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' t# x" f2 o; \9 U" G: ^
of a sample, especially one which requires signal averaging or phase2 a1 j7 [ b- H6 @$ l
cycling, you need to have a temporally constant and spatially5 z( W8 K' `) _+ H
homogeneous magnetic field. Consistency of the Bo- N- p& j4 u+ x0 A8 {% x# k+ q
field over time will be discussed here; homogeneity will be discussed
: {. I# a8 n) M3 pin the next section of this chapter. The field strength might vary over
0 c5 m( \5 u! j8 e& p- U; xtime due to aging of the magnet, movement of metal objects near the
) M: I. g' U! V( I+ X, Ymagnet, and temperature fluctuations. Here is an example of a one line
: _6 r' o* t4 F, xNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. : Y% g2 V9 b5 m3 d4 z
The field lock can compensate for these variations.
3 p: Y s. u5 L# C# QThe field lock is a separate NMR spectrometer within your spectrometer.
1 p5 l( P" O: Q3 \6 I0 z! C1 mThis spectrometer is typically tuned to the deuterium NMR resonance' {3 x& H# F1 O/ q7 U4 o- w
frequency. It constantly monitors the resonance frequency of the
1 r' M2 g4 V* C9 F" qdeuterium signal and makes minor changes in the Bo magnetic field to keep the x8 h- S9 e& d4 T8 K" m1 C
resonance frequency constant. The deuterium signal comes from the
$ ]5 I! X7 ]' Xdeuterium solvent used to prepare the sample. The animation window
, P5 U$ H+ B! @" {contains plots of the deuterium resonance lock frequency, the small
& r3 H" M a# `: S$ @7 X. padditional magnetic field used to correct the lock frequency, and the& N4 [) p8 R$ U3 J# [$ J
resultant Bo# Y/ P Y( |6 M! b$ C* @
field as a function of time while the magnetic field is drifting. The
& p7 H6 i) S! O3 Glock frequency plot displays the frequency without correction. In. m/ l" Z, ~$ i0 B. N" F
reality, this frequency would be kept constant by the application of; h& ]% a3 N# z; i& ?6 b) I/ G8 ~
the lock field which offsets the drift.
1 {) k! t H7 X9 l v3 [9 ?8 v% P2 |: N+ z0 \$ P8 c
. i, m0 r/ q I9 ]. _+ L. n" }& S
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
7 A+ E- Z6 _ W! p' S* Nreference. The resonance frequency of the deuterium signal in many lock
' T" \: T! v6 hsolvents is well known. Therefore the difference in resonance frequency
; \6 A& D0 O9 Z& M) lof the lock solvent and TMS is also known. As a consequence, TMS does# q0 ^! h: C! Q& P, \
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate , f/ y# o$ ~" z( C3 Z
=0.
| 
发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
