column slowly increases with use, as the stationary phase distributes itself more evenly throughout the packing. It may take several weeks of use for the column to give a constant maximum efficiency.   The filtration method gives a packing with the stationary well distributed over the support but the loading can not be accurately calculated. A known mass of stationary phase is dissolved in sufficient solvent to provide excess liquid when mixed with a weighed amount of the support. The mixture is filtered under vacuum and the volume of the filtrate measured. From the volume of filtrate, the amount of solvent remaining on the support can be calculated and hence this stationary phase loading can be accessed. The bed is then sucked dry, the solvent evaporated and the coated support packed into the column. The amount of stationary phase on the support is not determined accurately by this method due to solvent losses by evaporation

nbsp; Experimental Support for the Sorption and Displacement Process Scott and Kucera (15) carried out some experiments that demonstrated, sorption and displacement interaction. They dispersed about 10 gram of silica gel in a solvent mixture made up of 0.35%w/v of ethyl acetate in n-heptane. From the adsorption isotherm in figure 21, it is seen that at 0.35w/v of ethyl acetate more than 95% of the first layer of ethyl acetate has been established and very little of the second layer was formed. This

quantitative losses can occur. In the above example, the effect of the adsorptive sites on the support is reduced by blocking them with phosphoric acid. Phosphoric acid is very involatile and thus can tolerate the high temperature and although it is active enough to block the adsorption sites it is not active enough to cause sample decomposition. It is seen that the peaks exhibit excellent symmetry for free acids. Teraphthalic acid has also been used for this purpose to deactivate the support. The column was glass, 3 m long and 2 mm in diameter and packed with a silicone polymer, SP-216-PS on 100/120 mesh Supelcoport which is a proprietary support that has already been deactivated and treated with phosphoric acid. The column was temperature programmed from 130oC to 200oC at 15oC/min. Nitrogen was used as the carrier gas at a flow rate of 20 ml/min.. The separation is effective, relatively rapid, and accurate quantitative results should be easily obtainable from the

the aromatic nuclei in the polymer. Supports for GLC There have been a number of materials used as supports for packed GC columns including, Celite (a proprietary form of a diatomaceous earth), fire-brick (calcined Celite), fire-brick coated with metallic silver or gold, glass beads, Teflon chips and polymer beads. Today however, the vast majority of contemporary packed GLC columns are filled with materials that are either based on of Celtic or polystyrene beads as a support. Diatomaceous supports comprise the silica skeletons of microscopic animals that lived many millions of years ago in ancient seas and lakes. As food transfer through the cells could only occur by diffusion, the supporting structure had to contain many apertures through which the cell nutrients could diffuse. This type of structure is ideal for a gas chromatography support, as rapid transfer by diffusion through the mobile and stationary phases is an essential requisite for the

describes the temperature change in a theoretical plate, in terms of the physical properties of the plate and the volume flow of mobile phase that passes through it must be derived. Consider the (n)th theoretical plate in a GC column, as depicted in Figure 21. The properties of the plate are defined as follows, vg is the volume of gas in the plate, vl is the volume of liquid (stationary phase) in the plate, vS is the volume of support in the plate, Sl is the specific heat of the stationary phase, SS is the specific heat of the support, rl is the density of the stationary phase, rS is the density of the support, Xl(n) is the concentration of solute in the stationary phase in plate (n), Xg(n) is the concentration of solute in the mobile phase (gas) in plate (n), q is the

adsorption results from the presence of trace quantities of heavy metals such as iron. which can be largely removed by acid washing prior to silanization. All three types of support are commercially available. None of these supports, however, are completely devoid of adsorptive properties and in may cases the effect of the residual adsorption must be further reduced by suitable stationary phase additives.   To try to completely eliminate adsorption effects from the support, Teflon was explored as a possible alternative to a diatomaceous earth. Teflon powder proved to have little adsorption, but also proved to be extremely difficult to pack into a column. So difficult, that it is very rarely used in general GLC analyses. Its inert character makes it useful for the separation of certain highly corrosive materials. It has a temperature limit of about 250˚C. Glass beads have also been used as supports for packed GC columns and, if silanized