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
& u+ Z0 D# r' {1 y! ?. P vof a sample, especially one which requires signal averaging or phase) F" a. B9 W! C& P4 p
cycling, you need to have a temporally constant and spatially& V' S% Y: _7 w) |3 g. ?
homogeneous magnetic field. Consistency of the Bo: B- t" b4 `! E6 l
field over time will be discussed here; homogeneity will be discussed5 m5 t2 f, Q' {" M
in the next section of this chapter. The field strength might vary over8 O9 r; m! |% A( q3 y
time due to aging of the magnet, movement of metal objects near the! E) d0 P+ h# S z$ B0 E" f' G; ^0 C
magnet, and temperature fluctuations. Here is an example of a one line) r- p3 @9 k7 n' ?) t
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. ) G1 ]- ?6 o/ |+ a5 b% D( P7 Y" ~
The field lock can compensate for these variations.
! A( z; e5 Y4 E! |* H
The field lock is a separate NMR spectrometer within your spectrometer.6 a/ ^# ?( ?- ]
This spectrometer is typically tuned to the deuterium NMR resonance: u( Q7 N$ X& j: E8 v2 k7 n9 J! U
frequency. It constantly monitors the resonance frequency of the
7 Z% B! x/ ^" kdeuterium signal and makes minor changes in the Bo magnetic field to keep the- r" m! S, q7 C: c$ ?% a# G& E. d5 \
resonance frequency constant. The deuterium signal comes from the4 |; w$ U0 m+ B. G$ X1 H( M( x; ~ s; g
deuterium solvent used to prepare the sample. The animation window $ l+ c1 Z, m& {) I
contains plots of the deuterium resonance lock frequency, the small
8 |# p; Q- t; E: s8 J, \" K0 kadditional magnetic field used to correct the lock frequency, and the# u3 T* R0 v( L% y/ {1 e
resultant Bo
# q9 Z; X7 ~" N8 ]& `field as a function of time while the magnetic field is drifting. The
( O* u2 K/ ^( N/ h2 Llock frequency plot displays the frequency without correction. In; b- j+ P" `; Y7 {5 a. I1 ?
reality, this frequency would be kept constant by the application of f3 J3 d4 o H7 L3 ~* S# o
the lock field which offsets the drift.
2 r( H8 K+ k9 v& e& |
" v* @$ {" N6 v+ m
) n7 v; e, u0 qOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0* T3 ^( N& u5 z7 j Z- j
reference. The resonance frequency of the deuterium signal in many lock
0 K) f5 m7 g2 M" M( Msolvents is well known. Therefore the difference in resonance frequency
0 A- W- Z6 E1 O! u( `of the lock solvent and TMS is also known. As a consequence, TMS does
# j% A- L* A2 _4 ?not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate ' x: g& }$ w) t) U: j4 A. P8 h
=0.
| 
发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
