Eva Tesarova

Enhanced selectivity in CZE multi-chiral selector enantioseparation systems: proposed separation mechanism.

It has been reported many times that the commercial mixtures of chiral selectors (CS), namely highly sulfated β‐CDs (HS‐β‐CDs), provide remarkable enantioselectivity in CZE when compared with single‐isomer CDs, even single‐isomer HS‐β‐CDs. This enhanced enantioselectivity of multi‐CS enantioseparative CZE is discussed in the light of multi‐CS model that we have introduced earlier. It is proposed on a theoretical basis and verified experimentally that the two enantiomers of a chiral analyte under interaction with a mixture of CSs are very likely to differ in their limit mobilities, which is opposite to single‐CS systems where the two limit mobilities are likely to be the same. Thus while the enantioseparation is usually controlled by different distribution constants between the two enantiomers and CS used in single‐CS systems, an additional, electrophoretic, enantioselective mechanism resulting from different limit mobilities may play a significant role in multi‐CS systems. This additional mechanism generally makes the multi‐CS systems more selective than the single‐CS systems. The possible inequality of limit mobilities is also significant for optimization of separation conditions using mixtures of CSs. A practical example supporting our considerations is shown on enantioseparation of lorazepam in the presence of a commercial mixture of HS‐β‐CDs and a single‐isomer HS‐β‐CD, heptakis(6‐O‐sulfo)‐β‐CD.

Comparison of enantioselective HPLC separation of structurally diverse compounds on chiral stationary phases with different teicoplanin coverage and distinct linkage chemistry.

Enantioseparation performance of two teicoplanin-based chiral stationary phases, Chirobiotic T and Chirobiotic T2, with different teicoplanin coverage and distinct linkage chemistry was compared. Three structurally diverse groups of analytes, amino alcohols (beta-blockers), chlorophenoxypropionic acids, and branched-chain amino acids, in various mobile phase compositions/separation modes were examined. The amino alcohols showed better enantioresolution on Chirobiotic T2 in reverse-phase, hydrophilic interaction chromatography, and polar-ionic mode separation systems. The best results with these analytes were obtained in the polar-ionic mobile phase. In contrast, the chlorophenoxypropionic acids and the branched-chain amino acids yielded an improved enantioresolution on the chiral stationary phase with lower amount of teicoplanin in the hydrophilic interaction chromatography system. The comparison of separation of the chlorophenoxypropionic acids enantiomers in the reverse-phase and hydrophilic interaction chromatography environments showed completely opposite results. While better enantioresolution of chlorophenoxypropionic acids was achieved on Chirobiotic T2 in mobile phases with low methanol content, high methanol concentration was needed to reach baseline enantioseparation on Chirobiotic T.

Sterility testing by CE: A comparison of online preconcentration approaches in capillaries with greater internal diameters

Detection of microbial contamination is of critical importance in the medical and the food industry. Rapid tests for the absence or presence of viable microorganisms are in urgent demand. CE is a modern analytical technique that can be adapted for rapid screening of microbial contamination. However, the small dimensions of capillaries allow the introduction of only a small fraction of the sample, which can be problematic when examining large samples. In this article, we examine the possibilities of introducing larger sample volumes using capillaries with greater id together with different stacking techniques. The use of 0.32 mm id capillary and the injection of 60% of the capillary volume led to approximately 120‐fold improvement of the injected sample volume over the classical injection 5% of a 0.10‐mm id capillary. The setup we described opens new possibilities in sterility testing using CE.

Model of CE enantioseparation systems with a mixture of chiral selectors: Part II. Determination of thermodynamic parameters of the interconversion in chiral and achiral environments separately

The theoretical assumption that a multi-CS enantioseparation system, the model of which was described in Part I of this work, can be treated as a separation system with only one CS is confirmed by a set of experiments. The model assumes that each individual analyte-CS interaction is fast, fully independent on other interactions and the analyte: CS ratio is 1:1 and that the analyte is present in its concentration small enough not to considerably change the concentration of free CSs. An enantioselective environment in affinity capillary electrophoresis is created using a commercially available mixture of highly sulfated beta-cyclodextrines as chiral selectors (CSs) and lorazepam as an analyte that undergoes interconversion during the separation process. Dependencies of the electrophoretic mobilities of the two enantiomers on concentration of the CSs mixture are proved to follow the proposed multi-CS model. Global rate constants of interconversion are determined at various temperatures and concentrations of the CSs mixture. In accord with the proposed theory, linear dependencies of the global rate constants on CSs concentration are achieved. Intercepts and slopes of these plots correspond to local rate constants of interconversion in achiral (without the mixture of CSs in background electrolyte (BGE)) and chiral (with the CSs mixture in BGE) environments, respectively. The experimentally obtained electropherograms show an excellent fit with those resulting from the computer simulation based on our model.

Study on the aggregation of teicoplanin

The aggregation of teicoplanin was studied in four solutions: 0.06 M phosphate buffers pH 4.3 and 6.3, both with and without 10% (v/v) of acetonitrile. Conductometry, capillary electrophoresis, cyclic voltammetry, and static light scattering were employed to determine the critical micelle concentration (CMC) of teicoplanin. Dynamic light scattering was used to give an additional information on the size and the size distribution of the particles formed. While the CMC was found in solutions without acetonitrile, attempts to detect any CMC failed in solutions containing acetonitrile. The results point out to different solvating mechanism in solutions with and without acetonitrile, leading to two different schemes of association.

Chromatographic methods enabling the characterization of stationary phases and retention prediction in high‐performance liquid chromatography and supercritical fluid chromatography

In the scope of the present review, the current status of high-performance liquid chromatography/ultra-high performance liquid chromatography and supercritical fluid chromatography is briefly provided. These techniques and their retention mechanisms are compared. Various alternative approaches utilized for the determination and description of the retention processes in these two systems are mapped. Two frequently used concepts, linear-free energy relationships, and hydrophobic subtraction models, used for the characterization of the retention interactions, are discussed. Principles and selected applications of the both methods are also covered. Then the models applied for the prediction of retention behavior of solutes on stationary phases are outlined. The procedures utilized for the sorbent/column classification are also covered. Simple chromatographic tests frequently used for the basic characterization and mutual comparison of stationary phases are summarized and briefly commented on. The importance of a statistical evaluation of complex retention data obtained from the chromatographic measurements is outlined. Finally, computer simulations aiming at the facilitation of the quest to optimize separation conditions for a given mixture of analytes are touched upon.

Cyclic Oligosaccharide-Based Chiral Stationary Phases Applicable to Drug Purity Control; A Review

Oligosaccharide-based chiral stationary phases are frequently used for enantioselective separations by different chromatographic techniques, namely gas chromatography, high performance liquid chromatography, supercritical fluid chromatography or capillary electrochromatography. Their multimodal application potential (they are compatible with both polar and/or non-polar mobile phases) makes them suitable chiral selector candidates for separation of a wide variety of structurally diverse compounds. In this paper, separation systems utilizing cyclodextrin- or cyclofructan-based chiral stationary phases in analyses of pharmacologically active compounds are summarized. The review covers the period from 2000 to 2015. This review article can be helpful to analysts searching for an appropriate method for the separation/determination of pharmaceuticals of their interest.

Occurrence and behavior of system peaks in RP HPLC with solely aqueous mobile phases

System peaks are important but often also disturbing phenomena occurring in separation systems. Behavior of system peaks was studied in reversed phase high performance liquid chromatography (RP HPLC) systems consisting of an RP Amide C16 column and aqueous solutions of organic acids with alkaline metal hydroxides as mobile phases. Binary mobile phases, composed of benzoic acid and lithium hydroxide (LiOH) or cesium hydroxide (CsOH), yielded two system peaks. The first peak was stationary and the second one moved with dilution of the mobile phase or with changes of the alkaline metal hydroxide concentration. The latter changes affected dissociation of the benzoic acid present in the mobile phase and thereby its retention. The presumption that the first system peak is not influenced by the type of alkaline metal cation and that it is related to the non-adsorbed component of the mobile phase was confirmed by a cyclic procedure. Three-component mobile phases composed of benzoic acid, tropic acid, and a hydroxide gave rise to three system peaks as expected. The first peak was again stationary and the two others shifted depending on the concentration variation of both acids. Resonance causing a zigzag peak, well described in capillary zone electrophoresis (CZE), was observed if 1-pentanol was injected into a chromatographic system with one-component mobile phase.