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the original polymeric silicic acid remain. These residual hydroxyl groups confer upon silica gel its polar properties. These hydroxyl groups react with the silane reagents to form bonded phases. The silica surface is quite complex and contains more than one type of hydroxyl group, strongly bound or 'chemically' adsorbed water and loosely bound or 'physically adsorbed' water. There are three types of hydroxyl group. The first is a single hydroxyl group attached to a silicon atom which has three siloxane bonds joining it to the gel matrix. The second is one of two hydroxyl groups attached to the same silicon atom which, in turn, is joined to the matrix by only two siloxane bonds. These twin hydroxyl groups are called Geminal hydroxyl groups. The third is one of three hydroxyl groups attached to a silicon atom which is now only joined to the silica matrix by only a single siloxane bond. An example of each type of hydroxyl bond is shown in Figure 30. Figure 30. Different Forms of

Silica gel adsorbs relatively large quantities of water  which was explained on the basis of multi-layer adsorption. This concept was supported by Vleeskens (18,19) and experimentally validated by gravimetric measurements (20). An example of one type of multi-layer adsorption is shown in figure 32.   Figure 32. Multi-Layers of Physically Adsorbed Water The multi-layer adsorption depicted in figure 32 is much over simplified, as adsorption could also take place the surface of siloxane bonds as well

nbsp; Nonpolar or dispersive stationary phases are employed for the separation of hydrocarbons (e.g. OV101, which is also a polyalkyl-siloxane, is widely used in packed columns). The flow velocity of 20 cm/sec., appears to have been taken from the ratio of the column length to the dead time. Thus, due to the pressure correction the actual effective linear velocity would be much less than that (see Dispersion in Chromatography Columns ). Helium was used as the carrier gas which was necessary to realize the high efficiencies with reasonable analysis times. The FID detector provided the necessary to wide quantitative

Carborane Based Stationary Phases   An example of the use of a carborane phase at high temperature to separate the components of POLYWAX 1000 using a wide ranged temperature program shown in figure 37.     Courtesy of SGE Ltd.   Figure 10. The GC Analysis of POLYWAX 1000   The stationary phase used was the SGE HT5, a carborane modified siloxane stationary phase carrying 5 % phenyl groups. capable of operating at 480ûC. The column (aluminum clad as opposed to imide coated) was 12 m long, 0.53 mm I.D. and carried a film of stationary phase 0.15 mm thick. Initially, the column temperature was 200 ûC which was then raised to 480ûC at a rate of 10ûC/min.. A split injector was used to place the sample on the column. Hydrogen was used as the carrier gas at an inlet pressure of 5 psi. and the detector employed was an FID.  

described. The a-cyclodextrin structure is depicted in figure 20. The columns are usually 30 or 60 m long 0.25 mm I.D. and have an operating temperature range of 30˚C to 250˚C. Both the a and b forms are commercially available and both have been used very satisfactorily for the separation of the optical isomers of different flavors and fragrances. In order to employ the cyclodextrins as stationary phases for GC the permethylated a- or b-cyclodextrins are often embedded in a siloxane matrix (e.g. 35% phenyl-65% methyl polysiloxane) which is deposited on the walls of fused quartz capillary tubes

The procedure as outlined by Supelco Inc. is as follows. 1 g of tobacco (12% moisture) was placed in a 20 ml head space vial and 3.0 ml of 3M potassium chloride solution added. The fiber was coated with polydimethyl siloxane (a highly dispersive adsorbent) as a 100 mm film. The vial was heated to 95˚C and the fiber was left in contact with the head space for 30 min. The sample was then desorbed from the fiber for one minute at 250˚C.   Courtesy of Supelco Inc.   Figure 45 A Chromatogram of Tobacco Head Space   The separation was carried out on a column 30 m long, 250 mm I.D. carrying a 0.25 mm thick film of 5% phenylmethylsiloxane. The