NMR

Nuclear Magnetic Resonance (NMR) is an analysis technique that works by first exciting atoms in a sample using a high frequency magnetic pulse, then measuring the re-emitted radio signal from those same atoms as they return to normal. Since the way in which atoms respond depends on their overall environment, the technique reveals information about the chemical and physical features of the sample. .

The nucleus of a simple atom such as hydrogen has both electrical charge and spin. A spinning charge behaves like a tiny magnet, which like all magnets has a north and south pole. If a sample containing hydrogen atoms is placed in a magnetic field, then it's energetically favourable for each little "nuclear magnet" to align itself with the direction of the external magnetic field. If the sample is then subject to an additional radio frequency magnetic signal, each little nuclear magnet absorbs energy and is twisted through 90 degrees to the direction to the external field. The frequency of the applied signal needed to do this depends on the strength of the external field, since this controls the energy gap between the two orientation possibilities. When the radio frequency magnetic field is removed, the nuclear magnets revert back to their original orientations, emitting a decaying radio signal in the process. This signal is detected via the same coil that imparted the high frequency twisting field.

In the graphic, the black arrow really represents an averaged nuclear spin direction.

The chemical composition can be extracted because the response of each nuclear magnet is modified by the magnetic properties of any electrons nearby. This means that the frequency of the high frequency field required to twist the nuclear magnets, and the subsequent emitted radio signal, both depend on the chemical environment of each nuclear spin.

Other information can be extracted because the strength of the emitted signal depends on the number of nuclear magnets present. The variations in density of nuclear magnets in a sample can be measured as a means of imaging the sample, as for instance in hospitals. In this case NMR is called magnetic resonance imaging (MRI). The extra ingredient needed to achieve this is an additional externally applied field which varies from place to place in the sample. This in turn means that the energy gap between the two spin orientations depends on position, so the resulting radio frequency emission contains information about the spatial distribution of nuclear magnets in the sample.

One of IFR's NMR units.

At IFR, we have used NMR to monitor the ingress of water into dried foodstuffs such as pasta, of interest to companies who make "quick cook" products. The key quantity to measure here is the distance that each water molecule can cover by diffusion in a given interval of time. We have also used NMR in food authenticity applications, for example testing for clandestine mixing or adulteration in wine, fruit juice and coffee. This type of work typically relies on NMR's isotope sensitivities. Closer to the present work, we have also studied the microstructure in emulsion systems, where water diffusion measurements are sensitive to geometries that restrict water movement. NMR can see flocculation in emulsions, in which the droplets cluster together. Read more about this work in "NMR Q-space microscopy of concentrated oil-in-water emulsions", B.P. Hills, P. Manoj, C. Destruel, Magnetic Resonance Imaging 18 (2000) 319-333.

What you see here is the outer skin of the liquid nitrogen jacket that is part of the cooling system for the superconducting magnet within. There's a liquid helium cooling stage inside this. The superconducting magnet provides the aligning field for the nuclear magnets. The sample and the additional coils are inserted from below. All the processing and field control is done via separate boxes that are not in the picture.

The image to the right is our first attempt at demonstrating NMR imaging. The large disk is the interior of the sample holder. Inside this is a PTFE cylinder, which appears as an annulus in the cross-section image. A slice cut through the cylinder shows up clearly. Each pixel represents around 50 microns, and the narrow neck of the slice in the cylinder is around 100 microns across. This level of resolution allows us to pick out interesting small-scale features in both experimental and real food systems.