Free Books and Brochures

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

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

the column efficiency (unless it is accompanied by an corresponding increase in the particle diameter). However, increasing the column efficiency will have the opposite effect, as seen by equation (1), it will reduce the maximum sample load. Consequently, if the necessary efficiency to achieve the required separation has been obtained, then if the column is lengthened to increase the loading capacity for optimum performance, either the flow rate will need to be increased to reduce the efficiency and thus maintain the maximum loading, or the particle size will need to be increased to reduce the efficiency to its required value. However, an increased flow rate will also reduce separation time and thus increase sample throughput. Conversely, the alternative use of larger particles will increase column permeability and thus the column can be operated at a lower pressure and be constructed of lighter materials. Again, a an alternative, if the sample is merely a two component

needed at lower (k') values will require longer columns which will extend the analysis times. To resolve a solute pair with a separation ratio of 1.02, an efficiency of 360,000 theoretical would be required if the (k') value was 0.5. GC Capillary columns can provide such efficiencies but, in LC, such efficiencies would be extremely difficult and costly to produce. It follows that the phase system should be chosen so that the closest eluted solutes are not eluted at low (k') values. Less efficiency will be needed and, thus, shorter columns and consequently, shorter analysis times will be achieved. At (k') values that exceed 10, the required efficiency changes little as the capacity ratio increases. Thus, for fast analyses, the phase system provide a large separation ratio, but the first peak should elute at a (k') of 10 or more. The phase system should have high selectivity and retentive capacity so that minimum efficiency is required and the column can be as short as possible.&

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