Gas Chromatography - GC Columns > The Capillary or Open Tubular Column > Dynamic Coating > Page 29
Open tubular columns can be coated internally with a liquid stationary phase or with polymeric materials that can be polymerized to form a relatively rigid, internal polymer coating. There are two methods for coating a capillary column the dynamic method and the static method.
A plug of solvent containing the stationary phase is placed at the beginning of the column. The strength of the solution, among other factors, determines the thickness of the stationary phase film. In general the film thickness of an open tubular column ranges from 0.25 mm to about 1.5 mm.
Figure 15. The Dynamic Coating Procedure for an Open Tubular Column
In practice, a 5% w/w of stationary phase in the solvent will produce a film thickness of about 0.5 mm. However, this is only approximate, as the film thickness is also determined by the physical properties of the surface, the solvent and the stationary phase. The coating procedure is depicted in figure 15. After the plug has been run into the front of the column (sufficient to fill about 10% of the column length), pressure is applied to the front of the column to force the plug through the column at 2-4 mm per second (it will take about 5.5 hours for the plug to pass through a 60 m column). When the plug has passed through the column, the gas flow is continued for about an hour. The gas flow must not be increased too soon, or the stationary phase solution on the walls of the tube is displaced forward in the form of ripples, which produces a very uneven film. After an hour the flow rate can be increased and the column stripped of solvent. The last traces of the solvent are removed by heating the column above the boiling point of the solvent at an increased gas flow rate. Complete solvent removal can be identified by connecting the column to a detector and observing the baseline drift of the detector.