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 spectrum8 [& ? ~, ^3 X
of a sample, especially one which requires signal averaging or phase# s8 D! \& e# u2 E( D1 r" J' W
cycling, you need to have a temporally constant and spatially
' n& r& o; d: B* z! t( Thomogeneous magnetic field. Consistency of the Bo
8 s+ {) w& G% k0 |/ D7 Hfield over time will be discussed here; homogeneity will be discussed$ {" U+ M7 _3 l: ]
in the next section of this chapter. The field strength might vary over
! A1 ^: p0 G* A* v& l- m5 ^/ C* j1 N$ Atime due to aging of the magnet, movement of metal objects near the
% X$ @2 f# w9 u/ ]$ S, x' r% X/ lmagnet, and temperature fluctuations. Here is an example of a one line$ Y. V! ?9 y6 I1 x, C, |$ G- {
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
8 b5 l' A! x2 [The field lock can compensate for these variations.
9 I. B( b+ C B+ `$ {' v4 u1 g
The field lock is a separate NMR spectrometer within your spectrometer.5 _# ^2 U# s0 x2 _$ a$ p
This spectrometer is typically tuned to the deuterium NMR resonance( P* q* a* e/ i" K) W
frequency. It constantly monitors the resonance frequency of the ~( Q8 c% w& f( T7 P5 z/ x! f' Y
deuterium signal and makes minor changes in the Bo magnetic field to keep the" a+ j( a) E6 B v* B
resonance frequency constant. The deuterium signal comes from the
$ L2 C# k! }& h" ?4 k- q" N: Ydeuterium solvent used to prepare the sample. The animation window
) M3 w2 y) h0 O# }$ F# u* f: Zcontains plots of the deuterium resonance lock frequency, the small6 C+ b( ]6 j) A7 o% |/ Y3 w
additional magnetic field used to correct the lock frequency, and the
8 Y' d4 j. g( P" V1 x uresultant Bo
! J9 z1 i8 y6 F3 u* @6 l0 Ffield as a function of time while the magnetic field is drifting. The
. t8 E3 d$ R0 g$ ^6 w* Alock frequency plot displays the frequency without correction. In
' {, f+ A7 g% k. c1 Freality, this frequency would be kept constant by the application of; g" [3 \/ }7 m3 f
the lock field which offsets the drift.
# s, ^8 D1 T, c& N4 s
3 y, F* d8 B, C3 K
: u2 @8 P# r2 {9 B8 @8 c. N. {On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
2 Z" c! V$ r) O+ p S9 I( `# dreference. The resonance frequency of the deuterium signal in many lock
' {0 V7 @$ o, W# Q% csolvents is well known. Therefore the difference in resonance frequency1 w3 Y1 b' ^/ D* U+ Q8 d
of the lock solvent and TMS is also known. As a consequence, TMS does
8 H' D% ^: r5 B% o2 Nnot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate , s( t! S! d. b% K8 t( z ?
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
