When slurry packing preparative LC columns, care must be taken not to operate at pressures in excess of the bursting strength of the tube used for the column. As the column diameter increases, the maximum permissible pressure rapidly falls unless extremely thick walled tubing is employed. The safe maximum pressure for any tube can usually be obtained from the tube or column supplier. A slurry is made of the packing (110% of that needed to fill the column) and placed in the packing reservoir. The reservoir is rapidly connected to the pump (which must have both an adequate delivery rate and an adequate operating pressure - the delivery rate will depend on the column diameter and the applied pressure on the wall thickness and the column length). The pump is started and the column exit valve opened and the flow continued until it has been reduced to a constant value. The flow is then arrested and some packing will remain in the reservoir

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

the support and an accurate value for the stationary phase loading.   Column Packing Short columns are usually straight and can be packed vertically. The packing is added, about 0.5 ml at a time, and the column tapped until the packing had settled. Another portion of packing is then added and the process repeated until the column is full. U-shaped columns are packed in the same manner. Columns up to 50 ft long can be packed in a series of U's and then each U column joined with a low dead volume connection. If the columns were glass they were usually filled through an opening at the top of each U which was terminated in a plug of quartz wool and sealed-off in a blow-pipe flame. These long packed columns could be operated at a maximum of 200 psi. and could provide efficiencies of up to 50,00 theoretical plates. Such columns could tolerate charges of several microliters. A chromatogram of the isomeric heptanes and octanes obtained from a 50 ft

hydraulic jack, can be moved throughout the entire length of the column. It contains a porous frit at the top and a channel for passage for the mobile phase through its center. The channel is connected to the detector and thence to the fraction collector. To pack the column, the piston is withdrawn to bottom of the column and the column filled with the measured mass of packing. The amount of packing used will determine the effective length of the column. The hydraulic jack is activated and the packing compressed to the maximum pressure which is determined by the mechanical strength of the packing. If the packing is crushed by the use of too great a pressure, many fines will be produced, which cause the permeability to be extremely low and seriously reduce the column efficiency. The pressure to the column is maintained continuously and during each separation. In addition to stabilizing the bed when packed, the system allows the rapid packing and unpacking of media which can range from

13. Figure 13 An Example of a Column Packing Apparatus   The packing is placed in a reservoir attached to a gas supply that forces the packing through the column. The column exit is connected to a vacuum pump. A wad of quartz wool is placed at the end of the column, constrained by a small restriction, that prevents the wad from being sucked into the pump. The vacuum and gas flow are turned on simultaneously and the packing is swept rapidly through the column. This causes the material to be slightly compacted along the total length of the column and has been shown to produce well-packed columns. The procedure is a little tedious and the success rate is sometimes less than 90%. In addition, the process does not lend itself to automation. The difficulties involved preparing packed columns have also contributed to the preferential popularity of the open tubular columns. The production of capillary columns can be largely automated and

by increasing the dimensions of the column both in GC and in HPLC. However, this approach has distinct limitations. If the column radius is increased, unless special packing techniques are employed, the packing procedure becomes inefficient and the packing itself unstable. In addition to maintain the optimum mobile phase velocity, the flow rate will need to be substantially increased and the consumption of mobile phase will eventually become economically impractical. Conversely, if the column length is increased, then the impedance to flow will become greater leading to high column pressures. If large column radii are employed, then the mechanical strength of the column system will limit the maximum permissible pressure. Consequently, lengthening the column will eventually require the particle diameter to be increased to provide adequate permeability. Increased particle diameter will, in turn, reduce the column efficiency, which may impair the resolution of the compounds of