Probing chemical signatures in an earthy way

Expensive superconducting magnets are central to nuclear magnetic resonance (NMR) technology, one of chemistry’s workhorse methods for determining molecular structure and composition. Now, scientists in Germany have performed NMR analysis using the biggest and cheapest magnet of all—Earth.

The technique could lead to low-cost, portable NMR scanners useful to many industries, including oil exploration, food processing, and drug manufacturing, says Stephan Appelt of the Scientific Center of Jülich, who is developing the approach with his colleagues.

NMR works because many atomic nuclei behave as tiny bar magnets do. Scientists first place their sample in a uniform magnetic field that aligns the so-called spins of the nuclei. Then, the material is zapped with pulses of radio waves. Those bursts induce the spins to wobble, which causes the nuclei to emit their own radio signals. Because of the high strength of its magnetic field, a typical NMR machine can discern slight variations in signal frequencies to determine the kinds and locations of atoms in the sample (SN: 4/9/05, p. 229: Available to subscribers at Open Sesame: Portable devices may achieve magnetic resonance views).

In the May 20 Physical Review Letters, Appelt and his coworkers at Jülich and RWTH Aachen University describe a different tack. Instead of exciting nuclei in the sample material, they combine the sample with the gas xenon and measure the wobbles from the xenon.

Using a laser, they first efficiently align the nuclear spins in xenon. Rather than aligning one spin in a million as standard NMR does, this processes aligns the spins of almost one in three xenon nuclei. When the researchers then flow the xenon atoms over a sample, the sample molecules alter the xenon nuclei’s specific emissions. Because there are so many aligned nuclei, even Earth’s magnetic field can elicit detectable radio signals. The researchers have demonstrated the principle of the technique by using it to identify the organic solvents toluene and ethanol.