Gas Chromatography - GC Columns > The Packed GC Column > Adsorbents > Page 20

 

Adsorbents

There are two types of packing employed in GC, the adsorbents and the supports, on which the stationary phase is coated. There are both inorganic and organic types of GSC adsorbents, each of which have specific areas of application. All are ground and screened to provide a range of particle sizes that extend from about 30/40 mesh to 100/120 mesh. In general, the smaller the particle size the higher the column efficiency, but the packing procedure is more difficult. It is also essential that the particle size range should be as narrow as possible. Packing materials that have a wide size range not only produce columns with poor efficiencies, but again, are also far more difficult to pack.

Alumina, in an activated form, is used to separate the permanent gases and hydrocarbons up to about pentane. Alumina is usually activated by heating to 200˚C for about an hour. A common particle size is about 100/120 mesh and the pore size range from about 1 to 100,000. Silica gel in spherical form (prepared by spraying a neutralized silicate solution (a colloidal silica sol) into fine droplets, allowing the silica gel to be formed, and subsequently drying the droplets in a stream of hot air). Silica is produced with a wide choice of surface areas and porosity's, which can range from about 750 m2/g and a mean pore size of 22 , to a material having a surface area of only 100m2/g and a mean pore diameter of 300 . It is used for the separation of the lower molecular weight gases and some of the smaller hydrocarbons. In a specially prepared form, silica can be used for the separation of the sulfur gases, hydrogen sulfide, sulfur dioxide and carbon disulfide.

Molecular sieves are used for the separation of small molecular weight gases largely by exclusion. The naturally occurring aluminosilicates are called zeolites, the synthetic zeolites are the Linde Molecular Sieves of which there are a number of different types available for specific applications. The zeolites have a crystalline structure which does not collapse when dehydrated. When water is removed from the crystals, channels of uniform dimensions are left within the structure which becomes very porous and the size of the channels changes only slightly with temperature. Molecular sieves are used to separate substances of different molecular size and shape, e.g. straight chain hydrocarbons can be separated from their branched chain isomers. The molecular sieves designated 5A and 13X are commonly used for the separation of hydrogen, oxygen, nitrogen, methane and carbon monoxide and also argon, neon and the other rare gasses.