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drugs (16-18). Radial Flow Chromatography Another alternative chromatographic procedure for preparative separations is radial flow chromatography. The radial flow chromatography column consist of two concentric porous cylindrical frits between which the stationary phase is packed. basically, it is only effective when the separation ratios between the substance of interest and its neighbors is fairly high. This is because the column itself has a very limited length (equivalent to the radial thickness of the packing) and thus can produce relatively few theoretical plates. The efficiency can be significantly improved by using very small particles but the effect use of the radial column also requires a very homogeneous packing. A diagram of a radial chromatography column is shown in figure 32.. The mobile phase flows from the outer cylindrical frit, across the radius of the column, through the cylindrical bed of stationary phase, to the inner cylindrical frit. The radial gap

which is determined by the size of the particles selected for the packing. The larger the column diameter, the stronger must be the column and the thicker the walls. Large column operating at high pressures with relatively small particles can become extremely bulky and heavy. In addition, the construction of wide columns (3 in. O.D. and greater), irrespective of the packing, can be extremely expensive to both construct and to pack and it is essential to take cost into all design considerations. Columns having diameters greater than 0.5 in. need to have the frit supported on a suitable grid, as the frit material has limited strength and will fracture under pressure. The porosity of the frit will be determined by the particle size of the packing. In order to minimize the pressure drop across the frit at high flow rates, the frit porosity should not be made unnecessarily small. Radial dispersion of the ample can be extremely slow (see Dispersion in Chromatography Columns ) and

passes into the column and forms a lightly packed bed at the bottom of the column. The exit valve is hen rapidly opened and the sudden flow of gas packs and compacts the bed at the same time. After packing, the reservoir is carefully removed so as not to loosen the top of the packing and connected to the sampling system. LC Columns If particle sizes in excess of 20 mm are used, then the column can often be dry packed, with appropriate tapping, or, even better, with longitudinal and radial sonic vibration. The variance per unit length obtainable from a preparative LC column should be less than 2 particle diameters (determined using analytical scale samples). It is worth remembering that (as already discussed) when designing preparative columns, it is better to obtain the necessary efficiency using a longer column packed with larger particles, than the converse. The long column will permit much larger charges and, if pertinent for the sample concerned, will also allow

Before Radial Equilibrium is Achieved against Particle Diameter  Despite the lack of radial equilibrium, however, if the column packing is completely homogeneous throughout the column length, then the column efficiency should not be impaired. Unfortunately, ideal packing conditions are not always achieved and channeling often occurs, under which circumstances lack of radial equilibrium could result in the column efficiency being reduced with consequent loss in resolution. To ensure radial equilibrium, it must either be achieved on injection (using sample distribution device) or by employing narrow bore columns where radial equilibrium is more quickly reached. The latter alternative, however, will depend on the resolution required and the nature of the sample. Dispersion Processes that take Place in an LC Column There are four basic dispersion processes that can occur in a packed column that will account for the final band variance. They are, Multipath dispersion,

through the bed. The rearrangement of the more poorly packed areas results in the formation of voids or channels in the packed bed, while the migration of fines results in sudden increases in operating pressure with time which are sometimes a feature of large columns. The problem of bed stability in large diameter columns has been addressed by use of technologies which support the packed bed and allow changes in bed volume to be accommodated without effecting column efficiency. For production columns, the compensation for the variation of bed volume must be automatic, since it is clearly poor practice to wait for a problem to arise before taking avoiding action. Two techniques are in wide use, both relying upon some form of compression system to preserve the bed lifetime the radial compression column and the longitudinal compression column. The principles of both techniques are schematically shown in figures 17 and 18

The method used for packing radial columns is depicted in figure 33. The packing is prepared in the form of a slurry and is pumped directly into the column between the two frits. The column exit to the detector and fraction collector is closed, and, as a result, the slurry solvent passes through the outer frit and exits via the normal mobile phase inlet port. The columns are very easily unpacked by adopting the reverse procedure. Figure 34. The Separation of Some Large Biomolecules Using Radial-Flow Reverse Phase