nbsp; The sensor consisted of a vacuum tube containing a filament, grid and anode, very similar in form to the thermionic triode valve. The tube was operated under reduced pressure and an adjustable leak was arranged to feed a portion of the column eluent into the gauge. The sensor was fitted with its own pumping system and vacuum gauge and the usual necessary cold traps. Helium was used as a carrier gas and the grid collector–electrode was set at +18 V with respect to the cathode and the plate at -20 V to collect any positive

fitted with a gas diffuser through which helium can be bubbled. Many solvents and solvent mixtures (particularly aqueous mixtures) contain significant amounts of dissolved nitrogen and oxygen from the air. These gasses can form bubbles in the chromatographic system that cause both serious detector noise and loss of column efficiency. As helium is very insoluble in most solvents, it purges the oxygen and nitrogen from the solvent but does not produce bubbles in the system itself. Applying a vacuum to the reservoir is not a permanent solution to dissolved air as, on releasing the vacuum to allow the solvent to pass to the pump, air again dissolves in the solvent. The solvent is filtered through a stainless steel or sintered glass filter to remove any solid contaminants. Depending on the type of solvent programmer that is employed, the supply from each reservoir may pass either to a pump or to a valved blending device. Solvent reservoirs are not usually thermostatted but, when

to thermostat. The coiled column although more difficult to pack has been readily accepted due to the compact nature of their design. To obtain adequate efficiencies, however, a special packing procedure had to be developed. The apparatus used is shown in figure 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

The device consists of a plate with a small indentation, at the center of which is an aperture that can be connected either to a vacuum or to a source of gas pressure. A polymer film is placed over the plate surface and a vacuum applied to suck the film into the indentation. A small quantity of sample solution is placed in the indentation and the solvent evaporated. This procedure is repeated until sufficient sample is present on the film for a satisfactory TLC separation. It is important that the sample is not evaporated to dryness as the transfer of solid materials to the thin layer plate can be very inefficient

nbsp; Static Coating The entire column is filled with a solution of the stationary phase and one end is connected to a vacuum pump. As the solvent evaporates, the front retreats back down the tube leaving a coating on the walls. A diagram of the static coating procedure is shown in figure 16. The column is filled with a solution of stationary phase having a concentration appropriate for the deposition of a film of the desired thickness. Again the required concentration will depend on the stationary phase, the solvent, the temperature and the condition of the wall surface. Unfortunately, the optimum solvent

nbsp; A vacuum is then applied to the other end of the column. As the solvent evaporates, the plug front retreats back down the tube leaving a coating on the walls. A diagram of the static coating procedure is shown in figure 11.   This type of coating gives more reproducible films of stationary phase and films whose thickness varies little along the column. Most commercial capillary columns are prepared in this way and are then mounted in a supporting metal framework. A photograph of a modern