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
! R0 p% u2 b4 u) F; tof a sample, especially one which requires signal averaging or phase
) ?* ]) x+ h& Z8 P6 J6 Ocycling, you need to have a temporally constant and spatially
R$ X! @0 N: k0 n1 z; Hhomogeneous magnetic field. Consistency of the Bo
3 e2 X! H* p* g9 C5 vfield over time will be discussed here; homogeneity will be discussed% V3 ?7 U( Z, |
in the next section of this chapter. The field strength might vary over
$ w4 P- e1 D3 }8 ?( Z2 R- @time due to aging of the magnet, movement of metal objects near the- S: T# I- S( p/ a( Y U
magnet, and temperature fluctuations. Here is an example of a one line
3 g5 Z7 U" ^( ?* }6 qNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. & \$ b3 f- ]7 w6 t$ `7 H9 Y, M$ R
The field lock can compensate for these variations.
0 [8 g" D7 p" o; C( S' \" g H
The field lock is a separate NMR spectrometer within your spectrometer.
7 n; i# F2 Y& w( u& u+ pThis spectrometer is typically tuned to the deuterium NMR resonance C. I$ g8 ~: N+ [+ J# c
frequency. It constantly monitors the resonance frequency of the
2 B0 G, m# v5 Q& wdeuterium signal and makes minor changes in the Bo magnetic field to keep the E! k9 a3 P) s2 p3 u t
resonance frequency constant. The deuterium signal comes from the( B1 z6 s# |, e) l# W
deuterium solvent used to prepare the sample. The animation window
7 U; r8 a$ ?% y" rcontains plots of the deuterium resonance lock frequency, the small1 z, _8 f' S3 k8 _# l% e- ]
additional magnetic field used to correct the lock frequency, and the0 q p' y2 X; X2 v" b
resultant Bo7 W4 O0 O6 \+ ~1 O$ u q8 d
field as a function of time while the magnetic field is drifting. The
+ V7 h$ x5 ~' j4 f2 Y+ mlock frequency plot displays the frequency without correction. In/ D" _ ]9 N4 u* z# q
reality, this frequency would be kept constant by the application of
' P0 @' L# }4 W; R$ Hthe lock field which offsets the drift.
n6 z3 n0 r# d2 ^2 M; V) n
" J5 y5 X8 \ M' c/ i& L
! x: l+ r! W1 B- E4 F+ Q6 [) m9 h3 pOn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0& S% c* G! A5 D9 f/ ?& r7 I# k0 N$ v
reference. The resonance frequency of the deuterium signal in many lock* y' ` J8 d8 ]. W
solvents is well known. Therefore the difference in resonance frequency4 ~7 Y* h, [1 _, m. G% u
of the lock solvent and TMS is also known. As a consequence, TMS does4 n0 T3 r$ P! j1 @* b9 k+ H
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
+ c# z, Z0 Z2 ?; r+ Y! l=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
