of the silica layer must be made as small as possible and the layer must be spread in a thin, homogenous film on the supporting plate. TLC plate efficiency is measured in a similar manner to column efficiency but slightly modified. It is very difficult, if not impossible, to identify the positions of the points of inflexion on a TLC spot, but if the visible edges of the spot are assumed to occur at four standard deviations of the spot distribution, then it is still possible to assess the efficiency. In general it is considered that over 95% of the material in the spot is confined within 4 standard deviations of the spot dispersion. If the diameter of the spot (d), corresponds to four standard deviations, then applying the same rationale as with the packed column, where (Zs) is the retention distance of the solute. Thus, It shout be pointed out, however, the method contains

nbsp   The Effect of Temperature and Solvent Composition on the Required Column Efficiency Using the values for the capacity ratios and separation ratios derived from equations (47), (48) and (49) in equation (39) the efficiency necessary to ensure a separation of (6s) for the two enantiomers can be calculated over a range of temperatures and solvent compositions. Figure 22. Graphs of Required Efficiency against Temperature for Each Solvent Composition Curves relating required efficiency against temperature for each solvent composition, calculated in this manner, are shown in figure 22. As would be expected, the minimum efficiency is required at the lowest temperature and lowest ethanol concentration. As either the separation ratio and/or the capacity ratios decrease, the necessary efficiency to achieve a separation increases (as predicted by equation (39)). At one extreme, where the capacity ratio is very small (i.e. at 50% v/v ethanol and 50˚C), 15000

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

The packing is silica based but is contained in a short column 3.3 cm long, 4.6 mm in diameter and packed with particles 3 mm in diameter. The expected efficiency of the column (when operated at  the optimum velocity) would be about 5,500 theoretical plates. This is not a particularly high efficiency and so the separation depends on the phases chosen to provide the necessary selectivity and an appropriate gradient program. The selectivity was achieved using a complex mixture of ionic and dispersive interactions between the solutes and the stationary phase and ionic, polar and dispersive forces between the solutes and the mobile phase. The initial solvent in the gradient program was a 1% acetic acid and 1 mM tetrabutyl ammonium phosphate buffered to a pH of 2.8. The

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 possible to increase the inlet pressure to some extent, but ultimately the pressure will be limited and the effect of particle diameter will be the same but at higher efficiency levels

efficiency attainable at a given pressure. This is because, as the particle diameter is increased the column permeability is also increased allowing a longer column to be used. The permeability increases as the square of the particle diameter but the variance per unit length only increases linearly with the particle diameter. Thus, doubling the particle diameter will allow a column four times the length to be used but the number of plates per unit length will be halved. Consequently, the columnefficiency will be increased by a factor of two. It is also seen that the higher efficiencies will be obtained with mobile phases of low viscosity and for solutes of low diffusivity. Solvent viscosity and solute diffusivity tend to be inversely proportional to each other and so the sensitivity of the maximum obtainable efficiency to either solvent viscosity or solute diffusivity will generally not be large. The approximate length of a column that will provide the maximum column efficiency when