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The Knox Equation During 1972 and 1973 Knox and his co-workers (18), (19), and (20) carried out a considerable amount of work on different packing materials with particular reference to the effect of particle size on the reduced plate height of a column. The concept of reduced plate height (h ) and reduced velocity  (n) was introduced by Giddings (21) and (22) in 1965 in an attempt to form a basis for the comparison of different columns packed with particles of different diameter. The reduced plate height is defined as,                                             &

by a curve fitting procedure and not derived theoretically from a basic dispersion model; as a consequence the Knox equation has limited use in column design. It is, however, extremely valuable in accessing the quality of the packing. This can be seen from the diagram shown in figure 16.   Figure  16. Graph of Log. Reduced Plate height against Log. Reduced Velocity for Poor and Well Packed Columns The curves represent a plot of Log.(h ),(Reduced Plate height)against Log.(n ), (Reduced Velocity). The lower the Log.(h ) versus the Log.(n )  curve the better the column is packed. At low velocities the (B) term dominates and at high velocities the (C) term dominates as in the Van Deemter equation. The best column efficiency is achieved when the minimum is about 2 particle diameters and thus, Log (h ) is about 0.35. The minimum value for (H) as predicted by the Van Deemter equation has also been shown to be about two particle diameters. The optimum reduced velocity is

stationary phase will be reduced as the temperature rises and it will be seen in due course that this will cause the band velocity of all the solutes to be increased. Elution Development in Thin Layer Chromatography The development processes that take place on a thin layer plate is complicated by the frontal analysis of the mobile phase itself. The mobile phases used to elute the solutes in TLC are usually multi-component, containing at least three individual solvents. If the plate is not pre-conditioned with solvent, there is an elaborate modification of the plate surface which is depicted, for a ternary solvent mixture, in Figure 3. The edge of the plate is dipped into a tray of the solvent mixture which begins to migrate along the plate, driven by surface tension forces. The different solvents array themselves on the surface in the manner shown in Figure 3. Figure 3. The Development of a Thin Layer Plate

The Simulated Moving Bed Preparative Chromatography System The physical system described above was found to be extremely tricky to operate (although with modern computer control technology, the difficulties may well be significantly reduced) and this stimulated research into alternative moving bed systems. In 1971, Barker (13) and in 1973 Barker and Deeble (14) used a column in circular form to imitate the falling bed system. A diagram representing the wheel concept of Barker and Deeble is shown in figure 28. The actual column consisted of a peripheral groove in a large wheel in which the packing is placed and contained by a plate placed on the outside after packing. The wheel somewhat resembled the driving wheel of a

The more polar the substance, the larger is its dielectric constant. In normal chromatography (as opposed to reversed phase chromatography) the mobile phase is normally less polar than the solutes being eluted. Thus, the presence of a solute in the mobile phase will increase the dielectric constant of the mobile phase. Conversely, in reversed phase chromatography the solute is usually less polar than the solvent and the dielectric constant of the mobile phase is reduced by the presence of a solute. Thus. a device situated at the end of the column which responds to changes in dielectric constant would act as a chromatography detector. The sensor often takes the form of a cylindrical or parallel plate condenser. The volume of the sensor must be as small as possible to minimize dispersion. In addition, as the sensitivity of the device is proportional to the electrical capacity of the sensor, the capacitor plates must be very close together. A suitable

of the sample valve) to about 6000 p.s.i. Thus, if the pressure is limited, then to utilize longer columns the particle diameter must be increased to reduce the flow impedance and allow the longer column to be operated at the optimum mobile phase velocity. The use of larger particles to reduce flow impedance and thus permit the use longer column is possible because, at the optimum velocity, the inlet pressure decreases as the square of the particle diameter but the efficiency is only reduced approximately linearly with the particle diameter (thids is true for packed columns only). Thus, doubling the particle diameter allows the column length to be increased by a factor of four and as the plate height will be increased by a factor 2 the net result will be to double the number of theoretical plates