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Introducing NMR spectroscopy

In this video Dr Kate Kemsley introduces the concepts behind Nuclear Magnetic Resonance spectroscopy.
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In this week’s films, we will look at another technique that gives information about the chemical composition of different molecules called Nuclear Magnetic Resonance, or NMR spectroscopy. NMR is a well-established tool for research scientists used in chemistry labs worldwide. A related technology, magnetic resonance imaging, is familiar more widely from its use as a non-invasive diagnostic method in hospitals. The underlying principle of both approaches is the same, involving very careful measurement of the behaviour of atomic nuclei inside the sample when it is placed in a strong magnetic field. In the most common form of NMR, measurements are focused on hydrogen nuclei – that is protons.
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This is useful because hydrogen is present in nearly all carbon-containing compounds, and therefore, virtually all foods and drinks, such as orange juice. To understand how NMR gives rise to chemical information, start by picturing the proton at the centre of each hydrogen atom as a tiny spinning ball rotating on its own axis like a spinning top. Because a proton is a charged particle, this spinning motion causes it to behave something like a tiny bar magnet. When the sample is placed in a magnetic field, the proton’s spin axis starts to rotate around the field direction. This is called larmor precession. The rotation frequency is directly related to the field strength, and it’s in the same range as radio waves.
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Although protons are subatomic particles, their behaviour can be imagined as an analogue of a gyroscope spinning on its axis and precessing around the direction of the Earth’s gravitational field. The applied magnetic field also introduces a difference in energy between the two orientations of the proton’s spin axis. To collect an NMR spectrum, a short pulse of radio waves is directed into the sample. This flips some of the lower energy nuclei into the higher energy state, producing an overall magnetisation of the sample, which is measured by the detector coils.
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After the pulse is over, the measured signal, which is called the free induction decay, gradually decreases over a period of seconds or minutes, depending on the sample, until the protons have returned to their original state. However, not all protons in the sample relax at exactly the same rate. This is because the magnetic environment experienced by each nucleus is not only due to the applied field, but also to the motion of other charged spinning particles, electrons, and other nuclei within the sample. These effects are very much smaller than the applied field, but enough to encode the measured signal with a lot of highly detailed chemical information about individual proton environments present in the sample. This is a high field NMR spectrometer.
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At its heart is an electromagnet cooled by tanks of liquid helium and nitrogen to a superconducting temperature allowing the very high currents needed to generate a strong magnetic field. It’s often referred to as a 600-megahertz spectrometer, because this is the larmor resonant frequency of protons in its 14 tesla field. These are expensive pieces of equipment costing hundreds of thousands of pounds, but the amount of detailed chemical information that can be obtained using these instruments has made them an essential tool of analytical research labs worldwide. This is a proton NMR spectrum of sunflower oil measured using a 600-megahertz spectrometer.
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It’s so detailed that it’s difficult to show the entire spectrum with any clarity all at once, but by zooming in on individual regions, we can see separate peaks that come from the different proton environments along the fatty acid chains and in the glycerol backbone. This level of detail can be obtained across a huge range of different compounds, so it’s not surprising that high field NMR has been used to tackle a large variety of food authenticity problems. An example of an application that makes use of the sensitivity and specificity of high field NMR is the detection of pulp wash in orange juice. Pulp wash is the liquid obtained from further processing of orange pulp after the initial juice extraction process.
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It is economically attractive to extend concentrated orange juice by adding back pulp wash, but in most countries, the resulting product could not be declared as pure orange juice. Some years ago, it was discovered that proton NMR could detect market compounds of pulp wash in a relatively straightforward test. The hope is that tests of this kind and the publicity surrounding them will act as a deterrent to fraudsters. Although there is no doubt that high field NMR is a powerful analytical tool, the equipment needed to carry it out is expensive, technically complicated, and really only suitable for use in specialist labs. The arrival of a new technology called low field or benchtop NMR looks set to change this.
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In the next video, we will see how a benchtop spectrometer is being used to tackle food fraud issues.

This video introduces the concepts behind Nuclear Magnetic Resonance spectroscopy, a high end analytical technique favoured by chemists due to the detailed information it provides on molecular structure and composition.

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Identifying Food Fraud

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