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.
Dynamic Coating
Dynamic coating is carried out by injecting a plug of solvent in which the stationary phase is dissolved into the first meter of the column. Among other factors, the concentration of stationary phase in the solution, determines the thickness of the stationary phase film. The film thickness of stationary phase in an open tubular column can vary considerably and may range from 0.10 m to about 0.75 m. As an approximate guide, 5% w/w of stationary phase in the solvent will provide a stationary film thickness of about 0.5 m. However, this must be considered somewhat imprecise as the nature of the surface and type of the solvent will also effect the coating process and consequently the eventual film thickness. The coating procedure is depicted in figure 10.
Figure 10. The Dynamic Coating Procedure for an Open Tubular Column
The procedure is as follows. A plug of stationary phase solution is run into the front of the column (sufficient solution should be added to fill about 10% of the column length) and then connected to a gas supply. Pressure is applied to the front of the column and adjusted so that the plug velocity through the column is about 3 mm/second. After the plug has passed through and out of the column, the gas flow is continued for about an hour. The gas flow must not be increased too rapidly after the plug has left the column otherwise the film 0f stationary phase solution at the walls of the column will be displaced forward in the form of ripples and consequently produce a very uneven film. After an hour, the flow rate can then be increased and the column stripped of solvent by evaporation. This procedure requires some experience if evenly coated columns of the desired film thickness are to be produced. However, this method is the simpler of the two and the thickness of the stationary phase film can be controlled. The dynamic coating procedure is probably the most convenient for the inexperienced.
