12 April 2005— Scientists have used nuclear magnetic resonance for at least 50 years to analyze the chemical structure of materials and more recently to image the soft tissues of the body. It would be an ideal tool for identifying chemicals in the field, especially if you cannot get a sample back to a laboratory, as is the case for robots looking for evidence of life on Mars. The problem is, accurate NMR needs a large, heavy magnet surrounding the material to be examined, something you can't easily transport.
But a team of German and American researchers have now built the first portable resonance sensor with an open-sided magnet. Because of the magnet's U-shape, the device can identify samples that are simply placed next to it rather than inside a barrel-size, cylindrical electromagnet. The unique design will allow NMR to be used in situations where it's not possible to surround the sample, like the analysis of a tree or an artifact at an archeological dig.
"You could walk up to something and hold up your 'tricorder' as they do in Star Trek," says Daniel Weitekamp, an NMR researcher at the California Institute of Technology, in Pasadena. "That's tremendously simpler than if you have to grab the subject and stick it inside the [machine]."
Nuclear magnetic resonance occurs when certain atomic nuclei are placed in a static magnetic field and then exposed to an oscillating field. The nuclei have a property called spin, which makes them behave like tiny magnets and align themselves along the static field. The north-south orientation of these little spin magnets can match that of the field or oppose it, corresponding to a higher or lower energy state, respectively. A separate, oscillating field at a certain frequency causes the spin to flip between the lower and higher energy states.
This "resonance frequency" can be measured because the flipping of the spins creates a varying voltage in the coil, depending on the type of nucleus and the magnetic field the nucleus is experiencing. Analyzing a molecule in this way produces voltage spikes at frequencies corresponding to particular atoms. The relative height of the spikes gives an idea of how many of each atom the sample contains. NMR forms the basis of magnetic resonance imaging, a common medical imaging method used to diagnose muscle and brain diseases. And in molecular biology, NMR has recently overtaken X-ray crystallography as a standard technique for determining the structure of proteins.
Usually, NMR-based instruments are made with cryogenically cooled superconducting electromagnets that can weigh tons. The material sample is placed in the bore of the magnet, where the magnetic field is strong and uniform. A set of wire coils creates the oscillating field that tweaks the sample and picks up its identifying signal.
In the new sensor design, the researchers at the University of California, Berkeley, the RWTH Aachen University in Germany, and the City College of New York in New York City use 30-kilogram a permanent horseshoe-shaped magnet for the static field. This makes the system about the size of a DVD player.
But the magnetic field the horseshoe magnet makes is uneven, and NMR only works if you have a uniform field. If the external field isn't uniform, atoms in the sample experience different fields. "If different parts of the sample have different magnetic fields, what happens is you get a distribution of resonance frequencies. You don't get a single nice frequency anymore," says Vasiliki Demas, a graduate student in chemistry and materials science at UC Berkeley, who led the development of the sensor.
This has been a problem for portable systems designed in the past with permanent magnets. These systems have been used to estimate material structure for ten years, Demas says, but can't identify the chemicals in a sample precisely. "[They] can't actually tell that at this particular point of the sample there is water and at that particular point there is oil or some polymer," she says.
The German and American research teams accurately measured the fluctuations in the static field created by their permanent magnet. Based on this, they designed a coil that creates matching fluctuations in the oscillating field. This removes the relative difference between the two fields, effectively canceling out the static field's irregularity. To improve matching of the two fields, the position of the magnet and the size of coil have to be adjusted delicately.
Demas and her colleagues show that this arrangement can detect fluorine present in several different hydrocarbon molecules. This is "the first spectrum obtained by a single-sided system," Demas points out. Fluorine is relatively easy to detect with the frequency resolution that the portable system can achieve, but picking up fluorine is an esoteric application. "It's harder to think of interesting systems where you would have several different fluorine molecules in them," Weitekamp says.
Hydrogen, because of its abundance, is a more useful element to detect using NMR. The researchers believe that their sensor's resolution can be sharpened enough to detect hydrogen. Once that is done, Weitekamp says, the system could be a real hit with scientists. "There could be killer applications in biology," he says, "but as it stands now you'd have to think harder about what situation you'd have where this kind of resolution would be adequate."