Gas Chromatography - GC Columns > The Packed GC Column > Supports for GLC > Page 21
Carbon is also used as an adsorbent of which there are two types. The high surface area active carbon and the graphitized carbon (surface areas ranging from 5 m2/g to about 100 m2/g). The high surface area carbon, (ca 1000 m2/g) is used for the separation of the permanent gases and may need special treatment to modify its activity. The graphitized carbon adsorbents are much less active and separations appear to be based largely on exclusion. Macroporous Polymers such as the packings founded on the co-polymerization of polystyrene and divinylbenzene are also popular GC adsorbents. The extent of cross-linking determines its rigidity and the greater the cross-linking the harder the resin becomes until, at the extreme, the resin formed is very brittle. The macro-porous resin consists of resin particles a few microns in diameter, which in turn are composed of a fused mass of polymer micro-spheres, a few Angstroms in diameter. Consequently, the resin polymer has a relatively high surface area as well as high porosity. They exhibit strong dispersive type interaction with solvents and solutes with some polarizability arising from 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 efficient operation of the column. The original Celite material is too friable and the brickdust too active, and thus a series of modified Celites had to be introduced. There are two processes used to modify Celite. One was to crush, blend and press the Celite into the form of a brick and then calcine it at a temperature of about 900˚C. Under these conditions some of the silica is changed into cristobalite and traces of iron and other heavy metals interact with the silica causing the material to become pink in color. This material is sold under the trade name of Chromosorb P. The second process involves mixing the Celite with sodium carbonate and fluxing the material at 900˚C. This causes the structure of the Celite to be disrupted and the fragments adhere to one another by means of glass formed from the silica and the sodium carbonate. As the original Celite structure is disrupted, the material exhibits a wide range of pore sizes which differs significantly from the material that was calcined in the absence of sodium carbonate. This materials is sold under the name of Chromosorb W together with two similar materials called Chromosorb G and Chromosorb S. The residual deleterious adsorptive properties of the support are due to silanol groups on the surface and these can be removed by silanization. The support is treated with hexamethyldisilazane which replaces the hydrogen of the silanol group with a trimethylsilyl radical. The reaction proceeds as follows,