Polymer HPLC chromatographic system

As explained in Sections 16.4 and 16.5, the comprehensive characterization of complex polymer systems is hardly possible by the SEC alone. SEC employs only one retention mechanism which simnltaneonsly responds to all molecular characteristics of sample. Similarly, also the coupling of the different retention mechanisms within one single column only exceptionally allows fulfilling this task. Evidently several retention mechanisms should be applied in a tandem approach that is within at least two different on-line chromatographic systems. This is the basic idea of the two- and multidimensional polymer HPLC. In the present section, the principles of two-dimensional polymer HPLC, 2D polymer HPLC or (2D-LC) will be briefly elucidated. There are several reviews available [23-31,249,250] dealing with the 2D polymers. It is anticipated that also the three- and multidimensional polymer HPLC will be developed in future. 

In order to design the appropriate liquid chromatography separation system, it is necessary to nnderstand on molecular level some basic principles and tendencies of the processes taking place in the chromatographic column. Above processes resnlt in differences in retention of sample constituents to allow their mutual separation. Extent of retention of macromolecules within colutim reflects the volume of mobile phase needed for their elution, their abovementioned retention volume, V. For the sake of simplicity, let us consider constant overall experimental conditions that is the elnent flow rate, temperature and pressure drop. The latter two parameters are dictated not only by the inherent hydrodynamic resistance of colunm that is inflnenced by the eluent viscosity, size and shape of packing particles but also by the sample viscosity, which may be rather high in polymer HPLC. Further, only one variable molecular characteristic of separated macromolecules will be... 

In conclusion, large differences exist in the behavior of small molecules and large (chain) macromolecules in the chromatographic systems. These mainly result from substantial role of conformational entropy of macromolecules and are augmented by distinctions in the viscosity, flow patterns, as well as in the mobility (diffusibility) of solutes of different sizes. It is necessary to consider these differences in order to devise the appropriate chromatographic system for efficient HPLC separation of particular polymer samples. 

Dwyer [51] used a combination of chromatography and IR spectroscopy to provide a versatile tool for characterisation of polymers. HPLC-Fourier-transform IR spectroscopy interface systems deposit the output of a chromatograph on an IR optical medium, which is then scanned to provide data as a time-ordered set of spectra of the chromatogram. Polymer analysis applications described include the identification of polymer additives, the determination of composition/molecular weight distributions in copolymers, the mapping of components of polymer alloys and blends, molecular configuration changes in polymers, and component identification in complex systems. 

As one very striking example of the capabilities of the high-temperature gradient HPLC system, the separation of random ethyleneA inyl acetate copolymers is presented in Fig. 23. On silica gel as the stationary phase and using decaline-cyclo-hexanone as the eluent, full separation of copolymers of different compositions was achieved. In addition, the homopolymers PE and PVAc were well separated from the copolymers. This was the first time that a chromatographic system was available that separates olefin copolymers irrespective of crystallinity and solubiUty over the entire range of compositions. Namely, the mobile phase components used are solvents for both PE and PVAc. The non-polar solvent, decalin, supports adsorption of PVAc on the silica gel, while the polar solvent, cyclohexanone, enables desorption and elution of the adsorbed polymer sample firom the column [155]. 

Chiral separation of flavonoids has also been carried out by chromatographic systems by using a chemically bonded chiral stationary phase or by the addition of chiral mobile phase additives (reviewed by Yanez et al. ). These chiral polymer phases can be further subdivided into polysaccharide-derived columns, and cyclodextrin and mixed cyclodextrin columns. With regard to chiral mobile phase additives, the addition of an optically active molecule to the mobile phase can facilitate separation of enantiomers on conventional stationary phases. Cyclodextrin as a chiral additive is widely used to separate enantiomers mainly by capillary electrophoresis (CE), as discussed in Section 3.6.2.I. Table 3.7 summarizes the most habitual HPLC procedures employed for the analysis of various classes of food flavonoids. 

Whereas SEC is the dominant technique for the characterisation of polymers, various nonexclusion liquid chromatographic (NELC) methods, such as GPEC and LACCC offer equally valid possibilities for deformulation of complex polymer systems. In fact, molecular characterisation of polymers in the precip-itation/adsorption mode (gradient HPLC) enables differences in chemical structure and composition to be... 

In the mid-to-late 1990s, SFC became an established technique, although only holding a niche position in the analytical laboratory. The lack of robust instruments and the inflexibility of such systems has led to the gradual decline of SFE-SFC. Only a small group of industrial SFE-SFC practitioners is still active. Also the application area for SFC is not as clearly defined as for GC or HPLC. Nevertheless, polymer additives represent a group of compounds which has met most success in SFE-SFC. The major drawbacks of SFE-SFC are the need for an optimisation procedure for analyte recovery by SFE (Section 3.4.2), and the fair chance of incompatibility with the requirements of the chromatographic column. The mutual interference of SFE and SFC denotes non-ideal hyphenation. 

HPLC-QFAAS is also problematical. Most development of atomic plasma emission in HPLC detection has been with the ICP and to some extent the DCP, in contrast with the dominance of the microwave-induced plasmas as element-selective GC detectors. An integrated GC-MIP system has been introduced commercially. Significant polymer/additive analysis applications are not abundant for GC and SFC hyphenations. Wider adoption of plasma spectral chromatographic detection for trace analysis and elemental speciation will depend on the introduction of standardised commercial instrumentation to permit interlaboratory comparison of data and the development of standard methods of analysis which can be widely used. 

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