Silica polymerization reversibility

The CEC phases must be capable of carrying a charge to generate an EOE and appropriate moieties to facilitate the chromatographic processes. Silica-based reversed-phase packing materials have been most widely used in CEC. The use of polymeric and mixed-mode bonded particles has also been reported. Eor the silica-based phases, the carbon chains bonded on the silica surface provide the retention and selectivity for analytes, and the residual silanol groups on the surface of the silica are ionizable and generate the EOF. 

Chromatographic separation is generally carried out on octadecyl, octyl, phenyl, and polymeric reversed-phase columns, although polar stationary phases such as silica (153) or cyanopropyl (141, 142) have been also used in Ure analysis of sedecamycin and pirlimycin residues, respectively. 

A few polymeric reversed-phase stationary phases are available which provide the advantage that they can be operated over a wider pH range than the silica-based columns. Polymeric columns, however, tend to be less efficient than silica-based ones and are often less retentive. 

In addition to the silica-based, reversed-phase particles, polymeric particles are available that utilize a hydrophobic stationary phase, e.g., cross-linked polystyrene divinylbenzene. In these polymeric stationary phases, the backbone of the particle provides the opportunity for hydrophobic interactions. Although these packing materials are available in high-performance particles, the particles cannot withstand the high pressure that silica particles can and thus are typically used in medium- to low-pressure operations. The polymeric stationary phases usually have a series of aromatic... 

Depending on the surface modification reversed phase silicas can be grouped into (a) monomeric reversed phase silicas chemically modified with monofunctional silanes and (b) polymeric reversed phase silicas with a polymeric layer made by surface reaction with trifunctional silanes. 

Kabra, P.M. Wall, J.H. Evaluation of polymeric reverse phase extraction columns for liquid chromatographic analysis of cyclosporine in whole blood and comparison with silica based bonded reversed phase extraction columns. J.Liq.Chromatogr., 1989, 12, 1819-1834 [whole blood cyclosporin D (IS) column temp 70 LOD 10 ng/mL]... 

Recently, new approaches of sorbent construction for reversed-phase chromatography have been developed. Silicas modified with hydrocarbon chains have been investigated the most and broadly utilized for these aims. Silica-based materials possess sufficient stability only in the pH 2-8 range. Polymeric HPLC sorbents remove these limitations. Tweeten et al. [108] demonstrated the application of stroongly crosslinked styrene-divinylbenzene resins for reversed-phase chromatography of peptides. 

The analysis demonstrates the elegant use of a very specific type of column packing. As a result, there is no sample preparation, so after the serum has been filtered or centrifuged, which is a precautionary measure to protect the apparatus, 10 p.1 of serum is injected directly on to the column. The separation obtained is shown in figure 13. The stationary phase, as described by Supelco, was a silica based material with a polymeric surface containing dispersive areas surrounded by a polar network. Small molecules can penetrate the polar network and interact with the dispersive areas and be retained, whereas the larger molecules, such as proteins, cannot reach the interactive surface and are thus rapidly eluted from the column. The chemical nature of the material is not clear, but it can be assumed that the dispersive surface where interaction with the small molecules can take place probably contains hydrocarbon chains like a reversed phase. 

---