Dual-component mobile phase

For polymer systems without UV activity the combination of a RI detector with a density (D) detector can be used. The working principle of the density detector is based on the mechanical oscillator method. Since this detector yields a signal for every polymer, provided that its density is different from the density of the mobile phase, this detector can be regarded as universal [29,30,36]. The separation of mixtures of polystyrene and polybutadiene by SEC with dual den-sity-RI detection is presented in Figs. 7 and 8. In a first set of experiments, the response factors of both polymers in both detectors have to be determined. Then from the intensity of each slice of the elution curves in both detectors, the mass distribution of both polymers across the elution volume axis can be calculated. As can be seen in Fig. 7, a separation into the component peaks is obtained due to the fact that the molar masses of PS and PB are sufficiently different. For both components the individual elution profiles can be determined and using corresponding calibration curves for PS and PB the individual MMDs can be calculated. The same information can be extracted from an experiment where the molar masses of the components are similar and SEC separation does not work (see Fig. 8). Again the individual mass distributions are obtained and the MMDs for PS and PB can be determined. 

The analyses of aromatic components were performed on a 1525 Binary Waters HPLC with a dual absorbance detector using a Symmetry Cl 8 column and a mobile phase of methanol and water. Carboxylic acids analyses were performed using the same HPLC system with an Atlantis dC18 column and mobile phase. pH was monitored with a Coming 430 pH meter. For most experiments, the total organic carbon was also analyzed using a Shimadzu 5050 TOC analyzer equipped with a NDIR detector coupled with an autosampler ASI 5000. 

Cotton fibers are single cells composed primarily ( 96%) of the polymer cellulose. In our laboratory (5), cotton fibers were dissolved directly in the solvent DMAC-LiCl. This procedure solubilizes fiber cell wall components directly without prior extraction or derivatization, processes that could lead to degradation of high MW components. MW determinations have been carried out by a size-exclusion chromatography (SEC) system using commercial columns and instrumentation with DMAC-LiCl as the mobile phase. Incorporation of viscometry and refractive index (RI) detectors (6) allowed application of the universal calibration concept (7) to obtain MW distributions (MWDs) based on well-characterized narrow-distribution polystyrene standards (5). The universal calibration concept used by incorporation of dual detectors bypasses the need for cellulose standards. There are no cellulose standards available. Polystyrene standards for a wide range of MWs dissolved readily in DMAC-0.5% LiCl with no activation necessary. 

Overcoming interference with fluorescence detectors - use a dual flow cell to offset background fluorescence due to components of the mobile phase. 

Figure 11.36 Schematic diagram of the dual-column extraction system on-line via a Valeo 10-port switching valve with the chiral LC-MS/MS system. The components were as follows PI, a Leap Technologies autosampler with two HPLC pumps delivering mobile phases A and B to the extraction column EC-1 or EC-2 (Waters Oasis HLB 25 gm, 1 x 50 mm) P2, HPLC pump system delivering isocratic elution mobile phase through the extraction column to the chiral analytical column full bold arrows, pathway for mobile phase A (Table) used to load plasma sample onto extraction column dashed arrows, pathway for mobile phase (neither A nor B) used to elute analytes from extraction column to the chiral analytical column F, in-line filter G in-line guard column MS, a triple-quadrupole instrument in MRM mode. Reproduced from Xia, J. Chromatogr. B 788, 317, copyright (2003) with permission from Elsevier.

Dual-mode is an eluting mode applicable to CCC because only liquid phases are involved. It consists in switching the mobile and stationary liquid phase roles at a certain point within the experiment. This approach permits to ensure the complete elution of all components in complex mixtures comprising a broad range of... 

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