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Against the trend: NMR measurements in low fields

Today ever stronger static magnetic fields are used to increase the spectral resolution of nuclear magnetic resonance measurements (NMR measurements). It is, however, also possible to follow an opposite approach and to record NMR spectra in very low magnetic fields with superconducting quantum interferometers (SQUIDs) with excellent spectral resolution. At PTB, a low-field NMR spectrometer has been developed which achieves a resolution of far below 1 Hz in the new magnetically shielded BMSR-2 room. This opens up the possibility of performing improved and novel NMR measurements.

Measured NMR spectrum of distilled water, detection field: 900 nT. Contrary to high detection fields, the natural line width of about 0.15 Hz is broadened only insignificantly by the measuring device.

At first it seems surprising that at very low fields of a few micro- or even nanotesla flux density, a spectral resolution can be achieved which by far exceeds that of conventional high-field spectrometers. The reason is quite simple: The resonance lines are always broadened by field inhomogeneities which scale with the field strength. This is why, for example, recording the natural line widths of benzene, chloroform or distilled water around 0.1 Hz becomes possible for magnetic flux densities between 40 nT and 4 mT. As chemical shift displacement also disappears in the low-field range, the spectra of more complex molecules are simplified. In cooperation with the universities of Magdeburg and Jena, broad-band “low field” NMR spectra were measured and numerically simulated for a series of molecular systems which contain different nuclear moments. It turned out that the resolution of so-called hetero-nuclear J couplings was limited only by the natural widths of the resonance lines. The higher resolving capacity of the new technique thus allows a better analysis of reactions in which different nuclei are involved.

On the basis of these results it can be expected that in the low-field range, also MR imaging in the field of medicine will become possible. The magnetic fields produced naturally in the body and generated by bioelectric currents in the heart or in the brain are of the same order of magnitude and they can be recorded simultaneously by SQUIDs. This opens up the interesting new possibility of investigating the function and anatomy of organs with one single measurement.

Contact at PTB:

Phone: +49-531-592-0