Silica-based reversed phase media

Figure 2.23 Ion-pairing agents used in reversed phase chromatography. Ion-pairing agents illustrated in pink enhance the interaction of charged groups with the hydrophobic surface. They also suppress the ionization of residual silanols on silica-based reversed-phase media.

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... 

Part of this material (340 mg) was purified even more by reversed-phase medium-pressure liquid chromatography procedure using a column (2.5 x 45 cm) of Whatman LRP-I (C-18-bonded silica gel, 13-24 xm) eluted with a gradient from 15-60% acetonitrile in 0.1% TFA. Aliquots from the main peak were pooled and lyophilized to give 75 mg of almost pure peptide, which counted a 18.5% yield based on resin incorporation. The final purification was achieved by a preparative RP-HPLC... 

Alumina offers another alternative to silica because of its inherent higher pH stabihty. However, in contrast to its extensive use as a medium in column chromatography for purification purposes, or for separations in the normal-phase mode, there are still relatively few reports concerning alumina-based materials in the reversed-phase mode. 

In reversed phase liquid chromatography (RPLC) silylated silicas are preferred. The surface of these silicas is covered with chemically bonded non-polar groups such as alkyl chains or polymeric layers (Chapter 3.2.3). Silica modified with medium polar groups such as cyano, diol or amino might be used in NP as well as RP mode. Alternatively, cross-linked polymers such as hydrophobic styrene divinyl benzene-copolymers can be used (Chapter 3.2.4). Polymer packings show stability in a pH range 2-14 while silica based packings show limited stability for pH > 7. 

Reversed phase liquid chromatography is used in separating polar to medium polar components. Their separation is based on the interaction of the lipophilic part of the solutes with the non-polar surface groups. Retention depends on the nature of the active groups bonded on the silica surface as well as the functional groups of the solute. The hydrophobicity of reversed phase packings differs in the relative sequence ... 

The results described herewith demonstrate that the activity of Ru metathesis catalysts can be enhanced by introduction of EWGs without detriment to catalysts stability. This principle can be used not only to increase the catalyst activity, but also to alter its physical-chemical properties, such as solubility in given medium or affinity to silica gel. An example of novel immobilization strategy, based on this concept is presented. In fact, the possibility of reversibly binding catalysts to a solid phase is of major importance for industrial applications, particularly when continuous flow processes with immobilised homogeneous catalysts are pursued. 

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