Plate height thin-layer

An experiment to measure the viscosity of a fluid is shown schematically in Fig. 12.1. In this experiment the fluid is confined between two parallel plates. The bottom plate is held stationary, and the top plate, at a distance a away in the z direction, moves at constant velocity U in the x direction. The thin layer of fluid adjacent to each wall assumes the velocity of that wall that is, the gas at height z = 0 has zero velocity, and the layer of gas at z — a moves with x velocity u = U. At steady state, a linear velocity profile is set up across the gas, with the upper and lower limits just mentioned. Therefore the velocity gradient du/dz has the value Ula across the channel. It is found that that the force required to maintain the constant velocity of the upper plate is proportional to the area of the plates and the velocity U, and is inversely proportional to the separation a. Thus the retarding force of the fluid per unit area (of the plates) is proportional to the velocity gradient du/dz ... 

Approximate plate heights can also be determined for a given type of packing by thin-layer chromatographic measurements. Thus, for sample 2 in Figure 28-,3()a, the plate count is given by the equation... 

Liquid chromatography can also be used for the quantitative analysis of the separated compounds. In column chromatography, the detector response is normally related to the amount of sample in the effluent. Thus, the area under a chromatographic peak is useful for quantitative analysis in Fig. 21.2, the darkened area under peak C represents the peak area of that component. The peak height (distance h in Fig. 21.2) can also be used. In thin-layer or paper chromatography, the area of the spot is related to the amount of substance. The separated component can also be eluted from the plate or paper and measured externally by another technique (for instance, spectrophotometry). 

Poole, C. F. (1988). Band broadening and the plate height equation in thin layer chromatography. J. Planar Chromatogr.—Mod. TLC 1 H3-376. 

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