Polymers eluent compatibility

The eluent compatibility of a polymeric adsorbent will be dependent upon the chemical structure of the polymer backbone, chemical type of the cross-linking agent, degree of cross-linking, and any subsequent covalent or dynamic modifications carried out. The natural polysaccharide polymers in their native state are hydrophilic and are therefore compatible with aqueous eluents whereas the synthetic polymers can be hydrophobic, as in the case of polystyrene, and hence compatible with organic eluents, or hydrophilic, as in the case of polyacrylamide, and so be compatible with aqueous mobile phases. It is of course possible to modify the eluent compatibility of a polymeric matrix by surface coating or derivatisation. For example, the very hydrophobic maeroporous polystyrene matrices may be coated with a hydrophilic polymer to make ion exchange adsorbents or materials suitable for aqueous size separations [25]. 

In an ideal SEC separation, the mechanism is purely sieving, with no chemical interaction between the column matrix and the sample molecules. In practice, however, a small number of weakly charged groups on the surface of all TSK-GEL PW type packings can cause changes in elution order from that of an ideal system. Fortunately, the eluent composition can be varied greatly with TSK-GEL PW columns to be compatible with a wide range of neutral, polar, anionic, and cationic samples. Table 4.8 lists appropriate eluents for GFC of all polymer types on TSK-GEL PW type columns (11). 

An ELSD converts the HPLC eluent into a particle stream and measures the scattered radiation. It offers universal detection for nonvolatile or semivolatile compounds and has higher sensitivity than the RI detector (in the low ng range) in addition to being compatible with gradient analysis. ELSD is routinely used in combinatorial screening. Response factors are less variable than that of other detectors. An ELSD consists of a nebulizer equipped with a constant temperature drift tube where a counter-current of heated air or nitrogen reduces the HPLC eluent into a fine stream of analyte particles. A laser or a polychromatic beam intersects the particle stream, and the scattered radiation is amplified by a photomultiplier. Manufacturers include Alltech, Polymer Laboratories, Shimadzu, Waters, Sedere, and ESA. 

The solvent. The solvent should be chosen to best dissolve the sample, to be compatible with the column packing and to permit detection. The most commonly used eluents in SEC are tetrahydrofuran (for polymers that dissolve at room temperature), <9-dichlorobenzene and trichlorobenzene at 130 C and 150°C (for crystalline polyolefins) and phenolic solvents at 100 C (for condensation polymers, such as polyamides and polyesters). For the more polar polymers, dimethylformamide and aqueous eluents may be employed, but care is required in avoiding interactions between the polymer (sample) and the gel (packing). Flow rates of 1 ml/min are typical for SEC analysis. 

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