US 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.
This portable NMR sensor was developed by a collaboration of researchers with the
“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 Science journal paper describing the portable NMR device.
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 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 neighbouring 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.”
In 2001, an effort led by fellow researchers Alexander Pines, Carlos Meriles, and Dimitris Sakellariou, formerly with the UC Berkeley chemistry department, demonstrated a technique for obtaining high-resolution NMR spectroscopy data from samples in a grossly non-uniform magnetic field. In this “ex situ” method, the lack of uniformity in the magnetic field is compensated for by zapping a sample with a series of rf pulses that are precisely varied in energy, duration and timing.
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.”
The researchers applied the ex-situ technique in a portable single-sided sensor device, which, for these experiments, was configured for high resolution NMR spectroscopy. Two concentric U-shaped permanent rare earth magnets (Neodymium-Iron-Boron) were arranged to create a magnetic a “sweet spot” about seven millimetres above the active magnet surface.
In this sweet spot, the static magnetic field is parallel to the plane and reaches a field strength of 0.2 Tesla. The rf pulses were generated through a rectangular rf coil that is shaped and positioned with the inner of the rare earth magnets to optimise the interaction between the static magnetic field and the magnetic fields generated by the rf pulses.
When a sample is placed in the sweet spot and hit with a carefully orchestrated series of rf pulses the result is an NMR spectrum with a sharp enough resolution to reveal chemical shifts of eight parts per million within three minutes after sample testing begins.
“Eventually, we think we can attain a better than one part per million resolution, that is our goal for when this device becomes commercially available,” said Demas, who recently returned from