Height, average theoretical plate

According to van Deemter [27] the average theoretical plate height H, which is a characteristic of separation kinetics, is related to the factors influencing the mass transfer by the equation ... 

Figure 5 Correlation between the average theoretical plate height (H) and the distance ( c) traveled by the eluent on silica gel layers with various particle size and in different chamber system. Nus, normal unsaturated chamber N, normal saturated chamber UM, ultramicro chamber (totally closed layer, without overpressure).

Solving equation 12.12 for H gives the average height of a theoretical plate as... 

Commercial preparations of these supports are available in narrow mesh-range fractions to obtain particles of uniform size the material should be sieved to the desired particle size range and repeatedly water floated to remove fine particles which contribute to excessive pressure drop in the final column. To a good approximation the height equivalent of a theoretical plate is proportional to the average particle diameter so that theoretically the smallest possible particles should be preferred in terms of column efficiency. Decreasing particle size will, however, rapidly increase the gas pressure necessary to achieve flow through the column and in practice the best choice is 80/100 mesh for a... 

FIGURE 6.2 Height equivalent to a theoretical plate (H) of a GC-separated compound versus average carrier gas velocity for a 3-m-long silicon channel coated with non-polar dimethyl polysiloxane stationary phase (a) native silicon surface (b) oxidized silicon surface. Plots A are for hydrogen carrier gas, and plots B are for air carrier gas [598]. Reprinted with permission from the American Chemical Society. 

Figure 2-2. The experimental dependence of the theoretical plate height on the flow velocity for columns packed with same type of particles of different average diameter.

The average height equivalent to one theoretical plate (//) can be calculated from the relationship [15]... 

On HPTLC layers, the average particle size and the range of particle sizes are smaller than on TLC (e.g., 2-10 /rm and 5-17 /rm respectively for Macherey-Nagel HPTLC and TLC silica plates) and a marked fall in theoretical plate heights is observed (about one order of magnitude smaller than on standard silica layers). Consequently, smaller plate sizes (from 20 X 20 to 10 X 10 cm) and small sample volumes (from 1-3 to 0.1-0.2 juL or even less) and shorter migration distances (from 10-16 to 3-8 cm) can be employed. 

Suppose that the distance from where the product is removed at the top of the column to the surface of the liquid in the pot is 60 cm. The average distance between each theoretical plate in the example shown in Figure 3-9 (6) would be 10 cm. This is known as the Height Equivalent to a Theoretical Plate or HETP. HETP is a measure of the efficiency of distilling columns, and a standard equimolar mixture of hexane (b.p. = 69 C) and methyl cyclohexane (b.p. = 100.4 °C) often is used to determine HETP. Figure 3-11, p. 27, shows the effect of the number of plates on a separation. 

The flow of gas through the column and the diffusion of the solute in both gas and liquid all influence the solute band-width and, hence, efficiency and resolution. Early in the history of GC it was shown (by van Deemter and coworkers for packed columns, and by Golay for capillary columns) that H, height equivalent to a theoretical plate (HEPT) depends on the average linear gas velocity (u) through the column according to Equation 13.1 ... 

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