1 ~' I
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate 2 X1 ^9 v- I) D- W6 t) [
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
