Field Homogeneity and Spin Echo

FID signals, f is used to denote the time constant of a FID, which is the observed transverse relaxation time, T2, as described in Section 3.1.3.2 and can be expressed as 

One example of the use of the spin echo technique is to measure moisture and oil content simultaneously in many agriculture and food products such as oilseeds, nuts, chocolate, milk powder, cheese, cookies, and sausages. The total oil and moisture analysis using TD-NMR for seeds and seed residues have become international standard methods (ISO CD 10565 and ISO CD 10632). In seeds, oil and water molecules all have hydrogen and they all contribute to NMR signals. In dried seeds, the water molecules are bound, and the mobility of the water molecules is much more restricted than that of oil molecules, which remain free in the seeds. Therefore, the water in the 

Of course, this method cannot be used for samples with high moisture content because the relaxation time of free water is as long as that of the oil and the NMR echo contains signals from both oil and free water. The echo signal is no longer proportional to the oil content. Such samples (fresh olives, sausages, or meats) must be predried to remove the free water. Then the earlier method can be applied and will provide precise oil or fat content. 

The use of low-resolution TD-NMR for oil, fat, and moisture is often easier and more accurate than the use of near IR (NIR) (Chapter 4) because it is less dependent on the product matrix, so it is possible to calibrate an oil determination against 100% oil. In addition, NMR acquires the signal from the whole sample, not the surface. In some cases, if water is 10%, oil and moisture may be able to be determined simultaneously. Other applications of TD-NMR can be found on the websites of Oxford Instruments and Bruker and include oils in snack foods, chocolate and cocoa, milk powder, and so on. 

Similar to the determination of oil content in oilseeds, the spin echo technique is employed and the echo signal is used to measure the oil content in fiber, while the signals from the residual water and the fiber itself decay away before the refocusing pulse in the measurement. The TD-NMR analysis involves weighing a fiber sample of 2-5 g, putting it into a test tube, and placing the tube into the instrument. The measurement takes a few seconds to a few minutes with a precision of 0.02% in a spin finish range of 0.06%-1.00%. 

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