Capillary Chromatography - Capillary Tube Connections 1


A vacuum is then applied to the other end of the column. As the solvent evaporates, the plug front retreats back down the tube leaving a coating on the walls. A diagram of the static coating procedure is shown in figure 11.


This type of coating gives more reproducible films of stationary phase and films whose thickness varies little along the column. Most commercial capillary columns are prepared in this way and are then mounted in a supporting metal framework. A photograph of a modern capillary column is shown in figure 12.


Capillary Tube Connections


Columns are connected to the chromatograph by means of suitable compression fittings. Such fittings must be easy to assemble and must have appropriately low dead volumes. The basic single-ferrule type compression fitting arrangement is shown in figure 13A.


The important component of the fitting is the reducing ferrule. The ferrule usually consists of to parts a front and backing ferrule. The ferrules must be soft and are manufactured from a range of polymer-graphite combinations. The materials must be chosen to withstand the necessary high temperatures that are employed in capillary column gas chromatography. The cone shaped end to the ferrule allows it to be compressed against the capillary tube and thus form an airtight seal.


The connector shown in figure 13 B is used to either connect columns in series or to connect columns to injection system such as the retention gap sampling system. There is a central double ended ferrule in which the two sections of capillary tube are arranged to butt together in the center. The ferrule is then captured inside an outer metal tube with two compression nuts at either end. As a result of the tapered ends of the ferrule, the tubes inside are sealed to the ferrule as the cones shrink against the walls of the tubes under the applied pressure from either end.