Chiral stationary phases characteristics

Possible differences are also well illustrated by 3-thio- and 3-methyl-thiohex-anols and their esters (Table 1). Among these compounds, there is a tendency for the (R) enantiomers to have a typical, fruity aroma. However, for 3-methylthiohexanol (an aroma component of yellow passion fruit) this situation is reversed the (S) enantiomer had the characteristic fruity aroma ( exotisch, fruchtig ).52 For the separation of enantiomers of odorous compounds, enan-tioselective GLC with chiral stationary phases, and MGDC techniques using a conventional capillary column and an enantioselective column are commonly used.53... 

Although the Bilangmuir isotherm is an ideal model (in the sense that real surfaces with exactly the characteristics described by the Bilangmuir model do not exist), it is often successful in describing the adsorption of enantiomers on chiral stationary phase. The reason is that, because of their... 

The inherent chirality of proteins makes them very good candidates for the separation of enantiomers. Proteins which can tolerate organic solvents, as well as high temperatures, and which can function over a wide range of pH are useful as chiral stationary phases. Table 22-2 shows some of the characteristics of these proteins [138]. 

TABLE 22-2. Characteristics of Some Proteins Used as Chiral Stationary Phases... 

The first chiral separation using pSFC was published by Caude and co-workers in 1985 [3]. pSFC resembles HPLC. Selectivity in a chromatographic system stems from different interactions of the components of a mixture with the mobile phase and the stationary phase. Characteristics and choice of the stationary phase are described in the method development section. In pSFC, the composition of the mobile phase, especially for chiral separations, is almost always more important than its density for controlling retention and selectivity. Chiral separations are often carried out at T < T-using liquid-modified carbon dioxide. However, a high linear velocity and a low pressure drop typically associated with supercritical fluids are retained with near-critical liquids. Adjusting pressure and temperature can control the density of the subcritical/supercritical mobile phase. Binary or ternary mobile phases are commonly used. Modifiers, such as alcohols, and additives, such as adds and bases, extend the polarity range available to the practitioner. 

Alkylcarbonic acids have been separated into their enantiomers without any deriva-tization and their sequence of elution was assigned by co-injection with enantio pure references [17]. Latest results on stereoselective flavour evaluation revealed characteristic sensory properties for all the enantiomers of 2-alkylbranched acids, esters and corresponding alcohols. Tremendous differences between the mirror images of 2-methylbutanoic acid have been found. While the R-enantiomer exhibits a penetrating, cheesy-sweaty odour, the S-enantiomer emits a pleasant sweet and fine fruity note [87, 88]. All commercially available homologues of 2-methylbutanoic acid esters and 2-methylbutyl acetate are simultaneously stereoanalyzed, using heptakis (2,3-di-O-methyl-6-0-tert-butyldimethylsilyl)-P-cyclodextrin (DIME-P-CD) in PS 268 as the chiral stationary phase [88] (Eig. 6.33, Table 6.19). 

Chiral stationary phase used to separate optically active enantiomeric compounds, by bonding a stationary phase molecule that has enantiomeric properties to a solid support. The stationary phase therefore has specific optical retention characteristics see bonded stationary phases. 

The first effective chiral stationary phases for GC were the derivatized amino acids,which, however, had very limited temperature stability. The first reliable GC stationary phase was introduced by Bayer and coworkers,who synthesized a thermally stable, low-volatility polymer by attaching 1-valine-tbutylamide to the carboxyl group of dimethylsiloxane or (2-carboxypropyl)-methylsiloxane with an amide linkage. This stationary phase was eventually made available commercially as Chirasil-Val and could be used over the temperature range of 30°C to 230°C. OV-225 (a well-established polar GC stationary phase) has also been used for the synthesis of chiral polysiloxanes, which, in this case, possess more polar characteristics than the (2-carboxypropyl)-methylsiloxane derivatives. 

Characteristics of Optimization in Individual HPLC Modes chiral stationary phases... 

Miwa T, Miyakawa T, Miyake Y (1988) Characteristics of an avidin-conjugated column in direct liquid chromatographic resolution of racemic compounds. J Chromatogr 457 227-233 Erlandsson P, Marie I, Hansson L, Isaksson R, Petterson C, Petterson G (1990) Immobilized ceUulase (CBH I) as a chiral stationary phase for direct resolution of enantiomers. J Am Chem Soc 112 4573-4574... 

The various properties exhibited by ILs make them ideal stahonary phases in GLC. ILs exhibit a unique dual-nature selechvity that allows them to separate polar molecules like a polar stationary phase and nonpolar molecules like a nonpolar stationary phase. In addition, the combination of cations and anions can be tuned to add further selectivity for more complex separations. Viscosity, thermal stability, and surface tension are vital properties that dictate the quality and integrity of the stationary phase coating and are additional characteristics that can be controlled when custom designing and synthesizing ILs. Furthermore, thermal stability and the integrity of stationary phase film can be improved by immobilizing the IL by free radical polymerization to form stationary phases suitable for low- moderate-, and high-temperature separations. Chiral ILs have been shown to enantioresolve chiral analytes with reasonable efficiency. 

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