Comparison of vancomycin-based stationary phases with different chiral selector coverage for enantioselective separation of selected drugs in high-performance liquid chromatography

Emerging Developments in Separation Techniques and Analysis of Chiral Pharmaceuticals

Chiral separation, the process of isolating enantiomers from a racemic mixture, holds paramount importance in diverse scientific disciplines. Using chiral separation methods like chromatography and electrophoresis, enantiomers can be isolated and characterized. This study emphasizes the significance of chiral separation in drug development, quality control, environmental analysis, and chemical synthesis, facilitating improved therapeutic outcomes, regulatory compliance, and enhanced industrial processes. Capillary electrophoresis (CE) has emerged as a powerful technique for the analysis of chiral drugs. This review also highlights the significance of CE in chiral drug analysis, emphasizing its high separation efficiency, rapid analysis times, and compatibility with other detection techniques. High-performance liquid chromatography (HPLC) has become a vital technique for chiral drugs analysis. Through the utilization of a chiral stationary phase, HPLC separates enantiomers based on their differential interactions, allowing for the quantification of individual enantiomeric concentrations. This study also emphasizes the significance of HPLC in chiral drug analysis, highlighting its excellent resolution, sensitivity, and applicability. The resolution and enantiomeric analysis of nonsteroidal anti-inflammatory drugs (NSAIDs) hold great importance due to their chiral nature and potential variations in pharmacological effects. Several studies have emphasized the significance of resolving and analyzing the enantiomers of NSAIDs. Enantiomeric analysis provides critical insights into the pharmacokinetics, pharmacodynamics, and potential interactions of NSAIDs, aiding in drug design, optimization, and personalized medicine for improved therapeutic outcomes and patient safety. Microfluidics systems have revolutionized chiral separation, offering miniaturization, precise fluid control, and high throughput. Integration of microscale channels and techniques provides a promising platform for on-chip chiral analysis in pharmaceuticals and analytical chemistry. Their applications in techniques such as high-performance liquid chromatography (HPLC) and capillary electrochromatography (CEC) offer improved resolution and faster analysis times, making them valuable tools for enantiomeric analysis in pharmaceutical, environmental, and biomedical research.

Enantioseparation of β-Blockers Using Silica-Immobilised Eremomycin Derivatives as Chiral Stationary Phases in HPLC

The regularities of chromatographic retention and separation enantioselectivity of the selected β-blockers (propranolol, pindolol, alprenolol, atenolol, oxprenalol, metoprolol, clenbuterol, sotalol, pronethalol, salbutamol, and labetalol) were studied with eight chiral stationary phases (CSPs) in polar ionic mode (PIM) elution system. A range of novel CSPs was prepared by immobilisation of macrocyclic glycopeptide antibiotic eremomycin (E-CSP); structurally related antibiotics chloreremomycin (Chloro-E-CSP) and semi-synthetic oritavancin (O-CSP); and five eremomycin derivatives including amide- (Amide-E-CSP), adamantyl-2-amide- (Adamantylamide-E-CSP), eremomycin aglycon (EAg-CSP), eremosaminyl eremomycin aglycon (EEA-CSP), and des-eremosamynyl eremomycin (DEE-CSP) onto microspherical silica (Kromasil, particle size 5 micron, pore size 11 nm). The effect of different functional groups in eremomycin structure on chiral recognition of β-blockers was studied. The original E-CSP revealed moderate enantioseparation for all studied β-blockers. The presence of a free carboxylic group in a chiral selector molecule is found to be critical for the general retention of enantiomers as no separation enantioselectivity was recorded for Amide-E-CSP and Adamantyl-E-CSP. Modification of the aromatic system of eremomycin by the introduction of a chloro- substituent in the aromatic ring (Chloro-E-CSP) or a hydrophobic 4’-chlorobiphenylmethyl substituent to the disaccharide sugar residue (O-CSP) resulted in decreased enantioselectivity. The best enantioseparation of β-blockers was obtained for CSPs with eremosaminyl eremomycin aglycon and des-eremosamynyl eremomycin as chiral selectors.

Enantioseparation of linezolid and tedizolid using validated high-performance liquid chromatographic method

This paper reports the separation of two chiral antibacterial agents namely, linezolid and tedizolid using a validated high-performance liquid chromatographic method. In the current work, glycopeptide-based chiral column, CHIROBIOTIC® V2 (5-μm particle size, L × I.D. 25 cm × 4.6 mm) was employed with a mobile phase containing methanol and 0.15% aq. trifluoracetic acid (75:25%, v/v) in isocratic elution approach at flow rate of 1 ml min^-1 . The separation condition was customized (in terms of resolution values and retention times) was carried out by changing the content of the mobile phase, column temperature, flow rate, and so on. Results showed that the chromatographic separation was achieved within 15 min and average resolution values were 4.6 and 4.8 for tedizolid and linezolid, respectively. The detection limit values were 14.85 and 14.16 ng ml^-1 , respectively, for tedizolid enantiomers. Further, validation of separation parameters was performed by considering the international conference on harmonization guidelines, and ultimately, the mechanism of chiral recognition was also established.

'Ab Ovo' Chiral Phases and Chiral Reagents for Liquid Chromatographic Separation and Isolation of Enantiomers

The de-novo approach of mixing chirally pure reagents or Cu(II)-L-amino acid complexes in the slurry of silica gel for preparing TLC plates was reported from author's laboratory and was successful for separation and isolation of enantiomers. Using high molar absorptivity molecules, e. g., 1,5-difluoro-2,4-dinitrobenzene and cyanuric chloride, more than 38 new chiral derivatizing reagents were synthesized in our laboratory by straightforward nucleophilic substitution with simple chiral auxiliaries. Besides, (S)-naproxen, (S)-ketoprofen, and (S)-levofloxacin were used as chiral platforms. A conceptual approach using both achiral phases in chromatography for enantioseparation was also adopted. ¹ H NMR and DFT based software were used to explain structures of non-covalent and covalent diastereomeric pairs and determination of configuration and separation mechanism. The methods can be easily used to determine and control enantiomeric purity with advantages over a variety of commercial chiral phases.

Chiral separation of beta-blockers by high-performance liquid chromatography and determination of bisoprolol enantiomers in surface waters

Beta-blockers are chiral compounds with enantiomers that have different bioactivity, which means that while one is active, the other can be inactive or even harmful. Due to their high consumption and incomplete degradation in waste water, they may reach surface waters and affect aquatic organisms. To address this issue we developed a chromatographic method suitable for determining beta-blocker enantiomers in surface waters. It was tested on five beta-blockers (acebutolol, atenolol, bisoprolol, labetalol and metoprolol) and validated on bisoprolol enantiomers. Good enantioseparation of all analysed beta-blockers was achieved on the Chirobiotic V column with the mobile phase composed of methanol/acetic acid/triethylamine (100/0.20/0.15 v/v/v) at a flow rate of 0.5 mL/min and column temperature of 45 °C. Method proved to be linear in the concentration range from 0.075 µg/mL to 5 µg/mL, and showed good recovery. The limits of bisoprolol enantiomer detection were 0.025 µg/mL and 0.026 µg/mL and of quantification 0.075 µg/mL and 0.075 µg/mL. Despite its limitations, it seems to be a promising method for bisoprolol enantiomer analysis in surface water samples. Further research could focus on waste water analysis, where enantiomer concentrations may be high. Furthermore, transferring the method to a more sensitive one such as liquid chromatography coupled with tandem mass spectrometry and using ammonium acetate as the mobile phase additive instead of acetic acid and triethylamine would perhaps yield much lower limits of detection and quantification.

Progress in the Enantioseparation of β-Blockers by Chromatographic Methods

β-adrenergic antagonists (β-blockers) with at least one chiral center are an exceedingly important class of drugs used mostly to treat cardiovascular diseases. At least 70 β-blockers have been investigated in history. However, only a few β-blockers, e.g., timolol, are clinically marketed as an optically pure enantiomer. Therefore, the separation of racemates of β-blockers is essential both in the laboratory and industry. Many approaches have been explored to obtain the single enantiomeric β-blocker, including high performance liquid chromatography, supercritical fluid chromatography and simulated moving bed chromatography. In this article, a review is presented on different chromatographic methods applied for the enantioseparation of β-blockers, covering high performance liquid chromatography (HPLC), supercritical fluid chromatography (SFC) and simulated moving bed chromatography (SMB).

Surface Design of Enantiomeric HPLC Separation on Vancomycin and Teicoplanin-Based Stationary Phases, a Tool for Chiral Recognition of Model β-Blockers

A quality-by-design approach was adopted for enantioseparation of atenolol on Vancomycin and Teicoplanin-based chiral stationary phases using reversed phase (RP) mode and polar ionic mode (PIM), respectively to account for major forces involved in enantiorecognition of β-blockers on macrocyclics. A fractional factorial screening design for the two modes; followed by a central composite optimization design and regression analysis were able to point out critical factors and chromatographic responses and robust surface of the design. Within the studied range of flow the optimal was 0.3 mL/min for Chirobiotic T and 1 mL/min for Chirobiotic V. In PIM, a composition of 100% methanol was mandatory to compromise between best separation and least retention with equal amounts of the acid and base modifiers for enantiomers of atenolol, as model drug in addition to metoprolol and pindolol as structurally related compounds for possible extrapolation of results on members of the same class. However, in RP mode, only triethylamine acetate was needed as buffer for atenolol enantiomers. Chiral recognition of atenolol in both elution modes, further confirmed via extrapolation of the models on the two other β-blockers showed that ionic interactions rather than any other forces governed chiral recognition on the two macrocyclic stationary phases in both modes.

Thin Layer Chromatographic Resolution of Some β-adrenolytics and a β2-Agonist Using Bovine Serum Albumin as Chiral Additive in Stationary Phase

Direct enantiomeric resolution of commonly used five racemic β-adrenolytics, namely, bisoprolol, atenolol, propranolol, salbutamol and carvedilol has been achieved by thin layer chromatography using bovine serum albumin (BSA) as chiral additive in stationary phase. Successful resolution of the enantiomers of all racemic β-adrenolytics was achieved by use of different composition of simple organic solvents having no buffer or inorganic ions. The effect of variation in pH, temperature, amount of BSA as the additive, and composition of mobile phase on resolution was systematically studied. Spots were visualized in iodine vapors. Native enantiomers for each of the five analytes were isolated and identified and their elution order was determined. The limit of detection was found to be 0.7, 1.2, 0.84, 1.6 and 0.9 μg (per spot) for each enantiomer of bisoprolol, atenolol, propranolol, salbutamol and carvedilol, respectively.

Two Synthetic Methods for Preparation of Chiral Stationary Phases Using Crystalline Degradation Products of Vancomycin: Column Performance for Enantioseparation of Acidic and Basic Drugs

Two chiral stationary phases (CSPs) based on crystalline degradation products (CDPs) of vancomycin by using different synthetic methods were prepared and compared. Crystalline degradation products of vancomycin were produced by hydrolytic loss of ammonia from vancomycin molecules. Performances of two chiral columns prepared with these degradation products were investigated using several acidic and basic drugs as model analytes. Retention and resolution of these analytes on the prepared columns, as two main parameters, in enantioseparation were studied. The results demonstrated that the stationary phase preparation procedure has a significant effect on the column performance. The resolving powers of prepared columns for enantiomers resolution were changed with the variation in vancomycin-CDP coverage on the silica support. Elemental analysis was used to monitor the surface coverage of silica support by vancomycin-CDP. The results showed that both columns can be successfully applied to chiral separation studies.

Enantioselective biodegradation of pharmaceuticals, alprenolol and propranolol, by an activated sludge inoculum

Biodegradation of chiral pharmaceuticals in the environment can be enantioselective. Thus quantification of enantiomeric fractions during the biodegradation process is crucial for assessing the fate of chiral pollutants. This work presents the biodegradation of alprenolol and propranolol using an activated sludge inoculum, monitored by a validated enantioselective HPLC method with fluorescence detection. The enantioseparation was optimized using a vancomycin-based chiral stationary phase under polar ionic mode. The method was validated using a minimal salts medium inoculated with activated sludge as matrix. The method was selective and linear in the range of 10-800 ng/ml, with a R²>0.99. The accuracy ranged from 85.0 percent to 103 percent, the recovery ranged from 79.9 percent to 103 percent, and the precision measured by the relative standard deviation (RSD) was <7.18 percent for intra-batch and <5.39 percent for inter-batch assays. The limits of quantification and detection for all enantiomers were 10 ng/ml and 2.5 ng/ml, respectively. The method was successfully applied to follow the biodegradation of the target pharmaceuticals using an activated sludge inoculum during a fifteen days assay. The results indicated slightly higher biodegradation rates for the S-enantiomeric forms of both beta-blockers. The presence of another carbon source maintained the enantioselective degradation pattern while enhancing biodegradation extent up to fourteen percent.

Development of square-wave adsorptive stripping voltammetric method for determination of acebutolol in pharmaceutical formulations and biological fluids

A validated simple, rapid, sensitive and specific square-wave voltammetric technique is described for the determination of acebutolol (AC) following its accumulation onto a hanging mercury drop electrode in a Britton-Robinson universal buffer of pH 7.5. The optimal procedural conditions were: accumulation potential E_acc = - 0.8 V versus Ag/AgCl/KCl, accumulation duration t_acc = 30 s, pulse-amplitude = 70 mV, scan rate = 100 mV/s, frequency = 30 Hz, surface area of the working electrode = 0.6 mm² and the convection rate = 2000 rpm. Under these optimized conditions, the adsorptive stripping voltammetry (AdSV) peak current was proportional over the concentration range 5 × 10^-7 - 6 × 10^-6 M (r = 0.999). Recoveries for acebutolol from human plasma and urine were in the range 97-103% and 96-104% respectively. The method proved to be precise (intra-day precision expressed as %RSD in human plasma ranged from 2.9 - 3.2% and inter-day precision expressed as %RSD ranged from 3.4 - 3.8%) and accurate (intra-day accuracies expressed as % error in human urine ranged from -3.3 - 2.8% and inter-day accuracies ranged from -3.3 - 1.7%). The limit of quantitation (LOQ) and limit of detection (LOD) for acebutolol were 1.7 × 10^-7 and 5 × 10^-7 M, respectively. Possible interferences by substances usually present in the pharmaceutical formulations were investigated with a mean recovery of 101.6 ± 0.64%. Results of the developed square-wave adsorptive stripping voltammetry (SW-AdSV) method were comparable with those obtained by reference analytical method.

Effect of chromatographic conditions on liquid chromatographic chiral separation of terbutaline and salbutamol on Chirobiotic V column

In this study, a method for enantioseparation of terbutaline and salbutamol was established using Chirobiotic V column as a stationary phase. Polar ionic mode applying mobile phase containing ammonium nitrate in 100% ethanol, pH 5.1 was found to give the best separation. The salt concentration in the mobile phase and pH value were found to be the most important chromatographic factors affecting separation. Separation of enantiomers of these two basic analytes was complete in less than 10 min without applying ammonium trifluoroacetate (ATFA) or triethylamine (TEA) salts.

Enantioseparation of dansyl amino acids by HPLC on a monolithic column dynamically coated with a vancomycin derivative

In this work a chiral stationary phase was prepared by dynamically coating a monolithic reversed-phase HPLC column with a vancomycin-derivative as chiral selector. A hydrophobic alkyl-chain was attached to the vancomycin molecule, providing the immobilization of the chiral selector on the reversed-phase material. Dansyl amino acids were chosen as model analytes for testing the separation power of the dynamically coated phase. All investigated compounds were separated into their enantiomers. Compared with a conventionally packed vancomycin-CSP, a reversal of the enantiomer elution order was obtained.

Monitoring the effects of chiral pharmaceuticals on aquatic microorganisms by metabolic fingerprinting

The effects of the chiral pharmaceuticals atenolol and propranolol on Pseudomonas putida, Pseudomonas aeruginosa, Micrococcus luteus, and Blastomonas natatoria were investigated. The growth dynamics of exposed cultures were monitored using a Bioscreen instrument. In addition, Fourier-transform infrared (FT-IR) spectroscopy with appropriate chemometrics and high-performance liquid chromatography (HPLC) were employed in order to investigate the phenotypic changes and possible degradation of the drugs in exposed cultures. For the majority of the bacteria studied there was not a statistically significant difference in the organism's phenotype when it was exposed to the different enantiomers or mixtures of enantiomers. In contrast, the pseudomonads appeared to respond differently to propranolol, and the two enantiomers had different effects on the cellular phenotype. This implies that there were different metabolic responses in the organisms when they were exposed to the different enantiomers. We suggest that our findings may indicate that there are widespread effects on aquatic communities in which active pharmaceutical ingredients are present.

Retention mechanism of high-performance liquid chromatographic enantioseparation on macrocyclic glycopeptide-based chiral stationary phases

The development of methods for the separation of enantiomers has attracted great interest in the past 20 years, since it became evident that the potential biological or pharmacological applications are mostly restricted to one of the enantiomers. In the past decade, macrocyclic antibiotics have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance by means of high-performance liquid chromatography (HPLC), thin-layer chromatography and electrophoresis. The glycopeptides avoparcin, teicoplanin, ristocetin A and vancomycin have been extensively used as chiral selectors in the form of chiral bonded phases in HPLC, and HPLC stationary phases based on these glycopeptides have been commercialized. In fact, the macrocyclic glycopeptides are to some extent complementary to one another: where partial enantioresolution is obtained with one glycopeptide, there is a high probability that baseline or better separation can be obtained with another. This review sets out to characterize the physicochemical properties of these macrocyclic glycopeptide antibiotics and, through their application, endeavors to demonstrate the mechanism of separation on macrocyclic glycopeptides. The sequence of elution of the stereoisomers and the relation to the absolute configuration are also discussed.

Stereoisomer analysis of wastewater-derived β-blockers, selective serotonin re-uptake inhibitors, and salbutamol by high-performance liquid chromatography–tandem mass spectrometry

A reversed-phase enantioselective liquid chromatography-tandem mass spectrometry (HPLC-MS-MS) method was developed to measure enantiomer fractions (EF) and concentrations of pharmaceuticals in wastewater. Enantiomer resolution of six beta-blockers (atenolol, metoprolol, nadolol, pindolol, propranolol, and sotalol) along with two selective serotonin re-uptake inhibitors (citalopram, fluoxetine) and one beta(2)-agonist (salbutamol) was achieved with the Chirobiotic V stationary phase. Analyte recovery averaged 86% in influent and 78% in effluent with limits of detection ranging from 0.2 to 7.5 ng/L. These results represent an improvement in wastewater EF measurement for atenolol, metoprolol and propranolol as well as the first EF measurements of citalopram, fluoxetine, nadolol, pindolol, salbutamol and sotalol in wastewaters. Changes in EF through treatment indicate biologically mediated stereoselective processes were likely occurring during wastewater treatment.

Preparation and evaluation of a vancomycin-immobilized silica monolith as chiral stationary phase for CEC

Enantiomeric separations in CEC with the macrocyclic antibiotic vancomycin immobilized silica monolith as a chiral stationary phase are presented. The monolithic silica capillary columns were prepared by a sol-gel process in fused-silica capillaries with an inner diameter of 50 mum and subsequently in situ immobilization of vancomycin as a chiral selector by reductive amination. Enantioselectivity was obtained for eight pairs of enantiomers in nonaqueous polar organic or aqueous mobile phases and most of them were baseline-separated with high column efficiencies. It was observed that the organic modifier ratio (MeOH/ACN) in the polar organic mobile phase played a significant role in controlling the resolution and efficiency of the enantiomers. In enantiomeric separation of propranolol, repeatability for column efficiency and resolution in the nonaqueous mobile phase was given in terms of RSD values at 1.1 and 2.3% (n = 5) for run-to-run injections and 7.2 and 9.6% (n = 5) for column-to-column testing while repeatability for the separation of thalidomide in the aqueous mobile phase was given in terms of RSD values at 1.5, 2.8% and 6.1, 10.5%, respectively.

Determination of β-blockers and β2-agonists in sewage by solid-phase extraction and liquid chromatography–tandem mass spectrometry

A method using solid-phase extraction (SPE) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) has been developed for the determination of 12 beta-blockers and beta(2)-agonists in wastewater samples. Extraction of the drugs was effected by an Oasis MCX cartridge with a strong cation resin adsorbent. Matrix coextractives were removed from the SPE cartridge by methanol prior to the elution of the drugs with a mixture of dichloromethane, 2-propranol, and ammonium hydroxide. The extract was analyzed by LC-MS/MS with electrospray ionization operating in the positive mode. Recovery of the 12 compounds was in most cases better than 85% at the fortification levels of 500 and 50 ng/L, with standard deviations between 3 and 7%. Based on a concentration factor of 250, the method detection limits ranged from 6 to 11 ng/L for the target compounds. No degradation of these drugs in spiked sewage effluent samples was observed over a storage period of 7 days at 4 degrees C in the dark. Many beta-blockers were detected in the 14 wastewater samples collected from seven Canadian sewage treatment plants; those occurring at the highest concentrations were atenolol, acebutolol, sotalol, and metroprolol, with overall median concentrations of 1370, 339, 282, and 257 ng/L, respectively. However, there was only a small decrease in the median concentrations of these beta-blockers between the primary and final effluent, suggesting that these drugs are not easily removed by the sewage treatment processes. As a result of selective extraction and effective removal of coextractives, no matrix effect was observed for the samples during LC-MS/MS analyses.

Stereoisomer quantification of the -blocker drugs atenolol, metoprolol, and propranolol in wastewaters by chiral high-performance liquid chromatography–tandem mass spectrometry

A chiral liquid chromatography-tandem mass spectrometry (HPLC-MS-MS) method was developed and validated for measuring individual enantiomers of three beta-blocker drugs (atenolol, metoprolol, and propranolol) in wastewater treatment plant (WWTP) influents and effluents. Mean recoveries of the pharmaceuticals ranged from 67 to 106%, and the limits of detection of the analytes were 2-17 ng/L in wastewater effluents. The method was demonstrated by measuring, for the first time, the stereoisomer composition of target analytes in raw and treated wastewaters of two Canadian WWTPs. In these trials, racemic amounts of the three drugs were observed in influent of one wastewater treatment plant, but nonracemic amounts were observed in another. Effluents of the two plants contained nonracemic amounts of the drugs. These results indicate that biologically-mediated stereoselective processes that differ among WWTPs had occurred to eliminate individual enantiomers of the target analytes.

Capillary electrophoretic analysis of the antibiotic vancomycin in innovative microparticles and in commercial formulations

A new fast capillary electrophoretic method has been developed for the analysis of the glycopeptide antibiotic vancomycin in formulations. An electrophoretic run is completed within 3.0 min; fused silica capillaries (100 microm i.d., 8.5 cm effective length and 48.5 cm total length) and a background electrolyte consisting of 12.5 mM, pH 2.5 phosphate buffer are used. The applied voltage is -20.0 kV; samples are injected by pressure (30 mbar x 3 s) at the anodic end of the capillary. The method was successfully applied to innovative controlled release microparticles consisting of a coated albumin core containing vancomycin. A simple procedure has been developed to obtain complete vancomycin extraction from microparticles using a 5% (w/v) sodium dodecyl sulphate aqueous solution. The method has been validated in terms of linearity, precision and accuracy. Good linearity was found in the 0.25-5.00 microg/mL range. Satisfactory precision was obtained, with relative standard deviation values always lower than 3.9%; accuracy was satisfactory, with recovery values between 97.8 and 102.2%. The method is also suitable for vancomycin determination in commercial capsules.

Vancomycin as chiral selector for enantioselective separation of selected profen nonsteroidal anti-inflammatory drugs incapillary liquid chromatography

The chiral selector vancomycin was used either as mobile phase additive or bound as a chiral stationary phase (CSP) for the stereoselective separation of seven racemic nonsteroidal anti-inflammatory drugs (NSAIDs), fenoprofen, carprofen, flurbiprofen, indoprofen, flobufen, ketoprofen, and suprofen, by capillary liquid chromatography. The effect of the type of stationary phase, the chiral column Chirobiotic V or the achiral stationary phases Nucleosil 100 C8 HD and Nucleosil 100 C18 HD, and the concentration of vancomycin in the mobile phase on separation of the drug enantiomers were evaluated. All the drugs, except flobufen, were successfully enantioseparated on Nucleosil 100 C8 HD with 4 mM vancomycin present in the mobile phase (composed of methanol and buffer) in the reversed phase mode. On the vancomycin-bonded chiral stationary phase, it was difficult to get enantioseparations of the profen NSAIDs. However, flobufen gave better enantioseparation on the vancomycin CSP. The better enantioresolution of the majority of profen derivatives on the achiral columns with vancomycin added to the mobile phase can be attributed in particular to the higher separation efficiency of this capillary chromatographic system. In addition, vancomycin dimers, formed in the mobile phase, seem to offer a better steric arrangement for stereoselective interaction to these analytes than the vancomycin bonded on the CSP. These substantial differences in the CS structure significantly influence the chiral discrimination mechanism.