Preparative Chromatography - Preparative Chromatography Apparatus > Preparative Columns > Page 25
If the volume of sample is small enough then the sample can be placed on the column with a sample valve. Sample valves, with appropriate loops, can also be used for smaller preparative separations, and sample loop capacities of 20 to 30 ml are practical. Abovethesevolumes, however, a sample pump is recommended. It should also be pointed out that, as already discussed, there can be considerable dispersion in open tubes, and the sample loop should not be left in-line during development. The valve should be returned to the 'flush' position immediately after injection, so that the 'tail' of the sample left in the loop does not cause dispersion and tailing on the column. This procedure may entail sample recovery from the residue contained in sample valve.
Preparative columns (GC or LC) are usually made of glass or stainless steel the latter being used for high pressure systems. Preparative columns must be designed to accommodate the inlet pressure necessary to obtain the required flow rate through the packed bed 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 thus a device must be used to distribute the sample across the column so the full loading capacity of the column can be realized.