A

The problem is to achieve Nne with minimum effort and maximum convenience. It is more complicated than in column chromatography as nothing can be done about the speed of the eluent. This flow rate is obviously not constant and we shall consider the laws governing flow rate in TLC in the next section.

diffuse from the liquid to the wall of the capillary tube.

The height of ascent, zm, is governed by two forces acting on the liquid: capillary rise and hydrostatic pressure.

where y is the surface tension, r is the radius of the capillary, p is the density of the solvent, and 6' is the wetting angle.

The smaller the radius, the higher the ascent, and thus smaller particles will govern the rise of eluent. An order of magnitude of r is the radius of the largest sphere that can fall through the hole between three tangent spheres of diameter dp, i.e. dp/15. Equation [14] does not say anything, however, about the kinetics of the phenomenon.

According to Poiseuille's law, the flow rate a capillary is given by:

in thus R = (1 — RF)JWF. (Reproduced with permission from Guiochon G and Siouffi A (1979) Journal of Chromatographic Science 16: 368-386.)

The necessary plate number to achieve a given separation is well known in column chromatography:

— nr4 SP 8y Sx where y] is the viscosity of the liquid and SP/Sx is the pressure gradient along the tube.

As particles are of different sizes and r is not constant, there will obviously be a higher resistance to flow through the smaller tubes. The bulk of the flow will be through the larger capillaries, which fill more slowly behind the solvent front. Thus the volume of liquid is not evenly distributed along the plate; the wet area near the solvent front contains less liquid. A simple glance at a developing plate reveals this fact. In selected experiments the capillary rise can be monitored by UV recording of suitable solvents or, much better, by X-ray absorption. In this latter case the saturation of the porous layer is measured as a function of time and the relationship between the logarithm of the transmitted intensity I and the liquid content is linear. In all cases a plot similar to that of Figure 2 is obtained.

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Solar Panel Basics

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