Hybrid packings silica-based

The elution behavior of various polymers near their critical adsorption point with silica gel packings and various eluents has been studied (12). It was of interest to apply hybrid column systems composed of active ( critical ) packings (silica gels) in combination with nonactive (nonadsorptive) PS/DVB and DVB-based gels. Some PS/DVB and DVB gels exhibited rather strong... 

Reversed-phase LC is ideally suited for the analysis of polar and ionogenic analytes, and as such is ideally suited to be applied in LC-MS. Reversed-phase LC is the most widely used LC method. Probably, over 50% of the analytical applications are preformed by reversed-phase LC. Nonpolar, chemically-modified silica or other nonpolar packing materials, such as styrene-divinylbenzene copolymers (XAD, PRP) or hybrid silicon-carbon particles (XTerra), are used as stationary phases in combination with aqueous-organic solvent mixtures. Silica-based packing materials are used more frequently than polymeric packing materials. 

Figure 3.11. A partial list of popular silica-based columns including some recent new offerings. Columns based on high-purity silica are underlined. Columns packed with hybrid particles are in bold. Note that the ACQUITY (Waters) and Gemini (Phenomenex) are new second-generation hybrid columns.

Most high-performance packings are based on silica or related hard particles, such as inorganic-organic hybrid packings. Therefore, the focus of the following discussion is the performance of incompressible... 

Silica and silica hybrids are the bases for the most popular HPLC packings. The surface of silica is covered with silanol groups. These are subdivided into geminal, vicinal or bridged, and lone silanols, as shown in Figures 2A-2C respectively. 

Different bonded phases are used during method development because they provide different selectivity. Their different selectivity gives confidence in method specificity. Typically C18, C8, and phenyl are selected. In addition, mobile phase pH use range should be considered when selecting UHPLC columns. As mentioned, the silica-based packing columns are stable under acidic conditions but not at basic conditions. Therefore, the hybrid-based packing columns are selected under basic conditions. 

Chromatographic resolution is also dependent on column efficiency (i). Column efficiency is directly dependent on the nature of the support matrix and how well that support is packed in its column. Available chromatographic supports are based on dextran, agarose, polystyrene, acrylic, cellulose, silica gel and a variety of other polymers. Althou cellulosic supports are manufactured in both microcrystalline and leaded forms, most supports are beaded. Newer supports may use hybrid bead construction where the base support is coated with a second materid (e.g., dextran or silica coated with agarose). 

A few applications have employed conventional packed columns, although recent developments in new thermally stable stationary-phase materials have generated a renewed interest and the temperature stability of the different stationary-phase materials has been reviewed by Claessens and van Straten [43]. The new materials have included stable metal oxide materials, based on zirconia (Figures 18-4 and 18-5) and titania [44, 45] and hybrid phases combining silica and methylene or ethyl bridges [46]. These have been applied in a number of applications to pharmaceutical compounds (Table 18-1). 

Landers and colleagues have demonstrated [3, 4] a microchip-based DNA purification technique where DNA from biological samples such as blood has been isolated. The approach taken here was to maximize the surface area of interaction for DNA adsorption to the silica. Hence, silica beads of 10 pm diameter were packed into a microfabricated channel (Fig. 2) as a means to increase the silica-DNA interaction surface area. To pack the column, the silica beads are first introduced into the microfluidic channel and then the beads are immobilized within a solgel. In the silica particle/solgel hybrid approach... 

Capillary electrochromatography (CEC) is a hybrid of CE and liquid chromatography. The separation is carried out in capillaries that are either fully packed with octadecyT(ODS)-silica particles or partially packed having an open segment. The separation of the analytes is based on the difference in the magnitude of the distribution between the mobile and stationary phase. The flow of mobile phases is generated... 

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