Abstract
The main objective of field-cycling experiments [231, 367] is to obtain information on the spectral density of the fluctuating spin interactions in a frequency range as wide as possible.1 Typical examples are discussed in Chaps. 14 and 16 (see Figs. 12.1 and 14.2, for instance). The origin of the fluctuations is normally molecular dynamics so that these can be characterized in a direct and quantitative way.
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Notes
There are other variants of field-cycling experiments serving nuclear quadrupole resonance (NQR) spectroscopy [39, 135, 234], or zero-field spectroscopy [513, 529]. A promising biomedical variant of field-cycling experiments is field-cycled proton-electron double-resonance imaging (FC-PEDRI) which was developed to allow the distribution of free radicals in aqueous environments to be imaged via NMR signals [302, 303].
The RF pulse homogeneity and the pulse sequence employed for the signal detection are uncritical because the signal is proportional to the transverse magnetization anyway.
From a practical point of view, “field-cycling” means “field-energy-cycling.” Fast switching-up requires a device supplying a high peak power, fast switching-down needs a method for actively transferring the field energy away. Both requirements are best met by switching precharged high-voltage capacitors into the magnet circuit with a polarity matching the desired field change.
Note that phase-sensitive detection as required for signal accumulation stipulates a particularly good stability.
Zero-field NMR or NQR can also be performed by adiabatically switching off the external field and applying “90° pulses” at the beginning and at the end of the zero-field interval. A 90° pulse in the absence of the external field consists of a d.c. field pulse with zero carrier frequency, of course [342, 343].
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© 1997 Springer-Verlag Berlin Heidelberg
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Kimmich, R. (1997). Field-Cycling NMR Relaxometry. In: NMR. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-60582-6_15
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DOI: https://doi.org/10.1007/978-3-642-60582-6_15
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