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Column Efficiency The column efficiency is defined as the number of theoretical plates in the column. As discussed in the plate theory, the faster the equilibrium process, the smaller the plates and thus, the greater the number of plates in the column. It is therefore important to know how to determine the number of plates a column possesses and the relationship of the number of theoretical plates in the column to the properties of the chromatogram. Starting with the Poisson form of the elution equation, the peak width at the points of inflexion (which corresponds to

nbsp;                                     dXs = KdXm                                       (2) Consider three consecutive plates in a column, the (p-1), the (p) and the (p+1) plates and let there be a total of (n) plates in the column. The three plates are depicted in Figure 2.   Figure 2. Three Consecutive Theoretical Plates in a Column   Let the volumes of mobile phase and stationary phase in each plate be (vm) and (vs) respectively, and the concentrations of solute in the mobile and stationary phase in each plate be Xm(p-1), Xs(p-1), Xm(p), Xs(p), Xm(p+1), and Xs(p+1), respectively. Let a

The column was 10 m long, 1 mm I.D. packed with Partisil Silica Gel 20 mm particle diameter. At the optimum flow rate (i.e., 10 ml/min.) the column gave a quarter of a million theoretical plates. However, the chromatogram shown in figure 34 was obtained at a flow rate of 38 ml/min. and, thus, as it was operated well above its optimum velocity, the column only gave an efficiency of 160,000 theoretical plates. As the chromatographic data was acquired and processed by a computer portions of the chromatogram could be expanded and these are shown as inserts in the figure. It is seen that the apparently confused peaks at the start of the chromatogram are, in fact, well resolved into

;                                      Consequently,                         (57) Equation (57) shows the relationship between efficiency and 'effective plates'. The number of 'effective plates' in a column is not an arbitrary measure, but is directly related to efficiency as derived from the plate theory. Equation (57)shows that, as (k') becomes large, (n) and (NE) converge to the same value

will hold: The resistance-component of the cell reduces the bridge sensitivity to changes in capacity and thus the plates should be well insulated to prevent conductivity through the mobile phase. The capacity of the sensor can also be measured by making it one component of a resistance/capacity or an inductance/capacity oscillator. The frequency will depend, among other things, on the capacity of the sensor and, in turn, on the dielectric constant of the material between the plates. The frequency general can be heterodyned against a reference oscillator and the frequency difference will then be proportional to the change in capacity and hence the dielectric constant of the mobile phase. Poppe and Kunysten (28) described a dielectric constant detector which included a reference cell for temperature compensation. The cell consisted of two stainless steel plates 2 cm x 1 cm x 1 mm separated by a gasket 50 mm  thick. The two cells were identical and clamped

nbsp;      Figure 12. Graph of Log. Maximum Efficiency against Particle Diameter It is seen from figure 12 that changing the particle diameter from 1 to 20 micron results in an efficiency change from about 3500 theoretical plates to nearly 1.5 million theoretical plates and furthermore, this very high efficiency is achieved at an inlet pressure of only 3000 p.s.i.. It is also seen that the maximum available efficiency increases as the particle diameter increases. This is because, as already discussed, if the pressure is limited, in order to increase the column length to provide more theoretical plates, the permeability of the column must be increased to allow the optimum mobile phase velocity to be realized. It is