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 spectrum9 j* u: N; ]- g2 z9 s" a" e0 a, D* p+ E
of a sample, especially one which requires signal averaging or phase1 S3 e: y- x; A+ C' t
cycling, you need to have a temporally constant and spatially0 P+ r9 `! k* e, Q
homogeneous magnetic field. Consistency of the Bo" R% q+ G* r ^1 R. K
field over time will be discussed here; homogeneity will be discussed
* W+ a3 d( i- Q" ]in the next section of this chapter. The field strength might vary over. P* I9 Q5 z* E' P+ C! Q
time due to aging of the magnet, movement of metal objects near the
1 b( n1 m; D; k$ nmagnet, and temperature fluctuations. Here is an example of a one line
0 B6 m3 L. f1 y% M' u7 cNMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. : w: Y: I" n! w9 V5 m
The field lock can compensate for these variations.
; I# m$ v0 r3 g/ T v2 k# vThe field lock is a separate NMR spectrometer within your spectrometer.
& \/ w Z, }8 t1 X' V1 KThis spectrometer is typically tuned to the deuterium NMR resonance( l3 U$ F) a0 ?* ?2 w
frequency. It constantly monitors the resonance frequency of the
* \: r- B5 N. G6 r6 edeuterium signal and makes minor changes in the Bo magnetic field to keep the7 {4 S) _% m, s% H+ e
resonance frequency constant. The deuterium signal comes from the
/ k1 V0 x+ u2 B9 y$ Cdeuterium solvent used to prepare the sample. The animation window 0 b( g, Q3 k8 s
contains plots of the deuterium resonance lock frequency, the small! H v% T: ~' S$ ]2 \+ d
additional magnetic field used to correct the lock frequency, and the
6 ?5 _" r/ Q6 |5 ]resultant Bo
3 C9 i/ b# U9 [. d( ^6 z+ Gfield as a function of time while the magnetic field is drifting. The& q2 @* u" @1 b% O0 s. v, x
lock frequency plot displays the frequency without correction. In) \0 m* S- }' m) u/ G% e% q7 c0 f
reality, this frequency would be kept constant by the application of9 E- h# ~: e2 i* s1 h
the lock field which offsets the drift.
# C b: O& p! q8 Y
- O( y# S. d) v- b* \. E1 L4 a1 F" z* p9 p& T
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0 T% K- @8 e9 f+ @; |
reference. The resonance frequency of the deuterium signal in many lock" l1 B) t8 n4 K# s0 }# | W
solvents is well known. Therefore the difference in resonance frequency) ~) T5 o7 `0 D( A# j7 M0 B
of the lock solvent and TMS is also known. As a consequence, TMS does
& x4 n& K. q6 _5 R1 `7 \: z, Znot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate ) Y& u S5 L$ G
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
