Reversed-phase packings variation

It is common for SPE users to create their own packings by combining materials with different modes of separation. A common mixed-mode system is a weak cation exchange packing with a typical reversed-phase packing (e.g., Cl 8). Variations of this mixed mechanism approach have been described as shielded hydrophobic phases, semipermeable surfaces, dual-zone phases, and internal-surface reversed phases.8,9 This type of methodology has been used... 

FIG. 1 Phase diagram for Aerosol OT (AOT)-water-octane system. The boundaries of each individual phase were determined with 50 mM phosphate + 50 mM acetate buffer as an aqueous component (—). (From Ref. 2.) LI, L2 normal and reverse micelles of surfactant, respectively D, F liquid crystalline mesophases with lamellar and reverse hexagonal packing of surfactant molecules, respectively. Concentrations of all components are expressed as %(w/w). Cross-section of a type shows an example of the variation of water content at constant surfactant-to-organic solvent ratio cross-section of p type shows an example of the variation of organic solvent content at constant water-to-surfactant molar ratio. 

In the past HPLC separations of nucleosides have been carried out in a variety of modes, including anion-exchange (Floridi et al., 1977) and cation-exchange (Breter et al., 1977) however, since the introduction of stable packings reversed phase has become the preferred method. Typical chromatographic conditions for the separation of nucleosides include the use of dilute phosphate buffers with organic modifiers such as methanol or acetonitrile on ODS stationary phases. The effects of variations in these parameters is described below. 

This HPLC procedure uses weak" reversed-phase column packings and inverse-salt gr ents. While it can be consider as a kind of salting out" chromatography, it is also a variation of RPLC 48,49). Retention is related to salt concentration as... 

Comparison of the sterols and sterol esters of coconut and palm kernel oils have showed sufficient differences to form a basis for distinguishing between the two oils. Sterols were isolated as the digitonides and analyzed using packed-column GLC. Sterol esters, separated by preparative TLC, were analyzed by temperature-programmed capillary GLC (50% phenyl 50% methlypolysi-loxane stationary phase) and reversed-phase HPLC. Palm kernel oil displayed two major peaks, apparently due to campesteryl myristate and unresolved sitosteryl myristate/avenasteryl palminate, which were present at lower levels in the coconut oil. In addition, variations in the concentrations of other components were observed. 

Gas-solid-liquid PTC is a particularly interesting variation due to the absence of an organic solvent and the possibility of continuous operation in a plug-flow reactor packed with a solid such as inert alumina spheres. Other atypical variants include inverse PTC, where an organic-soluble reagent is transported by a suitable transfer agent into the aqueous phase, for reaction to occur there. Because of the reverse direction of catalyst transfer, it is appropriately called reverse PTC. Insoluble PTC results when the PT catalyst is immobilized on a solid support and used in a traditional liquid-liquid reaction system, or a three-phase liquid-liquid-liquid (L-L-L) systan is involved where the PT catalyst is concentrated in a third Uqnid phase. 

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