Transport, space, entropy, diffusion and flow. Elements underlying separation by electrophoresis, chromatography,...

We describe here some elements common among separation methods, including the requirement for separative transport, the sensitive utilization of space, and the dissipative (band broadening) roles of entropy, diffusion, and flow. We focus on the need to carefully manage the unceasing competition between separative and dissipative transport in all high resolution methods. We then examine the more specific roles these elements play in influencing separation in several important analytical separation techniques. We begin with electrophoresis and sedimentation, which are conceptually among the simplest of systems because they require no flow and only a single dimension of space. The conflict between separative and dissipative transport is examined closely for these systems and it is shown that the outcome is a theoretical plate number which can be expressed as a ratio of two energies: a structuring energy--delta mu ext, which organizes the separation, and thermal energy RT, which is responsible for its dissipation. It is explained why optimal separation is most often achieved in thin layers or in capillary tubes. Chromatography and field-flow fractionation are then described as two closely related methods in which flow is powerfully coupled with a simple enrichment process occurring at right angles to flow. Unlike electrophoresis, these systems are intrinsically two-dimensional. It is shown how flow and diffusion processes both assume two diametrically opposite roles: those of aiding separation and simultaneously causing its dissipation. With a few equations it is demonstrated that the dissipative role is best contained by reducing the thickness (diameter) of the system or of certain elements within the system.