Interesting research with implications for public safety is coming out of Berkeley:
Homeland security experts may soon be getting a valuable new tool for identifying the chemical constituents in suspicious substances. A portable device makes it possible for the first time ever to take high-resolution NMR spectroscopy – one of the principal tools for chemical analysis – out of the laboratory and into the field for use on samples of any size.
“Our device does not compete with the superconducting magnets that are used to study proteins, but there are many applications, besides homeland security, where you can’t bring samples from the field to the laboratory, including medical diagnosis, archaeological analysis, or the exploration of objects in space, like planets or moons,” said chemical engineering graduate student Vasiliki Demas, one of the co-authors of a paper describing the portable NMR device, which appears in the April 8, 2005 issue of the journal Science.
NMR is a phenomenon involving the atomic nuclei of molecules in which at least one proton or neutron is unpaired. The imbalance causes such nuclei to spin on an axis like miniature tops and gives rise to a magnetic moment, which means the nuclei act as if they were bar magnets with a north and south pole. When a sample is exposed to a strong external magnetic field, these spinning “bar magnets” attempt to align their axes along the lines of magnetic force. The alignment is not exact, resulting in a wobbly rotation about the force lines that is unique for each type of nuclei. If, while exposed to the magnetic field, the nuclei in a sample are also hit with a radiofrequency (rf) pulse, they will absorb and re-emit energy at specific frequencies according to their individual rates of rotation. These frequencies show up in an NMR spectrum as distinct peaks of varying height that, like a set of fingerprints, can be used to identify the sample’s constituent nuclei.
Because the rate at which resonating nuclei realign themselves with magnetic field lines is heavily influenced by their neighboring nuclei, NMR can also be used to provide detailed information on the structural, dynamic, and spatial relationships of atoms in a sample. Deviations from reference peaks on the NMR spectrum, called “chemical shifts,” reflect different concentrations of a sample’s constituent nuclei and can be used to positively identify the molecular composition and chemical nature of the sample.
Until recently, high resolution NMR spectroscopy could only be done by placing a sample inside the bore of a very large stationary magnet that produces a strong, uniform magnetic field. Portable NMR systems with open, single-sided probes, have been built, but the lack of uniformity in their magnetic fields limit them to low resolution.
“The variations within the magnetic fields of previous portable NMR devices are usually orders of magnitude too large to detect chemical shifts,” said Demas. “These devices mainly yield relaxation times as a crude estimate of a sample’s composition.”
Said Meriles at the time this technique was announced, “We have demonstrated that high-resolution NMR spectra can be recovered even with a strongly inhomogeneous magnetic field, which means it may be possible to develop a mobile magnet that can be scanned over otherwise inaccessible objects to get magnetic resonance information.”
In the picture above, you can see Vasiliki Demas, one of the co-authors, loading a sample for analysis inside a low-field electromagnet, currently being used as a testbed for obtaining high-resolution NMR spectra without the constraints of having to place samples inside the bore of a high-field magnet.
More at Berkeley Lab…
