Palm-sized magnet

German researchers have developed a light, permanent magnet that is suitable for NMR and fits in the palm of your hand. Writing in the journal Angewandte Chemie, they say it could be used for portable, high-resolution NMR instruments.

spectroscopyNOW readers will be well aware that high-resolution nuclear magnetic resonance spectroscopy is one of the most powerful analytical tools for investigating molecular structures and dynamics with great precision. The modern technique relies on very strong magnetic fields generated by superconducting electromagnets, which are very different from the much weaker permanent magnets used in the NMR spectrometers of the 1960s and 1970s. However, superconducting electromagnets are not insubstantial and so high precision comes at the cost of portability and low cost. Moreover, if the old Varian T-60 magnet, with a volume of about 1 cubic metre is scaled down to a palm-sized magnet, the sensitivity loss is about three orders of magnitude.

Now, Federico Casanova and his colleagues, Ernesto Danieli, Juan Perlo, and Bernhard Bluemich, at the Rheinisch-Westfaelische Technische Hochschule Aachen RWTH Aachen, Germany, have now developed a light, permanent magnet that is suitable for NMR and yet fits in the palm of your hand. The team suggests that the new magnets could lead to entirely portable, yet high-resolution NMR instrumentation.

Casanova explains that theoretically it should be possible to obtain NMR spectra that have sensitivity of approximately one third that possible with standard-sized superconducting magnets. "This would be an acceptable concession for a small and portable NMR system," he says. "However, there is one problem: As the magnet gets smaller, the dimensions of the homogenous (uniform) magnetic field also decrease, making the sample volume smaller. Reduction of the sample volume affects the signal-to-noise ratio."

With this problem in mind the RWTH Aachen team has now developed a small permanent magnet that weights just half a kilogram that nevertheless has an exceptionally homogenous magnetic field, which precludes the need for small samples and so can accommodate a standard-sized tube for proton NMR spectroscopy.

The new magnet is based on a so-called Halbach array, which is essentially an array of individual magnetic blocks assembled into a cylinder so that the direction of their magnetization is tuned to produce a homogenous field within. By connecting three Halbach rings of appropriate diameter, the team can optimized the field and compensate for the distortion at the ends of the cylindrical inner chamber. To further smooth out any inhomogeneities due to the granularity of the magnetic material itself, the team constructs each ring from trapezoidal magnetic blocks with gaps in between. In the gaps they include rectangular magnetic blocks that can be displaced radially to mechanically adjust, or shim, the magnetic field. The shim magnets can correct for inhomogeneities of the order of 20,000 ppm, the team explains, that limit is wholly unreachable with electrical shims because of the excessive current strengths required to make such an adjustment.

"Spectra we obtained [for water and toluene] show that our miniature magnet is suitable for high-resolution NMR spectroscopy with standard-sized sample tubes," reports Casanova. "It would be easy to transport together with the spectrometer. This could allow high-resolution NMR spectroscopy to develop into a portable analytical technique for use on samples in the field."

The team is now working on further improvements to make stronger and even more homogenous fields and to improve thermal stability. They point out that magnets made from SmCo are 1.5 T and corresponding to a proton frequency of 60 MHz can be obtained with an outer diameter of 75 mm. These magnets have better thermal stability than NdFeB magnets but field drifts arising from fluctuations of the magnet temperature must be eliminated. Whereas NdFeB magnets could produce field strengths as high as 2 T, which the team says is the likely upper field limit for this type of array.

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