is employed. Nevertheless, a liquid thermostatting medium introduces difficulties when changing columns and with column detector connections and is thus, not commonly used. The temperature program can be controlled by a microprocessor incorporated in the programmer or can be controlled from a central computer that governs the operation of the whole instrument. The GC column can be a packed or open tubular and thus the oven must be capable of taking both. The open tubular column is by far the most popular partly because they are considered state of the art and not because they necessarily provide an improved performance. Open tubular columns will always provide the highest efficiencies but, if correct operating procedures are adopted, in general, analyses carried out on packed columns, are likely to provide greater accuracy and better precision and repeatability. Packed GC columns are usually made of stainless steel or glass and open tubular column almost

through  column (1) to port (5), from port (5) to port (4) and out to the detector. Thus, the separation will take place in column (1). The ports connected to column (2) are themselves connected by the third slot and thus isolated. When the valve is rotated, the situation is depicted on the right hand side of figure (7); port (1) is connected to port (2), port (3) connected to port (4) and port (5) connected to port (6). This results in the mobile phase from either a sample valve or another column entering port (1) passing to port (2) through column (2) to port (3), then to port (4) and then to the detector. The ports (5) and (6) are connected, this time isolating column (1). This arrangement allows either one of two columns to be selected for an analysis or part of the eluent from another column pass to column (1) for separation and the rest passed to column (2). This system, although increasing the complexity of the column system renders the chromatographic process far more

and it becomes even more difficult and expensive for columns of wider diameter. There are other problems that need addressing, once packed, the practical lifetime of a column is also uncertain. The changes in performance of a preparative HPLC column that occurs with time depends upon the stability of the packed bed. Frequently, the bed settles after operation for even a short time and the top of the column needs to be repacked. Sometimes channels are formed in the bed, in which case the entire column has to be repacked. The rate of settling again depends upon the diameter of the column. This bed instability arises because there is a significant change in wall support as the column diameter increases. In analytical columns the walls are relatively close to the center of the column and 'bridges' of packing particles can be formed across the bed, as shown in Figure 16. These bridges allow the longitudinal forces acting on the packing within the column to be dissipated to the walls. When

The Maximum Sample Volume that Can Be Placed on a Chromatographic Column A sample placed on a column will initially contribute a starting variance to the peak which will continually increase during its passage through the column. The extent of the initial dispersion will depend on the sample volume and it is therefore important to know the maximum volume of charge that can be placed on the column before it significantly degenerates the column performance. Let a volume (Vi) be injected onto a column. The volume (Vi) will be dispersed on the column in the form of a rectangular distribution. The final eluted peak will have a total variance made up of that produced by the column and other parts of the mobile phase conduit system plus that due to the finite sample volume. For convenience, the extra column dispersion (except for that from the finite sample volume), will be considered negligible.  It is now possible to apply

separation of a complex mixture on a capillary column is shown in figure 17. The column used was designated as a VOCOL column and was 60 m long, 0.75 mm I.D. and carried a film of stationary phase 1.5 micron thick. The column was held a 10˚C for 6 minutes and then programmed to 170˚C at 6˚C per minute. The carrier gas was helium at a flow rate 10 ml/min. The detector employed was the FID. This chromatogram demonstrates the clear advantages of capillary columns over packed column. Not only does the column produce exceeding high efficiencies but they are also achieved with reasonable separation times. Open Tubular Column Types Open Tubular columns are broadly split into two classes, the wall coated open tubular columns or WCOT Columns (which have already been described and are by far the mot popular,) and the porous layer open tubes or PLOT Columns. The two types of column are shown diagramatically in figure 18. The PLOT columns are largely

and the stronger the ethanol concentration, the smaller the magnitude of (k'). Thus, more theoretical plates will be required to resolve the enantiomers and thus a longer column will be necessary. At a temperature of 5˚C and at an ethanol concentration of 5%v/v, the column need only be about 5 mm long(a length of column that is impractical to pack and operate). Contemporary columns, shorter that 2 cm are extremely difficult to operate efficiently.     Figure 24 The Minimum Column Length that will Produce the Required Efficiency The minimum column length that will provide the minimum analysis time for this particular separation is not in the practical range of column lengths normally available. Consequently, the optimum column length must be a compromise between, that which is theoretically desirable, and that which is practically possible, and thus the shortest column available would be chosen.