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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 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

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

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 column efficiency 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

, the capillary column is by far the most popular in current use in gas chromatography.   The capillary column was invented by Golay, the theory of which was presented at the 1958 Symposium on Gas Chromatography and published in 1958 (1). The efficiencies provided by the capillary columns were, at that time, startling, to say the least. It will be seen when the theory of capillary columns is considered, the plate height of a capillary column is not very different from that of the packed column, the main advantage of the capillary column being its low flow impedance. As result of its greater permeability relative to the packed column, much longer column scan be used thus, providing much higher efficiencies. In addition, because there is no multipath term (see Book 9 of this series) the optimum velocity is much higher and, thus, the longer columns do not proportionally increase the elution time. Capillary columns (with very few exceptions) are exclusively used for analytical

steel frit) having a precisely controlled internal diameter and finish. The column contains a close fitting piston. The piston, which is mounted on a constant-pressure 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

increases both the maximum sample volume and the maximum sample mass. It is also seen that increasing the column length will also increase 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