Gradient-plate technique

Rattanasomboon, N., Bellara, S.R., Fryer, P.J., Thomas, C.R., and McFarlane, C.M. 2001. The gradient plate technique as a means of studying the recovery of heat injured Brochothrix thermosphacta. International Journal of Food Science and Technology 36 369-376. 

The historical gradient plates, ditch-plate and cup-plate techniques (see Hugo 8c Russell, 1998) have been replaced by more quantitative techniques such as disc diffusion (Fig. 11.4), broth and agar dilution, and E-tests (Fig. 11.5). All employ chemically defined media (e.g. Mueller-Hinton or Iso-Sensitest) at a pH of 7.1—1 A, and in the case of solid media, agar plates of defined thickness. 

Palladium and palladium-silver alloy membranes on porous alumina tubes were prepared by means of simultaneous and sequential electroless plating techniques [234], The membrane reactor was used for the direct formation of hydrogen peroxide by catalytic reaction of H2 and 02 at 293 K. The concentration of H202 increased with increases in the transmembrane partial pressure gradient of H2. A high concentration of H202 was obtained with a membrane that consisted of a palladium layer on the outer surface, porous alumina in the middle, and a palladium-silver alloy layer on the inside. 

Single linear developments are mostly employed in the vertical mode. The apph-cabihty of the horizontal mode is discussed in Chapter 6. For circular and anticircular developments, the movement of the mobile phase is two-dimensional however, from the standpoint of sample separation it is a one-dimensional technique. Circular developments result in higher hRp values compared to linear ones imder the same conditions, and compoimds are better resolved in the lower-AR range. The same effect is noticed on plates with a layer thickness gradient (see Section 5.2.1). On the other hand, using antieircular developments, compounds are bettCT resolved in the upper-M range. 

In this technique, the development distance is increased linearly in 10- or 20-mm steps (with evaporation of the mobile phase from the plates after each step) by using the same solvent or a series of solvents for modified IMD technique. In gradient IMD, the eluent strength is rednced stepwise. 

Another possibihty to improve the temperature homogeneity is to introduce an additional polysiHcon plate in the membrane center. The thermal conductivity of polysilicon is lower than that of crystalline siHcon but much higher than the thermal conductivity of the dielectric layers, so that the heat conduction across the heated area is increased. Such an additional plate constitutes a heat spreader that can be realized without the use of an electrochemical etch stop technique. Although this device was not fabricated, simulations were performed in order to quantify the possible improvement of the temperature homogeneity. The simulation results of such a microhotplate are plotted in Fig. 4.9. The abbreviations Si to S4 denote the simulated temperatures at the characteristic locations of the temperature sensors. At the location T2, the simulated relative temperature difference is 5%, which corresponds to a temperature gradient of 0.15 °C/pm at 300 °C. 

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