Objectives and Methods

The aim of any separation, including particle size separation, is either analytical or preparative. Analytical separations are generally used to increase the sensitivity or selectivity of the subsequent analytical measurement, or to obtain more specific information about the analysed sample. Very often, the original sample is a complicated mixture making the analysis possible only with a prior separation step. Hence, the original multicomponent sample to be analysed must first be separated into more or less pure fractions. Whenever the samples are of particulate character and/or of biochemical or biological origin, direct analysis without preliminary separation is often impossible. An accurate analytical result can be obtained from any analytical separation method by employing an appropriate treatment and interpretation of the experimental data. Separation is usually based on the differences in extensive properties, such as the mass or size of the particles, or according to intensive properties, such as density, electrophoretic mobility, etc. If the relationship between the separation parameters and the size of the separated particles is known or can be predetermined by using an appropriate calibration procedure, the characteristics of an unknown analysed sample can be evaluated quantitatively. The particle size distributions of the analysed samples are determined conveniently from the record of a coupled detector: a fractogram. Detailed information concerning the associated properties of the separated and characterized particles and/or composition of the analysed system which can be extracted from the fractogram represents more sophisticated application of a particular separation method.

Preparative separations are aimed at obtaining a significant quantity of the separated fractions from the original sample. The fractions are subsequently used for research or technological purposes, for detailed analysis of various effective sizes, for the determination of the structure or chemical composition of the particles of a given size, etc. The practical preparative separations can range from laboratory microscale, which cannot be experimentally distinguished from analytical separations, up to industrial macroseparation units.

Analytical and preparative separations are fundamentally identical so that, consequently, we do not distinguish between them and all separation methods are described and discussed from the point of view of the principles involved by making comments on their specific applications only if the discussed technique exhibits particular characteristics predetermining it for a special analytical or preparative purpose.

The most suitable and widespread methodologies for particle size separations described below, starting from the most versatile to more specific ones, are:

• field-flow fractionation

• size-exclusion chromatography

• hydrodynamic chromatography

• centrifugation

• electrophoresis

Besides these modern techniques, some classical procedures mentioned above such as wet or dry sieving, filtration, etc., should not be forgotten.

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