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
+ n% Z! B; Z# ]3 {of a sample, especially one which requires signal averaging or phase! H, l$ L7 S y( I+ E6 i6 K4 K
cycling, you need to have a temporally constant and spatially
9 h6 D3 O2 `/ T( _. I: khomogeneous magnetic field. Consistency of the Bo
/ g. F8 m4 V, b% X" V) K. W- Xfield over time will be discussed here; homogeneity will be discussed
7 Q) l+ R, c3 v4 _( tin the next section of this chapter. The field strength might vary over$ s) ]6 P2 \( s5 D
time due to aging of the magnet, movement of metal objects near the) P5 D# @' ^. L* F8 f0 i7 c
magnet, and temperature fluctuations. Here is an example of a one line4 m: w* @, f' ?9 x9 ~
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount.
( J$ ~! [0 Q9 B/ {' HThe field lock can compensate for these variations.
: I' W2 \6 J% C3 e' \8 V( n: w
The field lock is a separate NMR spectrometer within your spectrometer.
5 j! K0 U, @; HThis spectrometer is typically tuned to the deuterium NMR resonance% ~3 P1 E: R5 K, X7 L" V2 f) `0 O
frequency. It constantly monitors the resonance frequency of the
& o) O* B! b" l/ i8 B' f+ W" i6 Ydeuterium signal and makes minor changes in the Bo magnetic field to keep the
# u; m. Y, F z Jresonance frequency constant. The deuterium signal comes from the
. W6 z- {! W; J: ^ w# Fdeuterium solvent used to prepare the sample. The animation window & M: b( e% v+ P1 S6 }
contains plots of the deuterium resonance lock frequency, the small
8 u8 H" ]; }/ y, U$ hadditional magnetic field used to correct the lock frequency, and the
6 C2 ~/ g. f" j, k. t/ H" uresultant Bo2 P! N. L7 v# C3 M
field as a function of time while the magnetic field is drifting. The& e' v" N6 R! [" r, G$ A0 H
lock frequency plot displays the frequency without correction. In k9 l# l/ D- P. `. Y2 E, O P4 U
reality, this frequency would be kept constant by the application of! _0 _# q. J, Q( L, v$ D9 T6 C" Q' R
the lock field which offsets the drift.
$ U0 s/ ~, N3 V& O6 O- K+ O; d0 j6 K! p. r& p
4 O0 L; p9 J- r# e! `
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0. h& l7 U, P4 c- w6 q
reference. The resonance frequency of the deuterium signal in many lock* Q; Q7 \$ E, L4 e8 S7 Q
solvents is well known. Therefore the difference in resonance frequency
2 w* V P+ }, F; n$ N- [" ]- lof the lock solvent and TMS is also known. As a consequence, TMS does
. D" N0 h( n1 q% K6 p$ S6 w! }not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate . h2 P8 y7 `" b8 U0 F! Q- ?, f
=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
