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 spectrum0 n9 f p$ }' I, b6 O
of a sample, especially one which requires signal averaging or phase [( ^9 r% Z% b, J& n
cycling, you need to have a temporally constant and spatially
7 A" T; N5 f$ A6 I: jhomogeneous magnetic field. Consistency of the Bo
6 W8 }7 A d2 ?6 Z9 A, Kfield over time will be discussed here; homogeneity will be discussed
! ]2 `1 Z9 f1 P: g y" j' a8 zin the next section of this chapter. The field strength might vary over8 K, P( j: b/ Q' T4 K0 j+ I! m7 b* E
time due to aging of the magnet, movement of metal objects near the& Q8 }+ r! g+ n4 V* ~
magnet, and temperature fluctuations. Here is an example of a one line1 k- I3 h3 ^1 x* [; ]
NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. / w1 _) S# f% Q4 @
The field lock can compensate for these variations.
1 [: V' _$ U; m3 B1 A
The field lock is a separate NMR spectrometer within your spectrometer., B2 W% E& _9 d# y! P* O7 h7 k6 w
This spectrometer is typically tuned to the deuterium NMR resonance. d: m. m }8 C" N" c% N5 B0 i1 p
frequency. It constantly monitors the resonance frequency of the
& K( J: e7 c4 B8 l5 n4 B1 @deuterium signal and makes minor changes in the Bo magnetic field to keep the" ]6 c- D5 L: s6 X! i" z- b' d% c
resonance frequency constant. The deuterium signal comes from the; n( ]3 R7 ?- N3 f5 _! \& E: F4 [
deuterium solvent used to prepare the sample. The animation window 1 \7 g, h. V! e9 A( {* z
contains plots of the deuterium resonance lock frequency, the small: n0 ~# c9 e6 E) |# q5 i- {! l+ `, M
additional magnetic field used to correct the lock frequency, and the# Q# A0 u" z+ m" D2 Y A
resultant Bo7 |5 B8 ^7 h* W z9 R Y* p
field as a function of time while the magnetic field is drifting. The
# V. x9 p9 V7 |$ c: o8 S* @; @lock frequency plot displays the frequency without correction. In
1 P$ ^- B8 n/ D r! X$ Z8 Areality, this frequency would be kept constant by the application of+ j9 I0 k$ B6 {' R: r h/ ~5 \
the lock field which offsets the drift.
6 a! \% _ j2 B4 |" R3 m _6 D
% O* t y2 f% {% t+ @6 q2 ~) u9 n# T. z9 h% a$ ^
On most NMR spectrometers the deuterium lock serves a second function. It provides the  =0
( v. ^9 B* S# P* ?reference. The resonance frequency of the deuterium signal in many lock
5 X# Y. F# {) F8 d# f" N( @+ R' Zsolvents is well known. Therefore the difference in resonance frequency8 ?" b4 w; O7 Z( O% j# B- Y9 j
of the lock solvent and TMS is also known. As a consequence, TMS does
9 d3 a2 l. | v- n4 {not need to be added to the sample to set =0; the spectrometer can use the lock frequency to calculate
& A. w: l0 Z% \, L, v# T7 F=0.
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发表于 2009-8-18 08:48:16
发表于 2009-8-18 17:25:58
