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( Y7 w8 F9 C" r' Z
of a sample, especially one which requires signal averaging or phase
6 u3 [$ [1 ^8 `cycling, you need to have a temporally constant and spatially
" j H& H; O2 Z: D5 t5 Jhomogeneous magnetic field. Consistency of the Bo0 i( k9 \# g9 s! f' z
field over time will be discussed here; homogeneity will be discussed
5 S2 |& [/ m; m* y; ? K3 cin the next section of this chapter. The field strength might vary over4 j: o% H9 g L* y4 A0 B# r. ]
time due to aging of the magnet, movement of metal objects near the4 z' o Z7 b6 x
magnet, and temperature fluctuations. Here is an example of a one line
; V5 t: \# X* M' D# _. s7 }NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. 9 |" @4 j$ i+ m
The field lock can compensate for these variations.
" _0 N; K- I7 ^. m1 S" ~# E' T0 VThe field lock is a separate NMR spectrometer within your spectrometer.' K4 L w) g2 _4 r5 ^9 K) {
This spectrometer is typically tuned to the deuterium NMR resonance
& h* f5 z2 q9 D- q! y+ nfrequency. It constantly monitors the resonance frequency of the) a; ?, s- w1 a" n' y& P/ f
deuterium signal and makes minor changes in the Bo magnetic field to keep the/ \0 g- {( V+ g2 Q( g! b
resonance frequency constant. The deuterium signal comes from the
9 r! f. f) F& S( A9 Ydeuterium solvent used to prepare the sample. The animation window + V I* O. ~7 n* g; g, A
contains plots of the deuterium resonance lock frequency, the small
- I% }, r( |& u& y" d( radditional magnetic field used to correct the lock frequency, and the
& S* `0 ]2 }3 `- T" Mresultant Bo
8 ~$ F$ |+ U% I z8 D8 Jfield as a function of time while the magnetic field is drifting. The# i4 L+ M0 p# N! d" n; ]; V
lock frequency plot displays the frequency without correction. In9 C2 q7 A, f4 X; Q' a
reality, this frequency would be kept constant by the application of3 S" B" J: E8 M- j9 u$ {& p" u
the lock field which offsets the drift.
. @5 z& H1 ~% q% r- C, J6 A% |; t4 y* H x6 E( l% c3 s9 E
' D: O/ @9 f2 Z0 h& W3 l
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
2 ~ @2 [: I" J& t3 {3 T( O& Q! T* W8 [reference. The resonance frequency of the deuterium signal in many lock D/ f' x& {' H, z" ^' n% G
solvents is well known. Therefore the difference in resonance frequency
+ h; v# N0 Q x% W0 Nof the lock solvent and TMS is also known. As a consequence, TMS does
) {- V- u1 H& Q6 vnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
' r, g# P, n! V7 v=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
