The
Capillary Tube
The critical
component of a capillary electrophoresis instrument is the capillary tube
itself that is usually made of silica coated with polyimide (the same type of
material used in gas chromatography capillary columns) and has an internal
diameter ranging from 0.2 to 0.01 mm. The smaller the diameter the less the
heat is generated which will degrade the separation. Very small diameters will
allow much higher voltages to be used permitting the use of considerably longer
tubes. Teflon and glass has been used for capillaries but silica is by far the
most popular material. One of the advantages of silica and Teflon is that these
materials are transparent to UV light, which facilitates certain methods of
detection. Small windows can be formed in the wall of the tubing by burning of
the polyimide at about 135oC with concentrated sulphuric acid; this
process does not appear to weaken the tubing. Windows can also be produced by
physical burning and by removing the polyimide by scratching, but these procedures weaken the tube and makes it vulnerable to
fracture. The columns can exhibit many thousands, if
not millions, of theoretical plates.
Note. The theoretical plates used to describe efficiency in capillary electrophoresis are those as defined by chromatographic theory not by distillation theory.