Microtesla nuclear magnetic resonance (NMR)

Microtesla nuclear magnetic resonance (NMR) has been demonstrated. In conventional NMR, a several-tesla magnetic field is used to orient atomic nuclei in the sample. The polarized nuclei can resonantly absorb a burst of radio waves, and precess around the imposed field. The spectral “chemical shift” information from reemitted radio waves is then used to identify molecules. NMR also lies at the heart of magnetic resonance imaging (MRI). Now, a team of scientists led by John Clarke and Alexander Pines (Lawrence Berkeley National Laboratory and the University of California, Berkeley) have exploited an often overlooked fact: For a homogeneous field, the NMR linewidth scales linearly with the field strength. Thus, a 1000-fold reduction in field strength produces a line both narrower and taller by that same factor. The researchers placed a small liquid sample of methanol and phosphoric acid in a polarizing field of only 1 mT and a much weaker orthogonal measuring field of 5 μT (Earth’s field is roughly 50 μT). The group then turned off the polarizing field and used a SQUID to detect not chemical shift but “J-coupling,” which can measure an atom’s chemical environment as well as its identity. In that way, they not only identified protons and phosphorous-31, but saw the signature—a doublet split by 10.4 Hz—of the covalent bonds in trimethyl phosphate. These techniques open the possibility for “pure J” spectroscopy and perhaps could form the basis of inexpensive MRI machines. (R. McDermott et al., Science 295 , 2247, 2002 http://dx.doi.org/10.1126/science.1069280 .)