in detecting an abnormal distribution of components in a series of routine samples. It is used widely for
the analysis of biologically active materials. Proteins, nucleic acids, enzymes, viruses and drugs are
among the many classes of compound amenable to separation by this technique. A significant
proportion of the applications of electrophoresis lies in the fields of clinical diagnosis and forensic tests.
It is particularly useful for the characterization of body fluids such as serum, urine, gastric juices, etc.
Two variations of the basic technique are isoelectric focusing and immuno-electrophoresis. The former
offers improved resolution and sharper bands in the separation of weak acids, weak bases and
ampholytes through the use of pH and density gradients superimposed along the potential gradient. The
latter employs specific antigen–antibody interactions (Chapter 10) to visualize the separated
components of serum samples.
High-performance Capillary Electrophoresis
The technique of HPCE, or CE, involves high-voltage electrophoresis in narrow-bore fused-silica
capillary tubes or columns and on-line detectors similar to those used in HPLC (p. 127). Components of
a mixture injected into one end of the tube migrate along it under the influence of the electric field
(potential gradient) at rates determined by their electrophoretic mobilities. On passing through the
detector, they produce response profiles that are similar to, but generally sharper than, chromatographic
peaks. Recorded as a function of time, the peaks of a capillary electropherogram resemble a very high-
efficiency HPLC chromatogram. Efficiencies measured in plate numbers (equation (4.42) or (4.43))
approach or exceed 10^6. This is firstly because the peaks are not broadened by mass transfer or
multiple-path effects, molecular or longitudinal diffusion being the only significant band-spreading
mechanism (p. 88), and secondly because flat flow-profiles are generated by a very pronounced electro-
osmotic effect (vide infra). The factors affecting efficiency, N, are given by the equation
where D is the diffusion coefficient of the migrating species, d the distance travelled and μ its
electrophoretic mobility, E being the applied potential gradient (field). Hence, a high field, short
migration time and distance, and small diffusion coefficients result in the highest efficiencies. In
general, diffusion coefficients are inversely related to the size of the migrating species; some examples
are given in Table 4.20. A particular advantage of capillary tubes over traditional electrophoresis
systems is their high electrical resistance. This facilitates the use of very high fields which generate only
a modest amount of heat that is readily dissipated through the wall of the capillary. High fields result in
fast rates of migration and hence short analysis times.