Polysaccharide-Coated Phases

The inherent chiral nature and availability of natural polymers, such as cellulose and amylose, were the primary reasons of their use in chiral separations. The ability of cellulose to separate racemic mixtures was first observed in paper chromatography [76,77]. The breakthrough point in the use of cellulose and amylose in modern HPLC was achieved with the development of CSPs where saccharides were adsorbed on the surface of aminopropyl-modified macroporous silica, [78,79]. 

Silica-based material provides mechanical stability and high efficiency, while adsorbed polysaccharide helices offer chiral selectivity to the wide range of enantiomeric analytes. 

Polysaccharide-coated CSP are prepared by coating of their benzoate or carbamate derivatives on substrate from a solution with the following evaporation of the solvent. These phases can be used under RP conditions with aqueous eluents or in normal-phase conditions with limited number of solvents to avoid dissolution of the coated layer [80, 81]. 

The nature and the position of the substituents introduced into the benzene ring of the carbamate derivatives essentially define the chiral recognition ability of these CSPs [82]. Higher long-term stability of polysaccharide phases was achieved with covalent bonding of polysaccharide to the surface of the support [83]. 

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The coated polysaccharide-based phases have mostly been used in normal phase conditions, but an increasing number of preparative applications have been reported in supercritical fluid chromatography [33] or reversed phase mode [34]. The broad applicability of the coated polysaccharide-based CSPs has made them very popular and they are now widely used for preparative separation of enantiomers and large-scale applications up to tonnes per year have been reported [35, 36]. The success of these CSPs is documented in numerous papers and these CSPs are the most used phases for analytical and preparative applications. 

The effects of coating the liposomes with HPs on the partition in two different aqueous two-phase systems are summarized in Tables 1 and 2 and Fig. 2. The conventional liposomes without any HP coat locate mostly at the interface between the two polymer phases as reported previously [5-7]. On the other hand, the HP-coated liposomes were significantly partitioned to the bottom polysaccharide-rich phase. The extent of the partition depended on the chemical structure and density of HP on the liposomal smface. 

In contrast to the previous report, in this study partition of the HP-coated liposomes to the bottom polysaccharide-rich phase occurred significantly. This suggests that a specific interaction takes place between the liposomal polysaccharide and the phase-forming polysaccharide in the bulk bottom... 

The partition of the HP-coated liposomes from the interface into the bottom polysaccharide-rich phase can be described by specific binding between the two carbohydrates [Eq. (1)]. 

By clicking the appropriate buttons on the form, the user can combine molecular structure queries of sample, CSP and solvent, using operators AND, OR, NOT with data queries in one search. A query for the search of chiral separations of alpha-aromatic acids on any polysaccharide phases coated on silica gel providing an alpha value superior to 1.2 is shown in Eig. 4-4. 

Cass et al. [66] used a polysaccharide-based column on multimodal elution for the separation of the enantiomers of omeprazole in human plasma. Amylose tris (3,5-dimethylphenylcarbamate) coated onto APS-Hypersil (5 /im particle size and 120 A pore size) was used under normal, reversed-phase, and polar-organic conditions for the enantioseparation of six racemates of different classes. The chiral stationary phase was not altered when going from one mobile phase to another. All compounds were enantioresolved within the elution modes with excellent selectivity factor. The separation of the enantiomers of omeprazole in human plasma in the polar-organic mode of elution is described. 

The ultimate combination of HPLC and AC is effectuated in High Performance Affinity Chromatography (HPAC).47 The development of this hybrid technique was highly assisted by the use of modified silica. Traditional polysaccharide supports may not be used for HPAC, because they lack mechanical stability to withstand the high pressure drops, inherent to this method. Modified silica beads are well suited. These may be coated with active groups as in normal AC applications. Additionally, if the separation requires the use of an organic stationary phase, the silica beads are modified with a silane or polymer with subsequent deposition of polysaccharides such as dextrans, agarose or cellulose.50... 

According to our earlier classification, the stationary phase can be a solid, a liquid, or a bonded phase. In the latter two cases, the phase must be coated on, or bonded to, particles of a porous solid support. Only a few materials have found widespread use as stationary solid supports they are silica, synthetic polymers such as the styrene-divinylbenzene copolymer, diatomaceous earths, and some polysaccharides. The most common types and uses are given in Table 2. 

Aldonitrile acetates of sugars have been applied to their GC analysis in different polysaccharides [442] on LAC-4R-886 polyester stationary phase (190°C)and to the analysis of polyols and aldoses in urine and crystalline lenses [444] on a capillary column of borosilicate glass (60 m X0.3 mm l.D.) coated with SE-30 containing a dispersion of Silanox 101 (temperature programming at l°C/min from 150°C). These derivatives were very stable and a uniform product was formed from every individual substrate. 

One considerable disadvantage of coated polysaccharide type CSPs, however, is the high solubility of the SO in many organic solvents, e.g. chloroform, ethylacetate, and tetrahydrofuran, restricting the choice of mobile phases that can be used. Accordingly, inflexibility in the optimization of separations and enantioselectivity is a considerable drawback this counts in particular for preparative separations, where often the solubility of the SAs in the mobile phase is limited and thus loadability and finally the productivity rate is reduced. 

Packed column SFC stationary phases are very similar or identical to those used for HPLC. With neat CO2 mobile phases, polymer or polymer-coated silica stationary phases have typically been used. With modified-C02 mobile phases, bonded-phase silica columns are typically used. For structural separations, diol, amino, or cyano stationary phases are most often used. For stereochemical separations, derivatized polysaccharide-bonded silica columns are most often the stationary phases of choice. A powerful feature of modified-C02 pSFC is the ability to serially connect different stationary phases to obtain enhanced or mul-... 

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