Silica supports, reversed phase

A particular problem with use of silica-supported reversed-phase media for trace analysis of surfactants is the presence of free Si-OH groups. These bind surfactants tightly, making complete recovery difficult, particularly for cationics (55). Phenyl-modified silica is excellent for SPE of NPE, but recoveries are low for AE, particularly more highly ethoxylated AE (55). 

Feibush, B., A new chemically deactivated silica-based reversed phase/ion exchange support, /. Liq. Chromatogr. Relat. Technol, 19(14), 2315, 1996. 

The reproducibility of results obtained with silica-based reversed phases, of the same declared characteristics but from different manufacturers, was sometimes poor, probably because of the properties of the silica used and the different reaction conditions in the alkylation of the support. 

The choice of reverse phase packing material will depend on the amount of information available on the component of interest and on other sample components. Initial tests such as solvent partitioning behavior, solubility m various solvents, and others see Chapter 1) can be used to estimate polarity and hence be of use in initial column/mobile phase selection. The most retentive of the silica-based reverse phase supports, Cl8 and C8, are a sensible first choice, as the retention of polar compounds is maximized, while the retention of nonpolar materials can be easily modulated by choice of eluent. If the compound of interest is very nonpolar (or the sample contains components that bind very strongly to retentive phases such as C8/C18), a shorter chain alkyl-bonded phase such as C6 or C4 may be more suitable. 

Most silica-based reverse phase chromatographic supports contain a small percentage of free (underivatized) silanol groups, and this can adversely affect... 

A saturation column situated between the pump and the injection device may be installed for two reasons. In a liquid-liquid partition system a saturation column containing a large amount of stationary phase on a suitable solid support may be used in order to ensure a proper equilibrium between the two phases. In other systems a saturation column containing bare silica may be installed in order to prevent dissolution of silica from the analytical column. This is an advantage even if the analytical colunm contains chemically modified silica for reversed phase chromatography. 

Chromatographic separations frequently rely upon interactions between analytes in a flowing liquid or gas stream (or mobile phase) and a solid surface which remains motionless. In traditional forms of chromatography, the motionless solid surface, or stationary phase, might be porous particles of a polar material like silica (a normal-phase chromatographic support), hydrophobically modified or coated silica (a reversed-phase chromatographic support), or a thin polysiloxane film on the inner lumen of a capillary (for capillary gas chromatography). 

Besides silica, other direct phase supports such as alumina, diatomaceous earth, cellulose, MgO-diatomaceous earth, and sucrose layers were employed for the separation of various pigment mixtures. Mixed-mode supports (cyano, diol, and aminosilica layers) were also employed in both the direct and reversed-phase elution modes for the separation of natural pigments, but their performance was markedly lower that those of traditional silica layers. Reversed-phase supports (polyamide, octadecylsilica, alumina, sihca, and diatomaceous earth impregnated with paraffin oil) were also applied for the separation of color pigments (carotenoids from Capsicum annuum, anthocyanins from red wines) and it was found that their separation... 

In contrast, the mono-layer of methanol is built up much more slowly and is not complete until the concentration of methanol in the aqueous mixture is about 35%w/v. The behavior of methanol on the reverse phase is reminiscent of the adsorption of chloroform on the strongly polar silica gel surface. The complementary nature of the silica gel surface and that of the reverse phase is clearly apparent. It is also clear that strongly dispersive solvents might form bi-layers on the reverse phase surface just as polar solutes form bi-layers on the highly polar surface of silica gel. In fact, to date there has been no experimental evidence furnished that would support the formation of bi-layers on the surface of reverse phases, although their formation is likely and such evidence may well be forthcoming in the future. 

The problems of stationary phase erosion can be largely overcome by solvent-generated LLC, where the stationary liquid phase is generated dynamically by the mobile phase, in this approach, one of the phases of an equilibrated liquid-liquid system is applied as a mobile phase to a solid support which is better wetted by the other phase of the liquid-liquid system. The support is usually silica when the stationary phase is aqueous or a polar solvent and a reversed-phase chemically bonded support when the stationary phase is a nonpolar solvent. Under these Conditions a multimolecular layer is formed on the surface of the solid support which has the properties of the liquid phase in... 

A PRP -1 (Hamilton Reno, NV) reversed phase column was coated with cetylpyridinium and eluted with tetramethylammonium salicylate acetoni-trile water.89 The separation was comparable to that observed on conventional ion exchange. Coated phases were also used to separate oxalate complexes of manganese, cobalt, copper, and zinc.90 Reversed phase silica supports were coated with poly(N-ethyl-4-vinylpyridinium bromide), poly(dimethydiallylammonium chloride), poly(hexamethyleneguanidinium... 

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