Introduction

Nuclear magnetic resonance (NMR) is now a mature spectroscopic technique and is widely recognised as being one of the most powerful spectroscopies available for the study of the structure and dynamics of condensed matter. Its origins are in the pioneering experiments of Bloch and Purcell in the mid 1940's (Bloch et al, 1946; Purcell et al, 1946). These experiments were followed by the discovery of the chemical shift (Proctor and Yu, 1950; Dickinson, 1950) which opened up the field of high resolution spectroscopy of the liquid state. Today high resolution spectroscopy is a routine analytical technique in almost every major chemistry laboratory and many others besides. The widespread introduction of pulsed techniques (Torrey, 1949; Hahn, 1950) led to the field of relaxation time analysis and so to the investigation of molecular dynamics on timescales covering at least eight orders of magnitude. These simple techniques are now widely used for bench top laboratory characterisation of the physical state and composition analysis of heterogeneous systems, including foodstuffs. As will be seen, they form the basis of most on line NMR applications. Subsequently, magic angle spinning (Andrew et al, 1958) and multiple pulse line narrowing (Waugh et al, 1968) extended the range of high resolution spectroscopy to the solid state. The first magnetic resonance images were published in 1973 (Lauterbur, 1973; Mansfield and Grannell, 1973) and over the subsequent years it has become possible to spatially resolve virtually all magnetic resonance parameters, initially from liquid samples but increasingly from solids as well. Imaging times have become progressively shorter so that 'real time' imaging is now possible and the scales of application have been extended from single cells to human bodies.

Throughout its development, magnetic resonance has remained largely a laboratory based technique. One reason is the common perception that magnetic resonance is 'complicated' or 'difficult'. Whilst the plethora of measurement techniques and possible outcomes and explanations requires that extreme care is exercised in interpreting new results, in many cases simple interpretations are possible. Another major reason is that many scientists continue to think of high resolution spectroscopy when they think of magnetic resonance. However, a high resolution spectrum is not the most obvious measurement to make for process control applications. It requires a very high magnetic field homogeneity and preferably also a high magnetic field in order to resolve the narrow resonance lines and hence chemical shifts and spin-spin couplings which serve to fingerprint the sample. In solids or in samples containing bound liquid fractions, or in porous samples with spatial heterogeneity of the magnetic susceptibility the observed resonance is broad and relatively featureless. Chemical shifts cannot be resolved without resource to complex line narrowing techniques which are not always applicable and rarely amenable to on line implementation. After spectroscopy, imaging comes to mind but there are problems with imaging as well, even if the magnetic field requirements can be met. Most standard imaging procedures take several minutes to acquire the three dimensional data sets necessary to inspect a sample in three dimensions. For most mass production purposes this is too long and in any case poses the problem of how to display, analyze and interpret the very large volume of generated data sufficiently quickly. Lower dimensional imaging, such as one dimensional profiling, is very much faster and generates much less data. Under suitable circumstances it can be applied usefully. This is most likely to be the case when looking for changes in density or solid content, which is invisible to conventional magnetic resonance imaging, such as stones in pitted soft fruits. In essence, profiling is no more than a sophisticated variant of free induction decay analysis discussed below, albeit that the signal is acquired in the presence of an applied magnetic field gradient.

By far the most appropriate technique for on line analysis is a measurement based on the characterisation of free induction decay signals and on relaxation time analysis, and in particular on spin-spin relaxation time analysis. These measurements are sometimes called low resolution NMR. Unlike high resolution spectroscopy where state of the art spectrometers use 15 T magnets, low resolution measurements can be made in low magnetic fields, say 0.15 T using magnets with relatively poor spatial homogeneity. Small, low field permanent magnets are ideal and the rapid and continuing advances in magnetic materials science and magnet design technology make these increasingly viable. Moreover, whereas high field spectrometers and imagers may be priced in millions of dollars, low field systems can be just a few tens of thousands of dollars. Other advantages of low resolution NMR are that the measurements can be made very quickly, typically within 1 s, and that in favourable circumstances simple interpretation is possible.

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