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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

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. Dynamic Coating 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

The liquid flows over the plate and the effective path length is the film thickness peculiar to the flowing solvent layer. The UV lamp is situated on one side of the plate and the photo cell on the other side, each facing normal to the plate surface. A reference photo cell (not shown) is placed close to the lamp to compensate for changes in light intensity that may arise from variations in lamp emission. Due to the very short path length (the thickness of the film of column eluent) the detector has the required low sensitivity and low flow impedance. The detector can operate very satisfactorily at concentrations as great as 10-2 g/ml (1% w/w), which is ideal for preparative chromatography. Another advantage of the device for preparative work, is its very low flow impedance and thus can easily cope with the high flow rates used in preparative LC. The film thickness does depend, among other things, on the column flow rate and thus

;                             (34) and letting Ds = xDm,     (35)   It is seen from equation (35) that the optimum velocity is, directly proportional to the diffusivity of the  solute in the mobile phase. To a lesser extent it also appears to be inversely dependent on the particle diameter of the packing and the film thickness of the stationary phase. The film thickness of the stationary phase is determined by the physical form of the packing, that is, in the case of silica gel, the nature of the surface and in the case of a reverse phase, on the bonding chemistry

work a portion of the eluent is split from the main stream, diluted with more mobile phase and then passed through the detector. In practice, this is a rather awkward procedure. As seen in figure 27 the column eluent passes through a delivery tube and onto a supporting plate that is usually made of fused quartz, so that  adequate UV light can reach the photo cell placed on the other side of the plate. The liquid flows over the plate and the effective path length of the sensor will be the film thickness which will be unique to the particular solvent used as the mobile phase. The UV lamp is situated above the upper side of the plate and the photo cell on the lower side. A reference photo cell (not shown) is situated close to the lamp and the output used to compensate for changes in light intensity from variations in lamp emission. The short path length results in a low sensitivity and the detector can operate satisfactorily at concentrations as high as 10-2 g/ml (1% w/w), which is

nbsp; Static Coating The entire column is filled with a solution of the stationary phase and one end is connected to a vacuum pump. As the solvent evaporates, the front retreats back down the tube leaving a coating on the walls. A diagram of the static coating procedure is shown in figure 16. The column is filled with a solution of stationary phase having a concentration appropriate for the deposition of a film of the desired thickness. Again the required concentration will depend on the stationary phase, the solvent, the temperature and the condition of the wall surface. Unfortunately, the optimum solvent concentration is not theoretically predictable and requires some preliminary experiments to be carried out to determine the best coating conditions.   Figure 16. The Static Method for Coating Open Tubular Columns After filling, one end of the column is sealed