Monolithic stationary phases properties

Monolithic stationary phases have to be regarded as the first substantial further development of HPLC columns, as they present a single particle separation medium, made up of porous polymer. As a consequence of their macroporous structure, they feature a number of advantages over microparticulate columns in terms of separation characteristics, hydrodynamic properties, as well as their fabrication ... 

Absorbance signals seen in NIR consist of combination and overtone bands of hydrogen bonds such as C-H, N-H, 0-H, and S-H, which are aroused by large force constants and small mass. NIR spectra thus cover precious information on chemical as well as physical properties of analyzed samples due to characteristic reflectance and absorbance patterns [121-123], which makes this analysis method applicable to the characterization of monolithic stationary phases. 

The ability to achieve precise and independent control over both the porous properties as well as the level of charged moieties of the rigid monolithic stationary phases opened new avenues for studies focusing on the effects these properties exert on the... 

Even if MIP and BET are widely accepted regarding the characterization of HPLC stationary phases, they are only applicable to the samples in the dry state. In order to investigate the impact of polymerization time on the porous properties of wet monolithic columns, ISEC measurements of 200 jm I.D. poly(p-methylstyrene-co-l,2-bis(vinylphenyl)ethane) (MS/BVPE) capillary columns (prepared using a total polymerization time ranging from 45 min to 24 h) have been additionally evaluated (see Table 1.2 for a summary of determined e values). On a stepwise decrease in the time down to 45 min, the total porosity (St) is systematically increasing to about 30% in total (62.8% for 24 h and 97.2% for 45 min). This is caused by a simultaneous increase in the fraction of interparticulate porosity (e. ) as well as the fraction of pores (Cp). The ISEC measurements are in agreement with those of the MIP as well as BET analyses, as an increase in should be reflected in an increase in 8p and as the relative increase in the total porosity (caused by decreasing the polymerization time... 

Such precise control of porous properties is expected to be very useful in the design of specialized CEC columns for separation in modes other than reversed-phase. For example, size exclusion chromatography (SEC) is an isocratic separation method that relies on differences in the hydrodynamic volumes of the analytes. Because all solute-stationary phase interactions must be avoided in SEC, solvents such as pure tetrahydrofuran are often used as the mobile phase for the analysis of synthetic polymers, since they dissolve a wide range of structures and minimize interactions with the chromatographic medium. Despite the reported use of entirely non-aqueous eluents in both electrophoresis and CEC [65], no appreciable flow through the methacrylate-based monoliths was observed using pure tetrahydrofuran as the mobile phase. However, a mixture of 2% water and tetrahydrofuran was found to substan-... 

It should also be mentioned that a variant of the dispersion polymerization technique, in which a porous stationary phase imprinted with the drug pentamidine was prepared directly in a column [18,19]. The polymerization procedure used did not produce a porous monolith but a macro-aggregate of micron-sized beads. A chromatographic evaluation of this column when eluted with a polar mobile phase showed good efficiency and molecular recognition properties comparable with more traditional imprinted columns. 

More recently, columns have been developed where the stationary phase is formed of a porous polymer network inside the capillary. These are called monolithic phases, and have emerged as an alternative to traditional packed bed columns for use in micro-HPLC. They hold many advantages over traditional packed bed columns, being easy to manufacture since the monolith is formed in situ, often via a one-step reaction process, and its properties such as porosity, surface area, and functionality can be tailored. Another major advantage is that they eliminate the need for retaining frits. These columns can be manufactured from a variety of materials, but the most common include sol-gel, methacrylate-based, acrylamide-based, and styrene-based polymeric structures. 

Separations are usually carried out in conventional octyl- and octadecylsiloxane (C8 and C18) columns. A significant number of surfactant molecules may be adsorbed on these stationary phases, giving rise to a structure similar to an open micelle. Consequently, column properties change radically, although the subjacent stationary phase (the bonded moiety) still plays a role in the interaction with solutes. Cyano-propylsiloxane-bonded columns are useful for some specific applications. Efficiencies are improved with fluorinated-bonded phases. More recently, ultrawide pore and monolithic octadecylsiloxane columns have been shown to enhance the eluting power of micellar mobile phases. 

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