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

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PRICE
Table 7 Summary of Gradient Calibration Methods
Method Range of Application ProsCons
Coil calculation Unlimited Generally applicable
Can be complicated to perform
Not very accurate
Echo shape gl2 receiver bandwidth Generally applicable
signal-to-noise Numerous systematic errors
1D Image gl2 receiver bandwidth Generally applicable
signal-to-noise Information on gradient linearity
Gradient pulse mismatch Similar to echo shape Similar to echo shape
Standard sample Need to have a relevant Simple
standard Includes gradient non-ideality
Few suitable and accurate standards
Need accurate temperature control
sion coefficients are listed in Table 8. Apart from
sample-dependent problems, the effects of eddy
currents andor mechanical vibrations, if present,
will result in this method giving only an apparent
calibration. If the sample experimental conditions
Ži.e., sample shape, delays, pulse lengths, gradient
strengths, etc. . are used in a subsequent experiment,
this calibration procedure has the advantage
of automatically including nonideal gradient
behaviour. However, because eddy current effects
increase with gradient strength, a calibration at
one current value cannot be used to determine
the gradient strength at another value of the
applied current. This method of gradient calibration
is further limited by the need to have a
compound containing a nucleus that can be observed
with the probe at hand and with a similar
diffusion coefficient and excellent temperature
control. Clearly, a multinuclear probe gives the
most possibilities. For lower diffusion coefficients,
suitable reference compounds become more
scarce. Glycerol has often been used as a reference,
but its diffusion coefficient is greatly affected
by water content as well as a highly temperature-dependent
diffusion coefficient and T2 Ž 4, 102 . .
Shape Analysis of the Spin-Echo and
One-Dimensional Images
It is possible to calculate the gradient strength
using the echo shape from a sample of known
geometry. This is easy to understand if you consider
that in the absence of a gradient there is no
spatial dependence of the resonance frequency,
but in the presence of a gradient there is a spatial
dependence. Thus, the observed FID and spectrum
will reflect both the gradient and the shape
Table 8 Some Selected Reference Compounds and Their Diffusion Coefficients at 298 K Useful for
Calibrating PFG Experiments
Diffusion 2 1
Ž .
Observe Nucleus Compound Coefficient m s Reference
1 9 H H O 2.30 10 Ž 118, 119.
2
2 2 9 H H O 1.87 10 Ž 120.
2
2 2
9
HO H in H 2O
1.90 10
7 Ž . 10
Li LiCl 0.25 M in H O 9.60 10 Ž 102.
2
13 9 C C H 2.21 10 Ž 81.
6 6
19 9 F C H F 2.40 10 Ž 102.
6 6
21 2 Ž . 9
Ne Ne 4 MPa in H O 4.18 10 Ž 121.
2
23 Ž . 9
Na NaCl 2 M in H O 1.14 10 Ž 122.
2
31 Ž . Ž . 10
P C H P 3 M in C D 3.65 10 Ž 102.
6 5 3 6 6
129 Ž . 9
Xe Xe 3 MPa in H O 1.90 10 Ž 123.
2
133 Ž . 9
Cs CsCl 2 M in H O 1.90 10 Ž 102.
2
A more comprehensive listing can be found in Holz and Weingartner ¨
Ž 102 . .