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Other application areas for low resolution NMR include the monitoring of dehydration and rehydration and of freezing and thawing (Monteiro Marques et al, 1991; Zagibalova et al, 1979). In these cases it should be noted that as well as the total liquid / solid content, it is sometimes possible to infer information about the molecular environment of the liquid from the T2 relaxation curve and hence information about the true reversibility of the process. The detailed shape of the very short T2 decay part of the relaxation curve due to solids is generally non exponential. Although it is hard to measure accurately, the shape is sensitive to the degree of crystallinity, so that amorphous and crystalline phases can, in principle, be separated. In favourable circumstances, the long T2 component is inversely proportional to liquid viscosity giving a further parameter which can be inferred (Mora-Gutierrez and Baianu, 1989). The application of magnetic field gradients leads to the NMR signal being sensitive to flow and diffusion (Watanabe and Fukuoka, 1992) and NMR flowmeters have been developed (Pryakhin et al, 1989). Droplet size in emulsions can also be monitored because the T2 relaxation of a liquid is sensitive both to restricted self diffusion and to variations in magnetic susceptibility. Finally, NMR can be used as a foreign body detector. However, the sensitivity depends very much on the existence of distinctly different relaxation behaviour in the sample and foreign body. Metals, which are usually an anathema to NMR spectroscopists, inhibit the working of the spectrometer by destroying the sample coil tuning and are easily detected.

3.3. Advantages and disadvantages

Four particular advantages of the low resolution NMR method as depicted by figure 1 over other non-NMR methods are generally recognised. Firstly, the NMR measurement is from the bulk sample rather than the sample surface, as is the case with infrared for example. Consequently, the measurement is unaffected by surface effects such as drying. Secondly, the NMR method is non invasive and non destructive and is thus suitable for on line analysis. Indeed it is not even necessary to touch the sample with a probe as with a capacitance test and so the sample may be packaged or in a pipe. Thirdly, no separate sample weighing is required since the solid to liquid ratio is determined directly. Finally, the method is very rapid. However, the sensitivity of the NMR signal to so many parameters (both sample related and experimental) implies that careful bench top characterisation of the product is required before on line use is begun. Moreover, it means that product formulation must be maintained for long periods without change if the technique is not to require regular recalibration. This may be a disadvantage. Under favourable circumstances the method is very accurate and moisture measurements to 0.4% accuracy have been suggested (Gribnau, 1992). NMR intensities and relaxation times are temperature sensitive and control, or at least monitoring, of the sample temperature greatly increases the accuracy of the NMR measurement. This may itself be done by NMR means. Appropriate methods include cross correlating two simultaneously measured NMR parameters and use of a calibrated NMR phantom in the spectrometer beside the sample.

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