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
- N0 |' ~& ~" }' dof a sample, especially one which requires signal averaging or phase
2 R# ~/ c+ ~* B) y! Wcycling, you need to have a temporally constant and spatially8 S3 Z/ G2 @6 l+ t/ N; r5 \' I! L
homogeneous magnetic field. Consistency of the Bo
/ K2 R5 e& d) Q/ f: Jfield over time will be discussed here; homogeneity will be discussed! K- v% O# ?; t3 L, Y/ B
in the next section of this chapter. The field strength might vary over* K. l, h# k) T
time due to aging of the magnet, movement of metal objects near the
. s% K$ H0 T' r, _/ \magnet, and temperature fluctuations. Here is an example of a one line
/ J0 {8 a+ u; E0 w; R: ONMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
/ P- Z+ F0 E6 ^' J4 mThe field lock can compensate for these variations.
) q' U b3 r# n2 MThe field lock is a separate NMR spectrometer within your spectrometer.
/ _. D6 ~! G1 C2 ^* X9 S4 UThis spectrometer is typically tuned to the deuterium NMR resonance! o6 v* Q8 K6 o; X5 {2 V; r
frequency. It constantly monitors the resonance frequency of the2 O6 a$ y' ~7 L3 ~
deuterium signal and makes minor changes in the Bo magnetic field to keep the
@- p' M: l* ]0 Eresonance frequency constant. The deuterium signal comes from the
3 j, l6 ~7 b: Y7 @& Fdeuterium solvent used to prepare the sample. The animation window * T! a/ N' f: F5 v9 Q% ^7 m- N
contains plots of the deuterium resonance lock frequency, the small7 T! {8 g. H0 z# Y/ u5 L
additional magnetic field used to correct the lock frequency, and the
/ o; d1 e$ m2 f' x( Y: ]resultant Bo
) q! b' R1 O9 c; [* Wfield as a function of time while the magnetic field is drifting. The+ y: x d3 {) e# ?+ F. j9 p
lock frequency plot displays the frequency without correction. In
& C: A; h- M' ireality, this frequency would be kept constant by the application of, _5 [+ g! D: o
the lock field which offsets the drift.
7 j$ H' |! u5 H) z
- E! j! ~" F. P; [1 Y0 t
) ~( Z8 r3 T% K5 N4 |( POn most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
. {2 Z$ F# C( sreference. The resonance frequency of the deuterium signal in many lock! R3 c/ r7 k8 Q
solvents is well known. Therefore the difference in resonance frequency
5 W1 b+ R6 ~0 }$ _. e) n7 G( ]of the lock solvent and TMS is also known. As a consequence, TMS does
; ?# v' N2 s' O0 C* @$ Anot need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
9 \) y& R& o; |3 ?5 S0 e, g" g=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
