Page 60 - Tandem Techniques

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diameters. The capillary column has not, at the time of writing this book, been developed sufficiently
for general analytical use in LC. There are two basic types of LC stationary phases: silica which
separates substances on the basis of polarity, and the reverse phase, that separates on the basis of
dispersive interactions. The latter is the most commonly used. Very fast separations can be achieved by
LC and efficiencies of three quarters of a million plates have been realized. Silica and to a lesser extent
reverse phases have a range of pore sizes and thus exhibit exclusion properties. Thin layer
chromatography is a lamina form of LSC, but has only very limited use in tandem systems. In any
column, the solutes are both retained and individually dispersed. Retention depends primarily on the
thermodynamic properties of the distribution system and the quantity of stationary phase in the column.
Dispersion, on the other hand, depends on the kinetic properties of the system, such as the mobile phase
velocity, the particle diameter of the support in a packed column, or the radius of a capillary column,
together with the physical properties of the solute and the two phases. There are three major
contributions to band variance: the multipath effect, longitudinal diffusion and the resistance to mass
transfer of the solute between the two phases. All three need to be minimized to produce an efficient
column. However, a high-efficiency column producing very narrow solute bands can make the design
of the interface between the column and the tandem spectroscopic system more difficult.

Capillary electrophoresis has only been used to a limited extent in tandem systems, but holds good
possibilities for the future. There are three electrophoretic methods of separation: zone electrophoresis,
where the components are separated on a basis of relative mobilities; isotachophoresis, where the
separation is again based on relative mobilities but where the solutes are sandwiched between leading
and terminating electrolytes; and finally, isoelectric focusing, where the solutes are separated according
to their isoelectric points. In all electrophoretic systems, movement of charged species is always
accompanied to a greater or lesser extent by electro-osmotic flow. Electro-osmotic flow is the
movement of a liquid when in contact with a charged surface under the influence of an electric field.
The flow is 'plug' form and is not accompanied by the parabolic velocity profile associated with
Newtonian