GARField

F. M. Garfield, Quality Assurance Principlesfor Analytical Eaboratories, 2nd ed., AO AC International, Arlington, Va., 1991 (Enghsh), 1993 (Spanish). 

Drying of Coatings and Other Applications with GARField... 

The relatively low entry level instrumentation cost and the relatively simple experimental methods associated with GARField - both comparable to a standard bench-top relaxation analysis spectrometer as commonly used by the food industry, for example, for water/fat ratio determinations - offer potential advantages to the industrial based user. Indeed, the overwhelming majority of the applications development work described here has been carried out in collaboration with major multi-national industrial corporations such as ICI Paints, Unilever and Uniqema, with industry sponsored research laboratories and associations such as Traetek, and with a range of small-medium sized enterprises. 

Fig. 2.3.1 A schematic diagram of GARField magnet pole pieces and the field pattern they produce together with a magnified sketch of the sample and sensor mounting showing the relative field, gradient and profile [/(r)] orientations.

The field orientation allows the excitation field, Blr of the sensor also to be oriented in the vertical direction parallel to the gradient. This differs from the situation in STRAFI. The advantage of the GARField layout is that a B1 excitation/ sensor coil can be made from a small surface winding below the sample, able to excite/sense a well defined central region of the sample away from edge effects. 

Detailed quantitative analyses of the data allowed the production of a mathematical model, which was able to reproduce all of the characteristics seen in the experiments carried out. Comparing model profiles with the data enabled the diffusion coefficients of the various components and reaction rates to be estimated. It was concluded that oxygen inhibition and latex turbidity present real obstacles to the formation of uniformly cross-linked waterborne coatings in this type of system. This study showed that GARField profiles are sufficiently quantitative to allow comparison with simple models of physical processes. This type of comparison between model and experiment occurs frequently in the analysis of GARField data. 

Fig. 2.3.7 Lower GARField profiles showing a UF (urea formaldehyde) glue line acting as a barrier to water transport for up to 24 h. The glue line is at 800 pm on the scale. Wood is above and below this. The water reservoir is beyond 1300 pm. The profiles shown were recorded after 20 (thin line), 100 and 1400 (thick line) min of exposure to water. Upper plots of the magnetization signal intensity in the lower and upper wood layers as a function of time for three glues urea formaldehyde (squares), phenolic resorcinol formaldehyde (triangles), and poly (vinyl acetate) (diamonds).

Human skin is the largest organ in the human body. It is fundamentally important to health as the semi-permeable barrier - the first line of defence - between the body and the external world. However, it remains relatively inaccessible to conventional magnetic resonance imaging, firstly because it is thin and therefore requires high spatial resolution, and secondly because it is characterized by relatively short T2 relaxation times, particularly in the outermost stratum comeum. Conventional studies have not usually achieved a resolution better than 70-150 pm, with an echo time of the order of a millisecond or so. As a planar sample, skin has proved amenable to GARField study where it has been possible to use both a shorter echo time and achieve a better spatial resolution, albeit in one direction only. Such studies have attracted the interest of the pharmaceutical and cosmetic industries that are interested in skin hydration and the transport of creams and lotions across the skin. 

Fig. 2.3.8 Lower GARField profiles of a human skin sample sandwiched between two glass slides, recorded immediately after the sample was floated onto the first slide and again approximately 90 min later. Upper increasing the pulse gap T from 150 to 500 ps increases mobility contrast and allows discrimination between the stratum comeum (right) and viable epidermis (left). Again two profiles are shown, recorded approximately 90 min apart.

Fig. 2.3.10 A schematic (top) and photograph (bottom) of the surface GARField magnet.

GARField has found a niche application area in the characterization of drying and film forming from aqueous dispersions and in skin-care. As a bench-top perma-... 

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