Size exclusion chromatograph compositional

This book covers some of the significant advances in hyphenated chromatographic separation methods for polymer characterization. Chromatographic separation techniques in this volume include size-exclusion chromatography, liquid chromatography, and field flow fractionation methods that are used in conjunction with information-rich detectors such as molecular size-sensitive or compositional-sensitive detectors or coupled in cross-fractionation modes. 

However, fractional separation has been the basis for most asphalt composition analysis (Fig. 15.5). The separation methods that have been used divide asphalt into operationally defined fractions. Three types of asphalt separation procedures are now in use (a) chemical precipitation in which n-pentane separation of asphaltenes is followed by chemical precipitation of other fractions with sulfuric acid of increasing concentration (ASTM D-2006) (b) adsorption chromatography with a clay-gel procedure in which, after removal of the asphaltenes, the remaining constituents are separated by selective adsorption/desorption on an adsorbent (ASTM D-2007 and ASTM D-4124) and (c) size exclusion chromatography in which gel permeation chromatographic (GPC) separation of asphalt constituents occurs based on their associated sizes in dilute solutions (ASTM D-3593). 

In this section we discuss the choice of a chromatographic system on the basis of, firstly, the type of stationary phase and, secondly, the composition of the eluent. A division in subsections is made from the kinds of stationary phases available, bearing in mind that the more specialized phases for ion-exchange, gel permeation (size exclusion), and bioaffinity chromatography are treated in separate chapters (4.5 to... 

Molecular weight distribution information obtained by size-exclusion chromatography on its own is insufficient to characterize the properties of complex polymers, such as copolymers and block and graft polymers [23,514,524]. For these polymers the chemical composition and functionality type distributions are equally important. A major obstacle to the characterization of these materials is that their molecular properties are present as joint distributions. Unlike the mass distribution the composition and functionality distributions can only be determined by separation methods that employ interactions with the stationary phase. To fully characterize a complex polymer it is not unusual to use manual or automated tandem techniques where the sample is fractionated according to its chemical or end group composition for subsequent further separation by size-exclusion chromatography to establish their mass distribution. Chromatographic methods may also be combined with spectroscopic methods to determine microstructural information. 

In a more general sense, the size exclusion mode of liquid chromatography relates to a separation regime, where entropic interactions are predominant and TAS > AH. In the reverse case, AH > TAS, separation is mainly directed by enthalpic interactions. As both separation modes in the general case are affected by the macromolecule size and the pore size, a certain energy of interaction may be introduced, characterizing the specific interactions of the monomer unit of the macromolecule and the stationary phase, e is a function of the chemical composition of the monomer unit, the composition of the mobile phase of the chromatographic system, and the temperature. 

Combination with other liquid chromatographic techniques is also reported by several workers. Orthogonal coupling of an SEC system to another high-performance liquid chromatography (HPLC) system K, Kj to achieve a desired cross-fractionation was proposed. It was an SEC-SEC mode, using the same polystyrene column, but the mobile phase in the first system was chosen to accomplish only a hydrodynamic volume separation, and the mobile phase in the second system was chosen so as to be a thermodynamically poorer solvent for one of the monomer types in the copolymer, in order to fractionate by composition under adsorption or partition modes as well as size exclusion. 

Figure 18 Size exclusion chromatography pattern of a wheat protein hydrolysate (left) and percent composition of the fractions having molecular mass within the known values of standard substances (right). Molecular weights of peptides are only approximately determinable, since their hydrodynamic properties are not necessarily identical with those of the marker substances. Chromatographic conditions stationary phase TSKgel Toyopearl HW-40F (Tosoh Co., Japan) 10 X 500 mm eluant 0.1 M NaCl in 0.1 M phosphate buffer pH 7.1 flow 0.4 ml/min detection UV 220 nm. (Unpublished data.)...

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