Gas Chromatography - GC Columns > The Capillary or Open Tubular Column > Page 28
Desty et al. (4), tried to eliminate the activity of the open tubular column surface by developing the first silica-based columns and invented an extremely clever device for drawing soft glass capillaries. Desty produced both circular rigid soft glass and circular rigid Pyrex capillary columns, but their permanent circular shape, made them difficult to fit to unions connecting columns to injector and column to detector. By careful surface treatment the rigid glass tubes could be coated with polar stationary phases such as CARBOWAX®. Dandenau (5) introduced flexible fused silica capillary columns using the quartz fiber drawing technique. The solid quartz rod used in quartz fiber drawing was replaced by a quartz tube and the drawing rates adjusted appropriately. The quartz tubes had to be coated on the outside with polyimide to prevent moisture attacking the surface and producing stress corrosion. Coating the capillary tube with a polyimide polymer immediately after drawing prevents moisture coming in contact with the surface and thus stabilizes the tube. Soft glass capillaries can be produced by the same technique at much lower temperatures (6) but the tubes are not as mechanically strong or as inert as quartz capillaries. Surface treatment is still necessary with fused quartz columns to reduce adsorption and catalytic activity and also make the surface sufficiently wettable to coat with the selected stationary phase. The treatment may involve washing with acid, silanization and other types of chemical treatment, including the use of surfactants.
Deactivation procedures used for commercial columns are kept highly proprietary. However, a deactivation program for silica and soft glass columns that is suitable for most applications would first entail an acid wash. The column is filled with 10% w/w hydrochloric acid, the ends sealed and the column is then heated to 100˚C for 1 hour. It is then washed free of acid with distilled water and dried. This procedure is believed to remove traces of heavy metal ions that can cause adsorption effects. The column is then filled with a solution of hexamethyldisilazane contained in a suitable solvent, sealed, and again heated to the boiling point of the solvent for 1 hour. This procedure blocks any hydroxyl groups that were formed on the surface during the acid wash. If the column is to be coated with a polar stationary phase, it may be advantageous to employ a polar or semipolar reagent as opposed to the dispersive silicone to facilitate coating. The column is then washed with the pure solvent, dried at an elevated temperature in a stream of pure nitrogen and is then ready for coating.