Gas Chromatography - GC Columns > Open Tubular Column Types > Page 32

Stationary phases that are polymeric can sometimes be formed on the wall surface by depositing the monomers or dimers on the walls and then initiating polymerization either by heat or an appropriate catalyst. This locks the stationary phase to the column wall and is thus completely immobilized. Polymer coatings can be formed in the same way using dynamic coating. The techniques used for immobilizing the stationary phases are also highly proprietary and little is known of the methods used by companies that manufacture the columns. In any event, most chromatographers do not want the trouble of coating their own columns and prefer to purchase proprietary columns.

Very difficult separations can be achieved using the capillary column, and in a relatively short time. An example of the separation of a complex mixture on a capillary column is shown in figure 17. The column used was designated as a VOCOL column and was 60 m long, 0.75 mm I.D. and carried a film of stationary phase 1.5 micron thick. The column was held a 10˚C for 6 minutes and then programmed to 170˚C at 6˚C per minute. The carrier gas was helium at a flow rate 10 ml/min. The detector employed was the FID. This chromatogram demonstrates the clear advantages of capillary columns over packed column. Not only does the column produce exceeding high efficiencies but they are also achieved with reasonable separation times.

Open Tubular Column Types

Open Tubular columns are broadly split into two classes, the wall coated open tubular columns or WCOT Columns (which have already been described and are by far the mot popular,) and the porous layer open tubes or PLOT Columns. The two types of column are shown diagramatically in figure 18. The PLOT columns are largely used for gas analysis and the separation of low molecular weight hydrocarbons. The external diameter of PLOT columns range from 320 to 530 mm with a porous layer that can be 5 to 50 mm thick.