Polymeric stationary phase, mass transfer

Another approach to preparing a stable reversed phase with fewer residual silanols is the use of polyfunctional silanes of the type R2SiX2. These react to form a polymeric stationary phase that shields the siloxane bonds and restricts access to residual silanols. Polymer phases have higher carbon loads and are typically more retentive than monomeric phases. However, they are more difficult to synthesize reproducibly and may exhibit batch-to-batch variability in their properties. They also exhibit poorer mass transfer kinetics and so provide poorer efficiency than monomeric phases. 

Chen YB, Kele M, Sajonz P, Sellergren B, Guiochon G. Influence of thermal annealing on the thermodynamic and mass transfer kinetic properties of D- and L-phenylalanine anilide on imprinted polymeric stationary phases. Anal Chem 1999 71 928-938. 

In practice, the less efficient mass transfer within polymeric stationary phases results in a lower optimum flow rate being obtained following a van Deemter plot. It is essential that the... 

Due to their defined monomodal macropore distribution (see Section 1.2.1), monolithic stationary phases, based on polymerization of organic precursors, are predestined for efficient and swift separation of macromolecules, like proteins, peptides, or nucleic acids, as their open-pore structure of account for enhanced mass transfer due to convection rather than diffusion. In fact, most of the applications of organic monolith introduced and investigated in literature are directed to analysis of biomolecule chromatography [29]. 

Sajonz P et al (1998) Study of the thermodynamics and mass transfer kinetics of two enantiomers on a polymeric imprinted stationary phase. J Chromatogr A 810(1—2) 1—17... 

Miyabe K, Guiochon G (2000) A study of mass transfer kinetics in an enantiomeric separation system using a polymeric imprinted stationary phase. Biotechnol Prog 16(4) 617-629... 

The difficulties encountered in LLC can be overcome by the use of chemically bonded stationary phases or bonded-phases. Most bonded phases consist of organochlorosilanes or organoalkoxysilanes reacted with micro-particulate silica gel to form a stable siloxane bond. The conditions can be controlled to yield monomeric phases or polymeric phases. The former provides better efficiency because of rapid mass transfer of solute, whereas the polymeric phases provides higher sample capacity. BPC can be used in solvent gradient mode since the stationary phase is bonded and will not strip. Both normal-phase BPC (polar stationary, non-polar mobile) and reversed-phase BPC (non-polar stationary, polar mobile) can be performed. The latter is ideal for substances which are insoluble or sparingly soluble in water, but soluble in alcohols. Since many compounds exhibit this behaviour, reversed phase BPC accounts for about 60% of published applications. The main disadvantage of silica bonded phases is that the pH must be kept between 2 to 7.5. However, bonded phases with polymer bases (polystyrene-divinylbenzene) can be used in the pH range of 0 to 14. 

As stated above, columns packed with irregular materials are less than ideal in terms of chromatographic performance. Thus, in recent years much effort has been dedicated to develop alternative methods to prepare imprinted stationary phases that are superior in terms of efficiency, mass transfer characteristics, and sample load capacity. Micrometer-sized spherical-imprinted polymers with narrow size distribution have been prepared through several techniques reported in Table 2. It should be considered that all these procedures show serious limitations. These are high sensitivity to small changes in polymerization conditions, a polymerization medium that is not compatible with weak noncovalent interactions between functional monomers and their template, high costs or procedure complexity (which can hinder a wide application of these techniques as valid substitutes to bulk polymerization method). 

Miyabe, K. Guiochon, G. Kinetic study of the concentration dependence of the mass transfer rate coefficient in enantiomeric separation on a polymeric imprinted stationary phase. Anal. Sci. 2000, 16, 719-730. 

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