Dynamic Coating

In dynamic coating, a plug of solvent, containing the stationary phase dissolved in an appropriate solvent, is placed in the beginning of the column. The strength of the stationary phase solution (among other factors) determines the thickness of the stationary phase film on he surface of the coated column. In general the film thickness ranges from 0.10 m to about 0.75 m. As an approximation, a 5% w/w solution of stationary phase in the solvent will provide a stationary film thickness of about 0.5 m. This data can only be considered approximate as the film thickness will also be determined by the nature of the surface, the nature of the solvent and the nature of the stationary phase. The coating procedure is depicted in figure 48.

Figure 48. The Dynamic Coating Procedure for an Open Tubular Column

A small quantity 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). A gas pressure is then applied to the front of the column so that the plug velocity through the column is about 3 mm/second. After the plug has passed through the column the gas flow is continued (at the same rate) for about an hour. It is very important that the gas flow is not increased too soon, as this will cause the stationary phase solution adhering to the walls of the tube to be displaced forward in the form of ripples, resulting in a very uneven film and poor efficiency. An hour after the plug of solvent has been expelled, the flow rate can be increased and the column stripped of solvent by evaporation. The satisfactory use of this procedure to provide evenly coated columns of the desired film thickness requires some practice but is probably the simpler of the two coating procedures to use.