Bonded phase brush type

The above data were obtained on a polymeric bonded phase and not a brush phase. The so-called brush phases are made from monochloro-sxlants, (or other active group) and, thus, the derivative takes the form of chains attached to the silica surface [2]. The bulk phases are synthesized from polyfunctional silanes in the presence of water and, thus, are cross linked and form a rigid polymeric structure covering the silica surface. These two types of phases behave very differently at low concentrations of moderator. 

In contrast, the alkane chains on the polymeric phase cannot collapse in an environment of water as they are rigidly held in the polymer matrix. Thus, the retention of the solute now continuously falls as the methanol concentration increases as shown in Figure 4. It should be pointed out that if the nature of the solutestationary phase interactions on the surface of a bonded phase is to be examined in a systematic manner with solvents having very high water contents, then a polymeric phase should be used and brush type reversed phases avoided if possible. 

The brush-type (Pirkle-type) CSPs have been used predominantly under normal phase conditions in LC. The chiral selector typically incorporates tt-acidic and/or n-basic functionality, and the chiral interactions between the analyte and the CSP include dipole-dipole interactions, n-n interactions, hydrogen bonding, and steric hindrance. The concept of reciprocity has been used to facilitate the rational design of chiral selectors having the desired selectivity [45]. 

The most popular bonded phases are, without doubt, the reverse phases which consist solely of aliphatic hydrocarbon chains bonded to the silica. Reverse phases interact dispersively with solvent and solute molecules and, as a consequence, are employed with very polar solvents or aqueous solvent mixtures such as methanol/water and acetonitrile/water mixtures. The most commonly used reverse phase appears to be the brush type phase with aliphatic chains having four, eight or eighteen carbon atom chains attached. These types of reverse phase have been termed C4, C8 and Cl8 phases respectively. The C8... 

Brush type phases always include dipole and hydrogen bonding interactions. 

Brush -type bonded phases of the type pioneered by Pirkle. 

Figure 3.8 Reactions used for the synthesis of polymeric or monomeric (or brush type) bonded stationary phases.

Mechanistic considerations (e.g., the extensive work published on brush-type phases) or the practitioner s experience might help to select a chiral stationary phase (CSP) for initial work. Scouting for the best CSP/mobile phase combination can be automated by using automated solvent and column switching. More than 100 different CSPs have been reported in the literature to date. Stationary phases for chiral pSFC have been prepared from the chiral pool by modifying small molecules, like amino acids or alkaloids, by the deriva-tization of polymers such as carbohydrates, or by bonding of macrocycles. Also, synthetic selectors such as the brush-type ( Pirkle ) phases, helical poly(meth) acrylates, polysiloxanes and polysiloxane copolymers, and chiral selectors physically coated onto graphite surfaces have been used as stationary phases. 

Modified-C02 mobile phases excel at stereochemical separations, more often than not outperforming traditional HPLC mobile phases. For the separation of diastereomers, silica, diol-bonded silica, graphitic carbon, and chiral stationary phases have all been successfully employed. For enantiomer separations, the derivatized polysaccharide, silica-based Chiralcel and Chiralpak chiral stationary phases (CSPs) have been most used, with many applications, particularly in pharmaceutical analysis, readily found in the recent literature (reviewed in Refs. 1 and 2). To a lesser extent, applications employing Pirkle brush-type, cyclodextrin and antibiotic CSPs have also been described. In addi-... 

The brush-type of CSP was introduced by Pirkle who was one of the pioneers of modern enantioselective liquid chromatography [55]. The most frequently used 7i-acceptor phases are derived from the amino acids phenylglycine (DNBPG) (Fig. 6.8) or leucine (DNBLeu) covalently or ionically bonded to 3-aminopropyl silica gel [56, 57]. These CSPs are commercially available for analytical or preparative separation of enantiomers. Further CSPs based on amino acid or amine chiral selectors such as valine, phenylalanine, tyrosine [58] and l,2-tr s-diaminocyclohexane (DACH-DNB phase) [59] and 1,2-traus-diphenylethylene diamine (ULMO phase) [60] were also developed (Fig. 6.8). These CSPs have been applied for the preparative separation of the enantiomers of a few racemic compounds, but the number of reported preparative applications has remained very limited over the last 10 years. 

Pirkle or brush type bonded phases Helical chiral polymers (polysaccharides) Cyclodextrins and crown ethers Immobilised enzymes Amino acid metal complexes Three-point interaction Attractive hydrophobic bonding Host guest interaction within chiral cavity Chiral affinity Diastereomeric complexation... 

The analytes listed in Tables 6.12 and 6.13 are flat, so steric hindrance can be neglected when studying their molecular interactions, therefore, a simple and homogeneous model was used instead of a brush-type bonded phase silica gel model whose blush types and density were designed to reduce the steric hindrance effects of the analytes. A model butyl phase with adsorbed naphthalene is shown in Figure 6.34. 

C2-, Cg-, and diphenyl-bonded layers are also manufactured and have been compared with Cis layers for the TLC of 30 compounds representing seven polar and nonpolar compound types (Heilweil and Rabel, 1985). Retention generally increased with longer bonded chain length, as did development times. Polymeric-bonded phases, made with a polyfunctional silane, were more stable than monomeric or brush phases. The aliphatic-bonded layers showed similar selectivit-ies with the various compound classes. Diphenyl and C2 layers had similar selec-tivities, leading the authors to propose a solvation-layer mechanism for reversed-phase separations. 

For example, cyclodextrins form chiral cavities which adsorb the corresponding enantiomers with different affinity while cellulose triacetate crystallizes in the form of helical substructures in which the enantiomers may be incorporated with different rates. For amino acid derived stationary phases there are two types of enantiomer differentiating interactions a brush-like hydrogen bond and dipole interaction plus a /[-complex donor or acceptor interaction with the aromatic residues in the amino acid. 

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