Reversed-phase types

This strategy is the one most commonly used for the analytical determination of ena-tiopurity. A given racemate is reacted with a unichiral derivatizing agent, and the resulting pair of diastereomers is separated on an achiral stationary phase, in most of the cases on a reversed-phase type (Fig. 7-2). 

A large range of stationary phases is available, and according to their polarity they can be divided into normal phase and reversed phase types. Silica gel, aluminium oxide, and a nitrile-bonded-phase are normal adsorbents used to separate carotenoids... 

Silica and alunlna with reversed-phase type mobile phases... 

Also separation of homologs is better achieved by applying bonded phases of the nonpolar or reversed phase type because the methylene groups are only weakly attracted by the polar stationary phase which is already covered by eluent molecules. 

Another approach is electrochromatography with capillary columns packed with an achiral stationary phase, preferentially a reversed-phase type material. The chiral SO is added to the background electrolyte, and may be adsorbed onto the stationary phase by a secondary equilibration process. Enantioseparations in this additive mode have been reported with cyclodextrin type SOs )504-507) and with a chiral ion-pair agent derived from quinine 1508) as mobile phase additives. 

Fig. 12.18. The nonlinear map of the normalised regression eoellicienls of Ihc general solvalion equation. Eq. (12.14), obtained for several reversed-phase types of HPLC systems, (a) Nonlinear map of all columns, (b) Nonlinear map of only the C-18 phases. (From Ref. 64 )...

In SPE, choice of an appropriate solid phase is based on their possible molecular interactions with the targeted compounds. For historical reasons, reversed-phase type adsorbents have been largely utilized to extract compounds from aqueous samples, given the fact that many of them have been developed already for use in reversed-phase chromatography applications and are commercially available. Ion exchange adsorbents have been used to retain counter ions based on electrostatic interactions. Solid phase adsorbents combining hydrophobic and ionic interactions are also present, for example graphitized carbon black (GCB) obtained by... 

Hydrophilic Interaction LC-MS-Based Approaches Hydrophilic interaction LC (HILIC) is a variant of normal-phase LC. HILIC uses hydrophilic stationary phases, but employs reversed-phase type eluents. Any polar chromatographic surface can be used for HILIC separations, even nonpolar bonded silicas. A typical mobile phase for HILIC includes acetonitrile with a small amount of water. However, any aprotic solvent miscible with water (e.g., tetrahydrofunan or dioxane) can be used. Alcohols can also be used with a higher concentration. Ionic additives, such as ammonium acetate and ammonium formate, are usually used to serve as the modifiers for controlling the pH and ion strength of the mobile phase. 

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