Compensation of Magnetic Field Instabilities in Field Cycling NMR by Reference Deconvolution
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Applied Magnetic Resonance
Compensation of Magnetic Field Instabilities in Field Cycling NMR by Reference Deconvolution Stefan Reutter • Alexei Privalov
Received: 29 June 2012 / Published online: 5 October 2012 Ó Springer-Verlag 2012
Abstract Performing a nuclear magnetic resonance (NMR) experiment in an unstable magnetic field causes fluctuations in the NMR frequency, leading to a loss of reproducibility and an effective shortening of the free induction decay after data averaging. Reference deconvolution allows the compensation of field fluctuations via simultaneous measurement of an internal or external reference signal. The technique was applied to compensate the effect of field fluctuations in a resistive electromagnet used for fast field cycling NMR. An external sample was chosen as the reference.
1 Introduction The external magnetic field used for nuclear magnetic resonance (NMR) experiments is, in general, subject to both spatial and temporal inhomogeneity. While spatial inhomogeneities may be addressed by appropriate shimming, temporal fluctuations of the external field are more difficult to handle and a large number of approaches have been devised to address the problem. In many cases, hardware approaches are a good choice to compensate field fluctuations. For low-frequency fluctuations, such as drifts, a field lock can be used that measures the position of a well-known NMR line (or uses a magnetic field sensor to detect the magnetic field) and feeds it back into a compensation coil [1–3]. For frequencies larger than a few kilohertz, passive shielding similar to a Faraday cage can somewhat alleviate the problem [4, 5], or the field changes can be detected using a pick-up coil and, again, used to control the current through a compensation coil [2, 6]. One of the major sources of field fluctuations in resistive electromagnets is a remnant of the frequency of the mains current, which is, in Europe, 50 Hz. S. Reutter (&) A. Privalov Institut fu¨r Festko¨rperphysik, Technische Universita¨t Darmstadt, Hochschulstr. 6, 64289 Darmstadt, Germany e-mail: [email protected]
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Fig. 1 Hundred accumulations of a simulated exponential decay from the quadrature signal (lower curve) and from its absolute value (upper curve)
Depending on the power source driven by this current, field fluctuations at 50 and 300 Hz can occur. This effect, sometimes referred to as ‘‘mains ripple’’, can be compensated either by a similar set-up as above, using a coil that generates a small additional field [6] anti-phasic to the effect of the ripple, or the experiment can be at least triggered to this frequency to improve the reproducibility of the experiment [7]. Another family of approaches relies on first detecting a ‘‘faulty’’ signal in an unstable field and correcting the NMR signal’s phase via comparing with a simultaneous measurement of a reference affected by the same field fluctuations. The most widespread of these approaches is generally ref
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