Pulsed-field gradient nuclear magnetic resonance as a tool for ...

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PRICE
and thus, at t 2qŽ G . , with respect to
the echo center, the phase-twisted echo will cause
a coherent superposition Žwhether
this is before
or after the echo center depends on the sign of
the mismatch . . Since E Ž q,. can be recovered,
0
it is possible to perform signal averaging even
though q and zo may fluctuate between scans.
Obviously, t will vary as q fluctuates; consequently,
the phase-twist analysis is performed after
every scan.
Apart from the signal-to-noise problem, a further
negative aspect of this method is that since
signal acquisition occurs in the presence of a
gradient, this method is not suitable for use with
spectra containing more than one resonance;
however, the serious gradient disturbances that
warrant the use of MASSEY are normally associated
only with measurements of large slowly diffusing
species Ž e.g., polymers . , and so spectral
resolution is less likely to be an issue.
SAMPLE PREPARATION AND
SPECTROMETER SETUP
Sample Preparation
The sample and, of course, the gradient probe
itself should be firmly held inside the magnet with
the sample maintaining a constant position with
respect to the gradient coil former. The sample
should be wholly contained inside the linear region
of the gradient coils, and thus typically the
sample is contained in a volume not more than
1 cm high. Such a sample, though, has large
changes in magnetic susceptibility close to the rf
coils. Accordingly, it is very difficult to achieve
good resolution Ž i.e., difficult to shim . . A solution
is depicted in Fig. 12. This method, compared to
just coaxially inserting a bulb into an NMR tube,
has an advantage in that it is easy to clean the
sample tube or work with viscous substances. It
also gives a precise shape with no meniscus effect.
Recently, two-piece susceptibility-matched microtubes
and inserts have become commercially
available. In passing, we note that instead of
physically restricting the size of the sample, a
slice-selective pulse Ž 55. could be used to restrict
the sample volume. However, this is possible only
on more sophisticated spectrometers, where the
diffusion gradients can be changed independently
of the slice gradient. Furthermore, it should be
noted that selective pulses are not of pure phase
and do not have sharp cutoff frequencies.
Figure 12 Ideally, all of the sample is contained within
the constant region of the magnetic field gradient Žsee
Fig. 3 . . Since the design of gradient coils are normally
limited by the probe dimensions, it is generally necessary
to keep the sample small so as to remain within
the Ž small. volume of constant gradient. A simple solution
is to place the sample in a cylindrical sample tube
and then cap the sample with a vortex plug, ideally of
the same magnetic susceptibility. This tube is then
coaxially inserted into a tube containing either an
NMR inert solvent with a similar magnetic susceptibility
or the same solvent but without the solute of
interest. Thus, the NMR-active part of the sample is
short, whereas the sample is still magnetically long and
allows easier shimming. A further advantage, especially
of this arrangement of the sample, is that it helps to
confine the sample to the region having the most
homogeneous rf.
For very strong Ž in the NMR sense. samples
Že.g., performing a diffusion measurement on pure
water . , radiation damping Ž 56. can be a problem.
Since the severity of the problem is related to the
magnitude of the initial magnetization, one simple
solution is to use a very small samplefor
example, by placing the sample in a small spheri-
Ž .
cal bulb e.g., Wilmad cat. no. 529A .
B Homogeneity and Field-Frequency
0
Locking
If the sample in question has only one or at least
well-separated resonances, a high degree of resolution
is of little consequence as long as there is
sufficient signal-to-noise. If it is necessary to use