Submicron and micron particles present in liquid environmental, biological, and technological samples differ in their dimensions, shape, mass, chemical composition, and charge. Their properties cannot be reliably studied unless the particles are fractionated. Synthetic particles applied as components of analytical systems may also need preliminary fractionation and investigation. The review is focused on the methods for fractionation and characterization of nanoparticles and microparticles in liquid media, the most representative examples of their application, and the trends in developing novel approaches to the separation and investigation of particles. Among the separation techniques, the main attention is devoted to membrane filtration, field-flow fractionation, chromatographic, and capillary electrokinetic methods. Microfluidic systems employing the above-mentioned and other separation principles and providing a basis for the fabrication of lab-on-chip devices are also examined. Laser light scattering methods and other physical techniques for the characterization of particles are considered. Special attention is given to "hyphenated" techniques which enable the separation and characterization of particles to be performed in online modes.

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SUMMARYTheoretical approaches to the description of ion retention in aqueous solution by soluble polymers during membrane filtration are proposed. Examination of the chemisms responsible for inorganic ion complexation and retention by membrane-based separations helps to select appropriate hydrophilic polymers and optimum chemical conditions for the process. The ion separation and enrichment data may be formally explained by using mathematical models based on the theory of kinetics of the reactions in partially open systems.

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