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) \7 L/ m. l; U( M; L
of a sample, especially one which requires signal averaging or phase
3 K, u1 f* Y& Z* O& p0 ]2 Hcycling, you need to have a temporally constant and spatially/ R. v9 C3 a# H1 ^
homogeneous magnetic field. Consistency of the Bo
# w7 g6 K6 k+ g2 x1 `& ?field over time will be discussed here; homogeneity will be discussed; Z+ M5 e6 p6 B1 y" B- l( B' y
in the next section of this chapter. The field strength might vary over
- X& u n$ i. d$ Z( l1 ?' C ^* j$ |time due to aging of the magnet, movement of metal objects near the
$ _ S |" Q$ V# o' O) L$ R3 v+ Smagnet, and temperature fluctuations. Here is an example of a one line
' e* d. f2 c2 P9 j' a" P# PNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
X0 p5 I" {8 {The field lock can compensate for these variations.
8 q, A. @: I0 [' h- c0 X& \9 \
The field lock is a separate NMR spectrometer within your spectrometer.
7 u) |( [: b' n$ ]This spectrometer is typically tuned to the deuterium NMR resonance* u# E2 r9 R8 F. R1 `+ a2 l
frequency. It constantly monitors the resonance frequency of the
* N( U3 L' Y2 s( c8 sdeuterium signal and makes minor changes in the Bo magnetic field to keep the
: R1 Z! p6 \' tresonance frequency constant. The deuterium signal comes from the
, Z9 s2 O9 v# \. mdeuterium solvent used to prepare the sample. The animation window / O# l) r" [; w% i( X
contains plots of the deuterium resonance lock frequency, the small& l! V% w* S) x+ l
additional magnetic field used to correct the lock frequency, and the
6 @9 @' |1 T) O) f# N6 \2 Wresultant Bo! g6 }( m' P$ Q0 H, g' M" d8 F
field as a function of time while the magnetic field is drifting. The; F1 Z1 Y4 l5 b# b! M4 n# a- _
lock frequency plot displays the frequency without correction. In: s' E0 _9 u. y# k
reality, this frequency would be kept constant by the application of5 R3 l1 b$ S. n& e
the lock field which offsets the drift.
+ i) M7 z/ p1 o `! T, ^/ ^' i3 ~. h/ `5 j$ J2 w
6 p3 o/ p1 f3 N7 O! j
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
6 T7 m' ~9 T0 `; l5 G7 [/ z" breference. The resonance frequency of the deuterium signal in many lock1 n+ m9 p5 \) B4 l6 T
solvents is well known. Therefore the difference in resonance frequency
' Q+ P; C. y5 @5 a7 yof the lock solvent and TMS is also known. As a consequence, TMS does
! z& \! s! c! r- Y: Enot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
. b/ Y) e. B$ ^4 B4 @( Q' k( `/ Z=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
