Capillary Chromatography - Open Tubular Columns
The majority of contemporary open tubular columns are made from fused quartz although, even today, in some applications (mainly hydrocarbon mixtures) stainless steel columns are still used providing equivalent performance. Nevertheless, fused quartz columns are perceived as "state of the art" columns. The internal surface of an open tubular column normally requires some deactivation and/or cleaning before it can be coated with stationary phase. Deactivation procedures (although usually very simple) are usually considered as highly proprietary. Under certain circumstances samples may need the column to be specially treated including particular column deactivation procedures, but most samples, can be successfully analyzed on columns that are deactivated by relatively simple clean-up procedures an example of which is the following.
A simple clean-up 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 heated to 100 ̊C for 1 hour. It is then washed free of acid with distilled water and dried. This procedure is thought to remove traces of heavy metal ions that are believed to cause adsorption effects. The column is then filled with a solution of hexamethyldisilazane in a suitable solvent, sealed, and heated to the boiling point of the solvent for 1 hour. The reagent reacts with any surface hydroxyl groups that were formed on the fused silica or glass surface during the acid wash and, in effect, chemically 'blocks' them from chromatographic availability. It may be advantageous to employ a polar or semipolar reagent as opposed to the dispersive silicone, if the column is to be subsequently coated with a polar stationary phase. The column is then washed with the pure solvent, dried at an elevated temperature in a stream of pure nitrogen and will then be ready for coating.
Open tubular columns can be coated internally with an appropriate stationary phase which should be dissolved in a suitable solvent. Alternatively, the surface can be coated with monomeric materials that are subsequently polymerized to form a relatively rigid polymer film on the column walls. There are two primary procedures for coating the stationary phase as a surface film on a capillary column, one is called dynamic coating and the other, static coating, the latter being the most commonly used.