Numerical analysis enantiomers

When using PFT with a neutral selector, it is quite difficult to avoid any entrance of the chiral selector into the ionization source, particularly at a high pH, where EOF is important. The use of BGE at low pH and/or coated capillary to minimize EOF is therefore mandatory. However, the coaxial sheath gas, which generally assists the ionization process, leads to an aspirating phenomenon of the chiral selector in the MS direction. Javerfalk et al. were the first to apply PFT with a neutral methyl-/i-CD for the separation of racemic bupivacaine and ropivacaine with a polyacrylamide-coated capillary and an acidic pH buffer (pH 3). Cherkaoui et al. employed another neutral CD (HP-/1-CD) with a PVA-coated capillary for the analysis of amphetamines and their derivatives. To prevent a detrimental aspiration effect, analyses were carried out without nebulization pressure. Numerous other studies presented excellent results such as the enantioselective separation of adrenoreceptor antagonist drugs using tandem mass spectrometry (MS/MS) the separation of clenbuterol enantiomers after solid-phase extraction (SPE) of plasma samples or the use of CD dual system for the simultaneous chiral determination of amphetamine, methamphetamine, dimethamphetamine, and p-hydroxymethamphetamine in urine. 

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]. 

We compare in Figure 10.12 the band profiles calculated for the (+) isomer of Troger s base using the forward-backward numerical method and an OCFE method. To avoid a circular argument, the isotherms were obtained by frontal analysis and the column efficiency was measiued imder linear conditions (from very small size injections) [59]. There is a significant difference between the band profiles, because the column efficiency is poor, 110 and 150 theoretical plates for the (-) and (+) enantiomers, respectively, under analytical conditions. As expected, the finite difference method introduces significant errors even in the case of a single component profile. 

The numerical results shown in Fig. 6.2 confirm that the rotation direction of n electrons temporally changes between clockwise and counterclockwise in the case of a single-pulse control. Switching of the rotation direction can be prevented efficiently, and unidirectional rotation of Jt electrons can be realized consecutively in a simple manner. In the three-level model analysis in a short-pulse limit, as already stated, the pulse with e+ (e ) creates a coherent superposition L) - - H) ( L) — H)), and L) - - H) created by a pump pulse with e+ evolves as L) - - H) L) — i H). Then the population in —) can be dumped to G) by applying a dump pulse with e just after the created state has completely shifted as L) — i H) L) — H). Thus, only clockwise rotation can be generated. Figure 6.3 shows the results of a pump-dump control simulation of an R enantiomer of DCPH. The values of the parameters of the pump pulse were/ = 2.24 GVm i, = 19.4 fs, co = 7.72 eVM, and e = e+, and those of the dump pulse were / = 2.37 GVm , tj = 19.4 fs, co = 7.72 eV/h, and e = e. The delay time between the pulses was 19.4 fs. 

After successful application of enantioselective GC to the analysis of enantiomeric composition of monoterpenoids in many essential oUs (e.g., Werkhoff et al., 1993 Bicchi et al., 1995 and references cited therein), the studies have been extended to the sesquiterpene fraction. Standard mixtures of known enantiomeric composition were prepared by isolation of individual enantiomers from numerous essential oils by preparative GC and by preparative enantioselective GC. A gas chromatographic separation of a series of isolated or prepared sesquiterpene hydrocarbon enantiomers, showing the separation of 12 commonly occurring sesquiterpene hydrocarbons on a 2,6-methyl-3 pentyl-p-cyclodextrin capillary column has been presented by Kbnig et al. (1995). Further investigations on sesquiterpenes have been published by Kbnig et al. (1994). However, due to the complexity... 

Using the indirect approach to achieve a chiral separation diminishes the need for a CS in the running buffer. Prior to analysis with CE, the enantiomers are derivatized with an optically pure agent to form diastereomers. An achiral environment, in which a pseudo-stationary phase is present, is sufficient to separate these diastereomers because they possess different physicochemical properties. The indirect separation is an efficient and versatile approach mainly because of the availability of numerous chiral derivatization reagents. These derivatizing reagents can contain chromo-phore, fluorophore, or electrochemical groups, which improves the detection of hardly detectable compounds. 

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