Preparative Chromatography - Packing Preparative Columns > LC Columns > Page 32
The pre-column is then carefully removed from the actual column and the top flange connected. The pre-column ensures that the top of the packing will be packed to a similar density to the bulk and will not be 'loose' and more porous and permeable.
For larger columns the apparatus on the left of figure 14 can be used. This procedure was developed by Filipi (9) for small bore GC columns but works equally well for large preparative columns. Sufficient packing is placed in the top reservoir to pack about 110% of the column volume, so that after packing the reservoir still contains a significant amount of packing. The reservoir is then connected to a reducing valve and gas cylinder (tank). The valve at the column exit is closed during this period. The pressure in the system is very slowly brought up to a pressure of about 50 p.s.i. (the optimum pressure will depend to some extent on the particle size of the packing and the column length and may need to be determined by experiment. During the initial pressure adjustment some of the packing 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.
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 multiple sample development techniques. In addition, the larger particles will provide greater column permeability, and thus lower pressures can be used. Lower pressures will, in turn, allow lighter and less expensive materials to be used in the construction of the preparative system.