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Palm-Size NMR

The portable but powerful magnet could be used to find archaeological artifacts or to detect contamination in products.
June 10, 2010

Room-sized nuclear magnetic resonance machines might shrink to handheld, portable devices thanks to a small, lightweight magnet design developed by German researchers.

Pocket NMR: A specially designed battery-sized permanent magnet generates a magnetic field that’s uniform enough to perform high-resolution NMR on chemical samples in a commercial NMR tube.

Nuclear magnetic resonance spectroscopy is a common tool for studying the structure of proteins and identifying the chemical composition of a material. It also forms the basis of the medical imaging technique magnetic resonance imaging, or MRI. However, bulky and expensive superconducting magnets are used to generate the strong magnetic fields (about seven tesla) needed for precision NMR.

The magnet, developed by Federico Casanova and his colleagues at the RWTH Aachen University’s department of macromolecular chemistry, is about the size of a standard D battery and weighs 500 grams. While portable magnets have been made before, the new one enables NMR measurements that are just as precise as the large commercial magnets. “This is a significant additional step toward mobile high-resolution NMR,” says Alexander Pines, a chemistry professor at the University of California, Berkeley, who is developing a new type of compact MRI designs.

As the size of a permanent magnet shrinks, it generates magnetic fields that are uniform over a smaller volume because of tiny imperfections in its material and shape. This means less of a material sample can be used, making the NMR measurements almost a thousand times less sensitive than if a superconducting magnet were used. The NMR signal then becomes comparable to the electronic noise, and the device can miss chemicals that are present in very small quantities.

The new magnet generates a 0.7 tesla magnetic field, but it generates an extremely homogenous field. As a result, it is the first portable magnet that works with the conventional five-millimeter tubes in which NMR samples are placed. “The goal of our work was to take this tube, keep the volume constant, and build the smallest magnet with the desired homogeneity,” Casanova says. “The important thing we did is to correct the inhomogeneity that comes from imperfections in the magnet.”

Calling the results impressive, Louis Bouchard, a University of California Los Angeles chemistry professor, says that no previous portable magnet design has achieved such good performance. Bouchard believes the cost of the magnet should be much lower than that of present-day commercial NMR magnets. “This will likely lead to such NMR units being much more widespread,” he says. “If these guys sold this product commercially, I would probably buy one.”

The portable magnet could make possible sensitive, high-resolution NMR devices that can be taken to an archaeological dig to identify artifacts and to a factory to detect contamination in products. It could be used in doctors’ offices to spot blood clots, bacteria, or cancer proteins in a patient’s blood. It could also allow portable NMR machines to monitor the production of drugs and chemicals in-line instead of taking chemical samples to NMR labs for analysis.

Casanova and his colleagues have tweaked a well-known magnet design known as a Halbach array, a special arrangement of many permanent magnets that focuses magnetic fields only on one side of the array. One common design is a Halbach cylinder, which has an intense magnetic field inside the cylinder. This is what the researchers start with. As they describe in a paper posted online in the journal Angewandte Chemie, they first stack three rings of samarium cobalt to make the cylinder. The cylinder’s outside diameter is 35 millimeters; the inside diameter of 15 millimeters is large enough to hold a standard NMR tube.

Each magnet ring is made of trapezoid-shaped pieces with gaps in between. These gaps are filled with rectangular pieces that move in and out by up to two millimeters. The researchers measure the inhomogeneity in the magnetic field created by the Halbach rings. Then, with the aid of sophisticated computer simulations, they calculate how much they need to move each of the rectangular pieces to adjust the magnetic field and smooth out inhomogeneities.

Even better magnets might be possible by fine-tuning the design, the researchers say. While the magnet’s field strength is 0.7 tesla right now, increasing the outer diameter of the magnet should make it possible to generate 1.5 tesla, the researchers say. What is more, using magnets made of other materials such as neodymium, as much as two tesla could be generated.

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