Macromolecular solutions, chromatography

The consideration made above allows us to predict good chromatographic properties of the bonded phases composed of the adsorbed macromolecules. On the one hand, steric repulsion of the macromolecular solute by the loops and tails of the modifying polymer ensures the suppressed nonspecific adsorptivity of a carrier. On the other hand, the extended structure of the bonded phase may improve the adaptivity of the grafted functions and facilitate thereby the complex formation between the adsorbent and solute. The examples listed below illustrate the applicability of the composite sorbents to the different modes of liquid chromatography of biopolymers. 

Particulate sorbents are available almost exclusively in the shape of micrometersized beads. These beads are packed in columns and represent currently the most common stationary phases for high-performance liquid chromatography (HPLC). Despite their immense popularity, slow diffusional mass transfer of macromolecular solutes into the stagnant pool of the mobile phase present in the pores of the separation medium and the large void volume between the packed particles are considered to be major problems in the HPLC of macromolecules, frequently impairing their rapid and efficient separation [1]. 

Rate equation analyses for classical size exclusion chromatography have been based on treating the porous matrix as a homogeneous, spherical medium within which radial diffusion of the macromolecular solute takes place (e.g. (28,30,31)) or If mobile phase lateral dispersion Is considered Important, a two dimensional channel has been used as a model for the bed (32). In either case, however, no treatment of the effects to be expected with charged Brownian solute particles has been presented. As a... 

These conclusions differ somewhat from those of Pirkle and Siegell in their analysis of adsorption chromatography in a crossflow magnetically fluidized bed (14). They found the dominant effects to be the width of the feed band and the external mass transfer resistance. It is not surprising that the effect of internal diffusion would be more important in size exclusion chromatography with macromolecular solutes. 

RI detectors are highly useful for preparative chromatography, where they can be operated at low sensitivity, and for polymer or macromolecular separations, where the change in refractive index from that of the mobile phase will be great, even for low concentrations of solute. 

In some cases, several secondary processes may manifest themselves simultaneously. A typical combined secondary process is the so-called concentration effect in gel chromatography, i.e., the often observed rise of the retention volumes of macromolecular substances with the increased sample concentration. The concentration effect is caused by a decrease in the macromolecular coils dimensions in solution with growing concentration, further by viscosity and osmotic effects as well as by secondary exclusion — and in the case of soft gels also by altering the pore geometry due to deswelling of gel particles in the zone of the sample. 

By means of gel chromatography, structure of macromolecules can be elucidated (e.g., their branching [53]), and in some cases also their tacticity [54]. GPC was applied fairly often to the study of the interactions of the macromolecules such as association and aggregation [54-59], the association with ligands [60,61], or the preferential solvation of macromolecules in mixed solvents [62], and the interactions in the polyelectrolyte-salt-solvent [63] systems. GPC can provide information concerning the polymer-solvent interactions [64] as well as about the changes in the macromolecular dimensions with concentration in solution [65,66]. 

Unlike other forms of chromatography that are driven by enthalpy terms in thermodynamic equilibrium, SEC is an entropy-controUed chromatographic separation technique that is, under ideal conditions, based solely on solute size. Although SEC is a relatively rapid and simple method for determining the MWDs and average molecular weights of macromolecular lignins, there are some strict requirements that must be met for proper use of this technique. 

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