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 V: s6 _+ r6 a. ?$ A! eof a sample, especially one which requires signal averaging or phase6 b1 S) X# {4 m, @" K( P
cycling, you need to have a temporally constant and spatially" c; P5 p2 T, F- A) s
homogeneous magnetic field. Consistency of the Bo! F P* B8 w7 y; O
field over time will be discussed here; homogeneity will be discussed
' o) ^$ {5 T" ^ ]% h& N! I( ein the next section of this chapter. The field strength might vary over# s9 n8 D% l( q4 s E+ v
time due to aging of the magnet, movement of metal objects near the
/ C/ r9 A0 d% lmagnet, and temperature fluctuations. Here is an example of a one line% D' b) R* d7 }" R/ n# p3 E
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. + @6 C2 T5 e7 ]! m: T
The field lock can compensate for these variations.
: p% D" x: l+ yThe field lock is a separate NMR spectrometer within your spectrometer.' x3 T/ ?4 x& s7 n7 j# l
This spectrometer is typically tuned to the deuterium NMR resonance
1 ]+ |. _# a* Afrequency. It constantly monitors the resonance frequency of the2 C- _, W2 X/ q
deuterium signal and makes minor changes in the Bo magnetic field to keep the+ e" W' `7 |0 t7 O) e. C2 b
resonance frequency constant. The deuterium signal comes from the
5 ?, h, K' `; }1 F+ W* @deuterium solvent used to prepare the sample. The animation window 5 p! v2 ]0 W" a
contains plots of the deuterium resonance lock frequency, the small
9 y5 s) Q% z5 v. k% @) @6 C radditional magnetic field used to correct the lock frequency, and the/ b6 a. Y5 Y# r% t7 F- Q) R: v
resultant Bo1 Y* z0 Y7 W& e$ P1 R7 [9 W
field as a function of time while the magnetic field is drifting. The% C% A0 }5 A' n/ ] J% ?! ]4 x. t
lock frequency plot displays the frequency without correction. In
% [0 ]1 ^' x4 ^( h% N; I8 ]reality, this frequency would be kept constant by the application of
& T, `5 r3 K4 Q! Z4 m8 Dthe lock field which offsets the drift.
8 s) @1 \, S- R; X% ~" @
. i) K* Y4 }0 O- O1 _: c
) B) |+ N! y0 Q! `; v& n" n+ _On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
( C* A( ]9 `2 x% areference. The resonance frequency of the deuterium signal in many lock
* a! E) c8 P& ~* u9 K# f6 |( xsolvents is well known. Therefore the difference in resonance frequency& o5 a p: I! s1 h' v/ P# l1 F0 n
of the lock solvent and TMS is also known. As a consequence, TMS does/ `9 d5 V! t! E/ Y* }, j5 d6 c) `# i. s1 X
not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate , C& n+ u3 G j+ ?. c8 W; A
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
