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 G* [4 X/ P; u1 F7 d6 }of a sample, especially one which requires signal averaging or phase
5 X* d- B, M/ }5 r0 fcycling, you need to have a temporally constant and spatially9 Q4 F( M" U9 P c" _
homogeneous magnetic field. Consistency of the Bo0 g. o" C/ w# x
field over time will be discussed here; homogeneity will be discussed
* y" l/ L# ?( @3 s0 G6 K+ }$ A& [$ uin the next section of this chapter. The field strength might vary over0 \4 V) {/ n$ T6 Y
time due to aging of the magnet, movement of metal objects near the+ ]# z! f _9 f. w5 i/ u
magnet, and temperature fluctuations. Here is an example of a one line
% V3 n0 g3 h7 bNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
* T+ F3 d5 t" \; i. q& T2 HThe field lock can compensate for these variations.
& X7 {. C8 n, h; r* f O5 sThe field lock is a separate NMR spectrometer within your spectrometer.
, q5 s3 Q; P# S& b. {+ DThis spectrometer is typically tuned to the deuterium NMR resonance3 a4 {% A$ Q0 ~9 `' }+ A% j
frequency. It constantly monitors the resonance frequency of the' l; ~- s5 ^$ P
deuterium signal and makes minor changes in the Bo magnetic field to keep the g+ {. l8 M3 o& s- M( X
resonance frequency constant. The deuterium signal comes from the
: J( ?# z+ a8 M: z1 T1 m4 E( Hdeuterium solvent used to prepare the sample. The animation window
. D7 S# X; C1 x; }. bcontains plots of the deuterium resonance lock frequency, the small/ D/ r. P! `8 {) W7 P; u
additional magnetic field used to correct the lock frequency, and the
4 r+ _4 @5 J* j6 Kresultant Bo
7 C/ u7 B c0 P2 ^! jfield as a function of time while the magnetic field is drifting. The
$ X; A" `( H+ o' M% w+ tlock frequency plot displays the frequency without correction. In
2 @ m# i$ G# ]6 f' Qreality, this frequency would be kept constant by the application of
1 v: w% l; g/ r: [; L- m: ~8 vthe lock field which offsets the drift.
, Z9 w2 i5 V* Y! A( ~' N3 E* l+ @0 T9 @
1 K9 Y* [) x0 i, f
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =05 V+ `' W% @& e6 k9 F. w. U
reference. The resonance frequency of the deuterium signal in many lock
+ o4 ?5 l L! d8 Z+ _solvents is well known. Therefore the difference in resonance frequency
* C* e" R' D; P' }: t" uof the lock solvent and TMS is also known. As a consequence, TMS does
i$ D/ i( K% _: L8 e. |not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate 5 e4 _( m1 l) V2 n/ W
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
