Figure 4.54
Schematic diagram of a capillary electrophoresis system.and an o.d. of about 400 μm. Like capillary GC columns, it is protected with an outer layer of a
polyimide. Potentials of between 10 and 30 kV are applied across the capillary during electrophoresis
creating fields of 100–500 V cm–^1. The on-line detector positioned close to the cathodic end of the
capillary is most commonly a UV absorbance or fluorescence monitor or a diode array spectrometer
providing absorbance data at multiple wavelengths. The detector cell is simply a small section of the
capillary tube from which the polyimide protective coating around the outside has been removed to
allow radiation from the detector lamp to pass through. The walls of the tube therefore serve as the
optical windows of the cell. The optical path through the tube is only 25– 75 μm, but it can be extended
by creating an enlarged region known as a 'bubble cell' thereby increasing sensitivity by a factor of
about three without any loss of efficiency. It should be noted that, although the absolute detection limits
in HPCE are extremely low, generally 10–^13 to 10–^20 g, the concentration limits are comparable to and
sometimes poorer than those in HPLC due to the very short optical path length through the capillary.
Fluorescence detection can be up to four orders of magnitude more sensitive than UV absorbance,
especially where laser induced excitation is used, mass detection limits being as low as 10–^20 – 10 –^21
mole. Pre- and post-column derivatization methods are being developed to extend the applicability of
fluorescence detection to non-fluorescent substances. Several types of electrochemical and mass
spectrometric detector have also been designed. Detector characteristics are summarized in Table 4.21.
Samples are injected into the capillary tube at the opposite end from the detector using one of two
methods, i.e. hydrodynamic or electrokinetic. These are illustrated in Figure 4.55. The end of the tube is
dipped into the