Static Coating

The alternative procedure of static coating is carried out by first, filling the entire column with a solution of the stationary phase, sealing one end and applying a high vacuum to the other. As the solvent evaporates under the high vacuum, the front of the plug retreats back down the capillary tube leaving a coating of stationary phase on the walls. A diagram of the static coating procedure is shown in figure 49.

Figure 49. The Static Method for Coating Open Tubular Columns

This method of coating is probably more repeatable than the dynamic method but, in general, produces columns having very similar performance. The static coating procedure is time consuming and often needs to be carried out overnight; in the static coating method the film thickness is also controlled by the concentration of the stationary phase in the solvent. Unfortunately, the optimum concentration for coating cannot be given and that concentration that will provide a defined film thickness must be determined by experiment. As with the dynamic method of coating, the film thickness will vary with the column type, its method of deactivation, the nature of the solute and the nature of the stationary phase.

Some stationary phases that are polymeric in nature can be formed in situ directly on the walls of the tube. In this procedure, the monomers or dimers are deposited on the column wall by one of the coating procedures described above and then polymerization is initiated either by heat, by UV light or by the use of an appropriate catalyst.