Plate height, definition

H is the plate height (cm) u is linear velocity (cm/s) dp is particle diameter, and >ni is the diffusion coefficient of analyte (cm /s). By combining the relationships between retention time, U, and retention factor, k tt = to(l + k), the definition of dead time, to, to = L u where L is the length of the column, and H = LIN where N is chromatographic efficiency with Equations 9.2 and 9.3, a relationship (Equation 9.4) for retention time, tt, in terms of diffusion coefficient, efficiency, particle size, and reduced variables (h and v) and retention factor results. Equation 9.4 illustrates that mobile phases with large diffusion coefficients are preferred if short retention times are desired. 

Partition coefficient, 9, 10 Partition ratio, 11 time optimization of, 57-58 Peak, definition of, 69 Peak capacity, 18, 19 Pellicular supports, 157 Permeability, 63-64 Phase selection diagrams, 218-219 Phase volume ratio, 11 Pinkerton (ISRP) columns, 225-226 Plate height, 17 Plate number, 14-16 Plate theory, 3, 28 Polarity index, 210, 211 Pore size of LC supports, 157 Porosity, 27 Precision, 99-100 Preparative scale ... 

In the form of equation (2-8) the definition of // is exactly identical to the plate height as it evolved from the distillation theory and was brought to chromatography by Martin and Synge [2]. If // is the theoretical plate height, we can determine the total number of the theoretical plates in the column as... 

Figure 30-11 Definition of plate height, H = (t IL. In (a), the column length is. shown as the distance from the sample entrance point to the detector. In (b), the Gaussian distribution of sample molecules is shown.

However, the definitions related to specific processes have to be kept in mind. In chromatography the plate height is a measure that lumps together the contribution of the fluid dynamic non-idealities (axial dispersion) and the mass transfer resistance... 

From the usual definition of equilibrium-staged separations, the plate height is calculated as // = L/N. The efficiency of chromatographic columns increases as the number of plates increases and as the plate height decreases. Both of these values are used widely in the literature as measures of column performance. 

The plate height, by the classical definition as shown in equation 17, is determined by dividing the column length, L, by the theoretical plate number,... 

N. This definition is somewhat artificial, because actually there is no fixed value of N on the column. The rate theory redefines the plate height (H or HETP) as follows ... 

A dimensional analysis of the perfonnance index shows that it has the dimension of an inverse viscosity. With a little algebra, using the equations for permeability [Eq. (2.51)3, die definitions of the flow-resistance parameter [Eq. (154)3, linear velocity [l s. (110 and (111)3. retention factor [Eq. (161)3 and the reduced plate height [Eq. (113)3, we can lit the performance index into its components ... 

From equations (2.54) and (253) and the definition of the reduced plate height, we can see that it depends primarily on the reduced plate height and the flow resistance parameter ... 

This definitional Eq. 1 is not very operative and is, thus, transformed to an expression which more clearly visualizes the dependence of the resolution on sample properties and experimental variables. The migration times are substituted for by the mobilities of the separands, and the standard deviations by the plate height, H, or the plate number, N, respectively. 

To determine the plate height, note that the nondimen-sional standard deviation (since z was nondimensiona-lized using Lt) of definition (6.3.16h) or (3.2.21) may he written in dimensional form, for any time f/separator length L, as follows ... 

The definition of separation efficiency known from chromatography, expressed as height equivalent to a theoretical plate (F1ETP), can be applied in the same way to CZE ... 

In rubber testing, the surface finish of metals is of importance, for example on mould surfaces and compression set plates. There are a number of standards in the ISO Geometric Product Specification series but the most relevant is ISO 428729 which covers terms, definitions and surface texture parameters relating to the profile method of measuring surface finish. There are apparently over 1000 different parameters to characterize surface finish30 but only a few are generally encountered. The most commonly found is Ra (previously called CLA) which is the mean deviation of the surface profile above and below the center line, followed by Rz, a measure of the peak to valley height. For example, the arithmetic mean deviation (Ra) of the compression plates for compression set tests must be better than 0.2 m. 

HETP Height equivalent to a theoretical plate. A measure of the combined effects of axial mixing and finite mass transfer resistance in causing deviations from ideal (equilibrium) behavior in a chromatographic column or in a countercurrent contact system. The definitions of HETP in these two cases are somewhat different,... 

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