i n s t ru m e n t a l s
NMR in the woods A
COURTESY OF STEPHAN APPELT
and cyclotetrasiloxane. Each mollthough most NMR spectrosecule showed a unique spectrum copists are confined to laboNMR probe and J-coupling pattern—distincratories, Stephan Appelt and coltions that would have been nearly leagues at the Central Institute for impossible to observe with a highElectronics and the Institute of field magnet. Furthermore, the Technical and Macromolecular silicone oil showed two sets of JChemistry (both in Germany) Spectrometer electronics coupled peaks due to the differtook their homemade NMR specence in chemical structure betrometer deep into a forest outside tween the interior O–SiMe2 and Jülich, Germany, and got to work. Thanks to an external prepolarizaterminal O– SiMe3 groups. The tion device, the new instrument relative intensities of the signals achieved a level of simplicity and provided insight into the degree utility greater than that demonof branching. Halbach magnet with sample strated by previous earth’s field Another measurable parameter NMR spectrometers (Nat. Phys. in earth’s field NMR is relaxation Researchers have developed an improved earth’s field NMR 2006, 2, 105–109). time. Because of its inherently low spectrometer with a Halbach magnet. Because the earth’s magnetic signal, lithium is particularly chal“beautiful idea, because you can actually field is too weak (2.5–7.5 10–5 T) lenging to study. The researchers successdesign independently and physically sep- fully detected lithium for the first time to adequately polarize spins for NMR, arate out the different parts of the specby earth’s field NMR. They showed that researchers must prepolarize spins to trometer system.” lithium’s relaxation time is highly sensiproduce a measurable signal. Previous “We call it ultrahigh resolution betive to temperature and concentration. earth’s field spectrometers contained cause its resolution is better than the They propose that earth’s field NMR electromagnetic coils within the probe, best superconducting magnets,” says could be used to generate a map of in addition to rf coils. To increase the Appelt. Line widths of ~0.03 Hz are neuron channels in the brain: Researchsimplicity and mobility of their instruachieved, allowing measurements with ers could inject lithium into the human ment, Appelt and colleagues used a an accuracy 10–100 greater than in body and directly monitor changes in its Halbach magnet to prepolarize the conventional NMR. However, NMR relaxation time as a function of location. nuclear spins. with earth’s magnetic field can’t resolve “It’s a very interesting suggestion,” reThe Halbach magnet is composed of spins from a single nuclear type, so promarks William Happer at Princeton Unieight permanently magnetized sections versity, “but they would need several orof a hollow cylinder. Within the cylinder, tons A and B cannot be differentiated. Heteronuclear J couplings, which are ders of magnitude greater polarization.” the magnetic fields are additive, and this produced by electronic interactions and The principal limitation with earth’s provides a 1-T field. However, outside manifested as peak splitting, are indefield spectroscopy is line broadening of the cylinder, each magnet cancels the due to stray fields. Appelt and colother magnets’ fields, and this eliminates pendent of field strength and are mealeagues performed their experiments in any stray fields that would broaden spec- surable with the technique. J couplings a forest because destructive stray magtral lines. Samples are quickly transferred vary as a function of the number of intervening bonds between coupled pairs, netic fields are present in and around (in 1–2 s) from the Halbach magnet to type of nuclei, bond orientation, and human environments. “For use indoors, the NMR probe for excitation and debond strength. J coupling is thus a usewe need to develop ways to shield tection. Joe Seymour, at Montana State ful probe of molecular structure. The re- against stray magnetic fields,” remarks University, assisted in the design of an searchers demonstrated this capability by Appelt. “But it’s still amazing that the earth’s field NMR spectrometer used in analyzing three silicon-containing comearth is as good as it is.” a Antarctica. He comments that including pounds: tetramethylsilane, silicone oil, —Erika Gebel the prepolarization Halbach magnet is a 2090
A N A LY T I C A L C H E M I S T R Y / A P R I L 1 , 2 0 0 6
© 2006 AMERICAN CHEMICAL SOCIETY
