Authored by J Beau W Webber
Abstract
Nuclear Magnetic Resonance (NMR) is a powerful technique that has many applications and a variety of techniques. It is often forgotten how simply it can be applied, particularly for material analysis. New highly portable NMR instrumentation based on Field Programmable Gate Array (FPGAs) is making the use of NMR in the laboratory and in the field fast and easy to use for materials research and characterization for biological and medical applications. It is particularly appropriate when nano-pores are present.Keywords: Nuclear magnetic resonance (NMR); NMR relaxation; NMR T1; NMR T2; Material science; Nanopores
Introduction
We have all see the amazing detailed 3D images of the insides of people, that the non-invasive technique NMR Imaging is bringing us. And increasingly, it is not just simple images but also images with parameterized information based on the two common NMR relaxation times: T2, the time for the atoms to “share” the spin information, and T1, the time for the spin information to be lost to the “lattice” – the rest of the sample [1]. But these imaging systems are highly complex. The chemist’s probe into molecules and pull out chemical structure and are always looking for magnets with higher and higher frequencies – now into the GHz region for water as well as extremely high homogeneity [2]. But let us not forget where NMR started, in the material sciences laboratories, with simple relaxation measurements of small samples at a few 10MHz. Is this still of any interest? Well it certainly should be.Discussion
There are a whole range of samples, from food, to biological, to rubbers and polymers, to medical materials, where the physical properties are open to study by NMR. In many of these materials these properties change with temperature, age or exact composition, and changes in these physical properties usually means changes in the NMR properties, particularly T1 & T2. NMR is particularly adept at studying the properties of liquids such as water (and ice) at interfaces, or in Nanopores, as in [3] (Figure 1).These studies can also throw light on the behavior of water at biological interfaces [4]. But you will say, even simple NMR instruments are still large and complex? Well no longer! Six components are required: a probe to hold the sample in the NMR coil; a magnet in which to put the probe and sample; a transmitter to create all the timing and pulse sequences to generate very short but powerful RF pulses (which are all contained inside the probe); a very sensitive and low-noise RF receiver, to capture and average the tiny NMR signals; and a console from which to control the experiment and analyses and display the results. This still sounds daunting, but two things have happened:
Firstly, magnetic materials have improved, and tiny but powerful permanent magnets suitable for NMR can be small enough to be hand-held, and can be taken to the sample being studied to measure properties such a aging in a racing car tire, or the maturity of “hung” meat. At its simplest the magnet can be just a cube of magnetic material. Slightly larger magnets but still smaller than a shoe-box, such as that shown in Figure 2, give results that are higher field/frequency, and have better homogeneity of field, so the results can be easier to interpret. (Figure 2).
Secondly, Field Programmable Gate Arrays (FPGAs) are revolutionizing many instruments in the medical field and in the research laboratory, and NMR is now no exception. Figure 2 shows the basic components of a NMR instrument suitable for measuring physical properties A magnet – the one shown gives 24 MHz for water; a probe – this sometimes needs to be customised for a particular experiment, and temperature control can be added; an NMR receiver (shown on the left) and an NMR transmitter (shown on the right), now both about the size of credit-cards (Figure 3).
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