Vacuum Other than for general purposes, vacuum has been used in two ways in chromatography procedures. Firstly, it has been used for degassing solvents in liquid chromatography. Dissolved gasses, usually nitrogen and oxygen from the air, tend to be evolved in the mobile phase as the pressure is reduced when the mobile phase leaves the liquid chromatography column and enters the detector. Gasses in the mobile phase in the detector can produce completely unacceptable noise and, thus, must be removed. The dissolved gasses were originally removed under vacuum but, unfortunately, are soon replaced if the solvent is left in contact with air at atmospheric pressure. For this reason degassing is now usually carried out by bubbling helium through the mobile phase reservoirs. Secondly, vacuum is used in the thermionic detector. This consists of a device, very similar in design to the thermionic valve which is attached to a vacuum and a small quantity of the eluent from a gas chromatography colum allowed to bleed through it. Helium is used as the carrier gas. The presence of solute vapor causes the thermionic current to fall. This type of detector tends to become contaminated rather readily. In the very early gas chromatographs (the fIrst commercial chromatograph was manufactured by Griffin and George) the elution process was not fully understood and a vacuum was applied to the column outlet to assist in carrier gas flow. The vacuum was soon found to be unnecessary and eliminated.

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Author: RPW Scott Book:Gas Chromatography Detectors
Section:GC-Detectors   Less-Common-Detectors   Thermionic-Ionization

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

GC-Detectors   Less-Common-Detectors   Thermionic-Ionization

Author: RPW Scott Book:Liquid Chromatography
Section:HPLC   Basic-HPLC   Gradient-Programmer   High-Pressure

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

HPLC   Basic-HPLC   Gradient-Programmer   High-Pressure

Author: RPW Scott Book:Gas Chromatography
Section:GC   GC-Columns   Packing

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

GC   GC-Columns   Packing

Author: RPW Scott Book:Principles and Practice of Chromatography
Section:Principles   TLC   Sample-Application

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

Principles   TLC   Sample-Application

Author: RPW Scott Book:Gas Chromatography
Section:GC   GC-Columns   Capillary   Static-Coating

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

GC   GC-Columns   Capillary   Static-Coating

Author: RPW Scott Book:Capillary Chromatography
Section:Capillary   Apparatus   Connections

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

Capillary   Apparatus   Connections

 
 
 

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