engleski

Additional separation mechanism in size-exclusion chromatography

Size-exclusion chromatography (SEC) is a modern separation chromatographic technic which provides the separation of macromolecular species from the macromolecular mixtures according to their size in solution. This technic belongs to the most frequent instrumental technics used in the molecular mass distribution (MMD) of homopolymers and copolymers. The theory of the separation mechanism in SEC has received considerable attention and many reviews include extensive discussions regarding the proposed theories of the separation mechanism. Equation (1) is an empirical expression relating the SEC eluation parameters: Ve = *Vo + K(SEC) Vi , where Ve is the peak eluation volume, Vo is the interstitial volume, V; is the volume of the mobile phase within pores of separation gel and K(SEC) is the distribution coefficient, and generally speaking K(SEC) varies between zero for excluded species and one for the totally permeating species. Separation according to the aforementioned is well established in case apolar and medium polar homopolymers and copolymers, and a correct relationship between the size (MM) and eluation volume is documented. But, there are many polymers which does not follow the mentioned separation mechanism or separation according to the size only: This is specially present in case of polar polymers and those bearing ionic harges (polyelectrolytes) and their characterisation is much more complicated. Two or more mechanisms are involved in separation and MMD is considerably more complex. Relation between SEC eluation parameters are expressed in equation : Vg = Vo + KSEC Kp V , where Kp non equal *1 (usually Kp > 1 ). These mechanism, named also secondary mechanisms, could suppress or eliminate separation according to the size. They were caused by ion inclusion, ion exclusion, association, adsorption, partition, Donnan equilibria etc. Manifestations of all these influences are observed as an earlier (later) eluation of chromatogram, chromatogram's spreading, bi-or-multimodal chromatogram, distortion of peak symmetry etc. Size exclusion mechanism and secondary mechanisms are discussed and supported by the MMD determination of poly(fluorostyrene-cochlorstyrene), polyethersulfone, polyimide, styrene and maleic anhydride copolymers and sulfonated styrene and maleic anhydride copolymers.

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