Polymer HPLC working mode

The dominating working mode of polymer HPLC is the straight elution of small volume of sample solution along the column. There were attempts to introduce the differential and vacancy procedures in SEC. In both cases, the eluent was a diluted solution of the polymer while either a proper sample or a reference polymer [314] or even pure eluent [315] was injected. A broader application of these proposals was hindered by both the high eluent viscosity and the large sample consumption. Moreover, the dependences of log M vs. Vr (Equation 16.12) obtained by the conventional and the differential/vacancy methods are mutually shifted. The deviation between both dependences was attributed to the nonequilibrium situation in the vacancy polymer HPLC [316],... 

Silica-base stationary phases have also been employed for enantiomeric separations in CEC [6,72-81]. In the initial work on chiral CEC, commercially available HPLC materials were utilized, including cyclodextrins [6,74,81] and protein-type selectors [73,75,80] such as human serum albumin [75] and ai-acid glycoprotein [73]. Fig. 4.9, for example, depicts the structure of a cyclodextrin-base stationary phase used in CEC and the separation of mephobarbital enantiomers by capillary LC and CEC in a capillary column packed with such a phase. The column operated in the CEC mode affords higher separation efficiency than in the capillary LC mode. Other enantiomeric selectors are also use in CEC, including the silica-linked or silica-coated macrocyclic antibiotics vancomycin [82,83] and teicoplanin [84], cyclodextrin-base polymer coated silicas [72,78], and weak anion-exchage type chiral phases [85]. Relatively high separation efficiency and excellent resolution for a variety of compounds have also been achieved using columns packed with naproxen-derived and Whelk-0 chiral stationary phases linked to 3 pm silica particles [79]. Fig. 4.10 shows the... 

Absorbance detection, either UV/visible or photodiode array, has broad but specific applicability, especially for styrenic polymers, epoxies, pheno-lics, polycarbonates, polyurethanes, aromatic polyesters, and many additives. When other HPLC modes are used, additional separating capability is sometimes achieved by changing the solvent composition during the analysis (gradient elution). For this work the UV or PDA detector is essential, since the RI detector would drift excessively as the composition, and therefore the refractive index, changes. See Sec. ILF. 

Modes of HPLC other than GPC can be used to evaluate polymer mixtures, copolymer composition, and to analyze additives. As indicated in Fig. 8 and in Secs. II.A and II.D, for this work a number of changes are made to the system, especially the type of column and detection methods. The mechanism of separation is no longer exclusion, in which no adsorptive interaction between the sample components and the column packing material is tolerable. Instead, there is deliberate adsorptive interaction between the samples and the column packing in order to achieve separations based on differences in chemical composition of the sample components, independent of molecular size. Sec. III.C. 

Column. As mentioned above, columns can be either packed or capillary. The majority of the early work in SFC was performed with conventional HPLC columns, 25 cm or 10 cm x 4.6 mm i.d. and packed with either 5 pm silica or bonded silicas (Cig, CN or diol). However, narrower-bore columns (2-0.5 mm) with 3 pm packing materials are now being considered. Most packed-column SFC uses carbon dioxide plus a polar modifier such as methanol, acetonitrile, tetrahydrofuran, 1,4-dioxan, methylene chloride or formic acid, typically in the range of 1-20%. Separations are usually effected in the normal-phase mode. This technology has been used successfully in the determination of polymer additives, where good quantitative data can be obtained. More efficient separations can, however, be obtained by the use of... 

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