Chiral stationary phases components

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... 

Analysis using a CMPA is usually resolved on a nonchiral column. A transient diastereomeric complex is formed between the enantiomer and the chiral component in the mobile phase, similar to the complexes formed with chiral stationary phases. A review by Liu and Liu (2002) cites several papers where addition of CPMAs has been used in analyzing amphetamine-related compounds. Some CPMAs include amino acid enantiomers, metal ions, proteins, and cyclodextrins. Advantages of this method of analysis include the use of less expensive columns and more flexibility in the optimization of chiral separation (Misl anova and Hutta, 2003). 

Capillary gas chromatography (GC) using modified cyclodextrins as chiral stationary phases is the preferred method for the separation of volatile enantiomers. Fused-silica capillary columns coated with several alkyl or aryl a-cyclo-dextrin, -cyclodextrin and y-cyclodextrin derivatives are suitable to separate most of the volatile chiral compounds. Multidimensional GC (MDGC)-mass spectrometry (MS) allows the separation of essential oil components on an achiral normal phase column and through heart-cutting techniques, the separated components are led to a chiral column for enantiomeric separation. The mass detector ensures the correct identification of the separated components [73]. Preparative chiral GC is suitable for the isolation of enantiomers [5, 73]. 

Generally, CD-based chiral stationary phases have been used in the reversed-phase mode. Earlier, it was assumed that in the normal phase mode, the more nonpolar component of the mobile phase would occupy the CD cavity, thereby blocking inclusion complexation between the chiral analyte and CD [4,11], But with the development of CD derivatives, it has become possible to use the normal phase mode too [45,74], Among the various CSPs based on CD derivatives, one based on a naphthylethyl carbamoylated derivative has shown excellent enantioselectivity in the normal phase mode [46,59]. Armstrong et al. [45] synthesized several /CCD derivatives and had them tested in the normal phase mode to resolve the enantiomers of a variety of drugs hexane-2-propanol (90 10, v/v) served as the mobile phase. The authors discussed the similarities and differences of the enantioselectivities on the native and derivatized CD phases. 

Aroma compounds originate from biosynthetic pathways inside an animal, a botanical body, and other life-forms as well as enzymes and thus frequently carry chiral components within the molecule. Determination of such enantiomeric properties can, in many cases, be accomplished using a GC column with a chiral stationary phase (CSP) application.75-79 These columns, usually called chiral GC column, will provide diastereometric interaction that could lead to resolution of enantiomers. Commercially available chiral GC columns predominantly utilize cyclodextrin derivatives as CSPs. Chiral columns consisting of multiple cyclodextrin derivatives intending synergic effect in resolution property80 are also successful in the market. In practice, these columns are mainly operated as secondary columns in MDGC technique. 

Chromatographic enantioseparation of chiral xenobiotics and their metabolites is a versatile tool for process studies in marine and terrestrial ecosystems [235]. In 1994, three papers focused on the enantioselective determination of toxaphene components [120,236,237]. Buser and Muller found that technical toxaphene mixtures are not necessarily racemic [237]. This observation was supported after isolation of non-racemic B7-1453 from the product Melipax which had an excess of ca. 25% of the dextrorotary enantiomer [27, 238]. The enantioselective separation of toxaphene components is almost restricted to chiral stationary phases (CSPs) based on randomly derivatized ferf-butyldimethyl-silylated /1-cyclodextrin (commercially available from BGB Analytik, Adliswil, Switzerland). So far, only a few toxaphene components were enantioseparated on other CSPs [239, 240]. Some of these CSPs are not well defined as well, and for this reason a test mixture called CHIROTEST X was suggested for initial column testing [241],... 

The separation of enantiomers is especially important in the pharmaceutical field, because drag enantiomers may produce different effects in the body. Enantiomer separations by chromatography require one of the components of the phase system to be chiral. This can be achieved by (a) the addition of a chiral compound to the mobile phase, which is then used in combination with a nonchiral stationary phase, or (b) the use of a chiral stationary phase in combination with a nonchiral mobile phase. The chiral phase can either be a solid support physically coated with a chiral stationary phase liquid or a chemically bonded chiral phase. For mobile-phase compatibility reasons, a chiral stationary phase is preferred in LC-MS. However, most chiral stationary phases have stringent demands with respect to mobile-phase compositiorr, which in turn may lead to compatibility problems. Three types of phase systems are applied in LC-MS ... 

This isotherm model has been used successfully to accoimt for the adsorption behavior of numerous compounds, particularly (but not only) pairs of enantiomers on different chiral stationary phases. For example, Zhou et ah [28] foimd that the competitive isotherms of two homologous peptides, kallidin and bradyki-nine are well described by the bi-Langmuir model (see Figure 4.3). However, most examples of applications of the bi-Langmuir isotherm are found with enantiomers. lire N-benzoyl derivatives of several amino acids were separated on bovine serum albumin immobilized on silica [26]. Figure 4.25c compares the competitive isotherms measured by frontal analysis with the racemic (1 1) mixture of N-benzoyl-D and L-alanine, and with the single-component isotherms of these compounds determined by ECP [29]. Charton et al. foimd that the competitive adsorption isotherms of the enantiomers of ketoprofen on cellulose tris-(4-methyl benzoate) are well accounted for by a bi-Langmuir isotherm [30]. Fornstedt et al. obtained the same results for several jS-blockers (amino-alcohols) on immobilized Cel-7A, a protein [31,32]. 

Recently, Lindholm et al. [24] measured by the perturbation method the quaternary isotherms of the two enantiomers of methyl- and ethyl- mandelate on a chiral stationary phase. The method developed earlier by Forss6n et al. [47] consists in injecting perturbation pulses that contains concentrations of the different components that are alternately higher and lower than the plateau concentrations of the different components (see Eq. 4.104). This method ensures that the different perturbations (four in the present case) are easily detectable and that their retention times can be measured accurately. The isotherm data were modeled using a quaternary bi-Langmuir model and excellent agreement was reported between experimental and calculated band profiles. 

In the former case [32], the production rate of 99% pme enantiomers from the racemic mixture of R- and S-2-phenylbutyric acid was maximized as a function of the sample size and the mobile phase composition. The calculations were based on the column performance and the equilibrium isotherms of the two components (bi-Langmuir isotherms. Chapter 3). The separation was performed on immobilized bovine serum albumin, a chiral stationary phase, using water-methanol solution as the mobile phase. The retention times decrease with increasing methanol content, but so does the separation factor. For this reason, the optimum retention factor is around 3. Calculated production rates agree well with those measured (Table 18.4). The recovery yield is lower than predicted. 

Despite all that we know of the essential oil components, many new compounds are constantly being isolated and their structures determined. The determination of the two-dimensional structures of small molecules is becoming much easier based on the intensive development of NMR spectroscopy techniques. On the other hand, the most important breakthrough in the past decade for essential oil study has probably been the development of various chiral stationary phases useful for the separation of enantiomeric mixtures [68-72],... 

The first effective chiral stationary phase, as already referred to in chapter 1, utilized derivatized amino acids to provide chiral selectivity [7] and this was achieved as early as 1966. These types of stationary phases however, had very limited temperature stability and the optimum temperature for separation was often in excess of that at which the stationary phase was stable. The first relatively stable stationary phase, as already mentioned, was introduced by Bayer [6], who combined the derivatized optically active component of the stationary phase in a polysiloxane gum. Nevertheless, a number of small molecular weight materials were used as stationary phase in the early days of chiral gas chromatography. The first type introduced by Gil-Av are shown below. 

Each of the five main types of LC chiral stationary phase have fairly wide areas of application. As a result, there is much overlap in the use of the different stationary phases, and many specific chiral separations can be accomplished by any one of the five stationary phases, providing the optimum operating conditions for the particular stationary phase are identified. Probably the most important applications come from the pharmaceutical and biochemical industries and, to some extent, from the clinical laboratories. The other important application source for chiral chromatography is the essential oil industries, but, as most of the components of essential oils are, by definition, fairly volatile the applications from this source are usually separated by GC. 

To accomplish this objective, a correct selection of the column is key the ones which are the most used for essential oils are the polar, and in particular, have chiral stationary phases those essential oils which have components of interest that present optical isomerism. 

Owing to its intermediate position between GC and LC, SFC can be performed equally well in open capillaries and packed columns. The separation can be influenced by the type of stationary phase and of modifier, by pressure, pressure drop, and temperature. In contrast to GC, SFC can also be used for the separation of nonvolatile or thermally labile compounds (although some temperature compatibility is necessary). The separation of enantiomers on chiral stationary phases can be very attractive because the temperature is lower than in GC, which increases the separation factors. SFC is an alternative to normal-phase LC because it is fast and carbon dioxide is ecologically sound. An example of an SFC separation can be found in the previous article. Principles, where Figure 2 shows the separation of orange oil components. 

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