Open Tubular Columns

The majority of contemporary open tubular columns used today are made from fused quartz, although in some applications stainless steel columns or even soft glass can be successfully used and provide equivalent performance. Fused quartz columns are perceived as the "state of the art" columns and are extremely effective for chiral separations in GC. Quartz tubing is drawn at extremely high temperatures using argon arc heating systems. After the tube is drawn, the outside skin of the tube cools more rapidly than the inside causing stresses in the outside skin. These stresses allow the adsorption of water on the surface to initiate cracks in the fused quartz and render it extremely friable. To eliminate this problem the outside of the capillary tube is coated with a polyimide polymer immediately after drawing. This layer of polyimide is thought to prevent contact with water and, thus, stabilizes the tube and renders it mechanically strong. Rigid soft glass tubes do not need coating to prevent fracture but thin, flexible soft glass tubes do, unless very carefully annealed immediately subsequent to drawing.

After drawing the internal surface of an open tubular column requires deactivation and/or cleaning before it can be coated with stationary phase. The cleaning and deactivation procedures used in the preparation of commercially available columns are seen by many to be "shrouded with mystery". This is partly because the procedures are usually considered as highly proprietary. It is true that certain special samples may, indeed, need special treatment and complete column deactivation, most samples, however, including those for chiral analysis, can be analyzed on columns that are deactivated by a relatively simple clean-up procedure. An uncomplicated column preparation procedure for silica and soft glass columns would first entail an acid wash.

The column is filled with 10% w/w hydrochloric acid, the ends sealed and the column heated to 100^oC for 1 hour. The column is then washed free of acid with distilled water and dried. Treatment with hydrochloric acid is thought to remove traces of heavy metal ions on the internal surface of the column that are believed to cause adsorption effects.

The next procedure is to fill the column with a solution of hexamethyldisilazane, seal it, and then heat it to the boiling point of the solvent for 1 hour. Hexamethyldisilazane reacts with the surface hydroxyl groups formed on the fused silica or glass surface during the acid wash and, in effect, chromatographically 'blocks' them. For some applications, if the column is to be subsequently coated with a polar stationary phase, it may be advantageous to employ a polar or semipolar silanizing reagent as opposed to the dispersive silane. After silanization the column is then washed with the pure solvent and dried at an elevated temperature in a stream of pure nitrogen. Open tubular columns are coated internally with the appropriate stationary phase dissolved in a suitable solvent, or, alternatively, with monomeric materials that are subsequently polymerized to form a relatively rigid polymer film on the column walls. There are two methods for depositing the stationary phase as a surface film on a capillary column, dynamic coating and static coating.