Comparative study on the application of capillary liquid chromatography and capillary electrochromatography for investigation of enantiomeric purity of the contraceptive drug levonorgestrel

Recent developments in electromigration techniques related to pharmaceutical and biomedical analysis - A review

The analysis of pharmaceutical compounds is an important research topic as the use of different drugs affects people's daily life for the treatment of diseases. In addition to the widespread use of the internet, counterfeit drugs have appeared in the market. The development of modern analytical techniques, reliable, precise, sensitive, and rapid methods, has provided powerful means of analysis used in various fields such as drug production, quality control, determination of impurities and/or metabolites, biochemistry, pharmacokinetics, etc. Analytical techniques so far used in the pharmaceutical analysis include high-performance liquid chromatography (HPLC), gas chromatography (GC), super/sub-critical fluid chromatography (SFC), and capillary electromigration techniques such as capillary electrophoresis (CE) and rather rarely capillary electrochromatography (CEC). CE has some advantages over other techniques, e.g., very high efficiency, reduced costs (use of minute volumes of solvents and samples), the possibility to use different separation mechanisms, etc. In this review paper, the main features and limitations of the capillary electromigration techniques (especially CE) are discussed. Some selected applications of CE to the analysis of pharmaceutical compounds published in the period 2021-2023 (May) are reported.

Our research cooperation with Professor Yoshio Okamoto

This article summarizes our cooperation with the research group of Prof. Yoshio Okamoto at Nagoya University during the period of time between 1992 and 2005. Although the text deals entirely with enantioseparations in high-performance liquid chromatography, capillary electrophoresis, and capillary electrochromatography, this is not a detailed review in any of these areas. The text highlights selected aspects of these techniques, which have been the subject of our joint research and in part their reflection in follow-up research by our and other research groups. Together with more systematically studied topics, aspects such as ultrafast separation of enantiomers, uncommonly high separation factor of enantiomers and other related issues are also addressed.

History, advancement, bottlenecks, and future of chiral capillary electrochromatography

Capillary electrochromatography (CEC) represents a technique with less than 30 years of intense development and in this period this technique has seen huge promise, fast development, stagnation, and significant decline of innovative activity. The major goal of the present overview is not to present an extensive review of the literature on chiral CEC but to analyze the reasons for this dramatic development and attempting to answer questions such as: 1) Was the potential of CEC reasonably evaluated in 1990s before starting the explosive development in this field? 2) Did the development of this technique take the right track? 3) What other developments and competitive trends led to stagnation in the advancement of CEC? 4) Why is the activity in this field currently decreasing? 5) What are the current challenges and promises and what is the future of chiral CEC?

Enantiomers separation by capillary electrochromatography using polysaccharide-based stationary phases

The separation of chiral compounds is an interesting and important topic of research because these compounds are involved in some biological processes, fundamentally in human health. Among the various application fields where enantiomers are remarkable, drug analysis has to be considered. Most of the drugs contain enantiomers and very often one of the two isomers could be pharmacologically more active or even dangerous. Therefore, the separation of these compounds is very important. Among the different analytical techniques usually employed, capillary electrochromatography has demonstrated great capability in enantiomers resolution. The great potential of this electromigration technique stands mainly in its high efficiency due to the use of an electrosmotic flow (flat flow profile) and on the high selectivity because of the use of a stationary phase. Chiral separation can be obtained utilizing several chiral stationary phases including a polysaccharide derivative. The aim of this review paper is to summarize the main features of capillary electrochromatography and polysaccharide derivatives of chiral stationary phase. It also report examples of practical applications utilizing this approach.

High Throughput Analysis of Chiral Compounds Using Capillary Electrochromatography (CEC) and CEC-Mass Spectrometry with Cellulose Based Stationary Phases

To fulfill the ever growing demand for rapid chiral analysis, this research presents an approach for highthroughput enantiomeric separations and sensitive detection of model chiral analytes using capillary electrochromatography (CEC) with UV and MS detection. This was achieved utilizing a short 7 cm CEC columns packed with cellulose tris (3,5-dimethyl-phenylcarbamate) (CDMPC) or sulfonated cellulose tris (3,5-dimethylphenylcarbamate) (CDMPC-SO₃) chiral stationary phases (CSPs) applying outlet side injections in CEC-UV. The separation performance was compared between CDMPC and CDMPC-SO₃ CSPs for rapid enantio-separation in CEC-UV mode. In addition, using a high sensitivity UV-flow cell in combination with outlet side injections, the S/N and hence the limit of detection of chiral drug could be improved. The 7-cm packed column was also used with traditional inlet injections for CEC coupled to a low-cost single-quadrupole MS. While outlet side injection was not possible in CEC-MS due to instrumentation constraints, the combined use of a short 7 cm column packed with CDMPC-SO₃ CSP provided several fold higher throughput. Both CEC-UV and CEC-MS with short packed bed has the potential for a simple, sensitive and cost-effective method for enantiomeric drug profiling in biological samples.

Preparation of perphenylcarbamoylated β-cyclodextrin-silica hybrid monolithic column with "one-pot" approach for enantioseparation by capillary liquid chromatography

Perphenylcarbamoylated β-cyclodextrin-silica (Ph-β-CD-silica) hybrid monolithic columns for enantioseparation in capillary liquid chromatography (cLC) have been prepared by a "one-pot" approach via the polycondensation of alkoxysilanes and in situ copolymerization of mono (6(A)-N-allylamino-6(A)-deoxy)-Ph-β-CD and vinyl group on the precondensed siloxanes. The morphologies of the Ph-β-CD-silica hybrid monolithic columns were characterized by optical microscopy and scanning electron microscopy (SEM), showing the uniform monolithic matrixes tightly bonded onto the capillary wall. The content of Ph-β-CD incorporated in monolithic matrix by the "one-pot" approach was ca. 2.9 times higher than that by postmodification method. The permeability of the Ph-β-CD-silica chiral hybrid monolithic column was 3.63 × 10(-14) m(2), and the minimum plate height was 12 μm corresponding to 83,300 theoretical plates/meter. Enantioseparations of 13 racemates were achieved by the Ph-β-CD-silica hybrid monolithic column. In this work, since the prepolymerization system mainly consisted of organic solvent (methanol (MeOH), N,N-dimethylformamide (DMF)), the limitation and difficulty of the use of water insoluble organic monomers in the previously reported "one-pot" method was circumvented. Therefore, various β-CD derivatives as well as other hydrophobic monomers could thus be used to prepare organic-silica hybrid monolithic columns with the "one-pot" process.

Separation of catechins and methylxanthines in tea samples by capillary electrochromatography

In this paper, the simultaneous separation of several polyphenols such as (+)-catechin, (-)-epicatechin, (-)-epigallocatechin, theophylline, caffeine in green and black teas by capillary electrochromatography (CEC) was developed. Several experimental parameters such as stationary phase type, mobile phase composition, buffer and pH, inner diameter of the columns, sample injection, were evaluated to obtain the complete separation of the analysed compounds. Baseline resolution of the studied polyphenols was achieved within 30 min by using a capillary column (id 100 microm) packed with bidentate C(18) particles for 24.5 cm and a mobile phase composed of 5 mM ammonium acetate buffer pH 4 with H(2)O/ACN (80:20, v/v). The applied voltage and the temperature were set at 30 kV and 20 degrees C. Precision, detection and quantification limits, linearity, and accuracy were investigated. A good linearity (R(2) > 0.9992) was achieved over a concentration working range of 2-100 microg/mL for all the analytes. LOD and LOQ were 1 and 2 microg/mL, respectively, for all studied compounds. The CEC method was applied to the analysis of those polyphenols in green and black tea samples after an extraction procedure. Good recovery data from accuracy studies ranged between 90% and 112% for all analytes.

Sulfated and sulfonated polysaccharide as chiral stationary phases for capillary electrochromatography and capillary electrochromatography-mass spectrometry

The applications of polysaccharide phenyl carbamate derivatives as chiral stationary phases (CSPs) for capillary electrochromatography (CEC) are often hindered by longer retention times, especially using a normal-phase (NP) eluent due to very low electroosmotic flow (EOF). Therefore, in this study, we propose an approach for the aforementioned problems by introducing two new types of negatively charged sulfate and sulfonated groups for polysaccharide CSPs. These CSPs were utilized to pack CEC columns for enantioseparation with a NP eluent. Compared to conventional cellulose tris(3,5-dimethylphenyl carbamate) or CDMPC CSPs, the sulfated CDMPC CSP (sulfur content 4.25%, w/w) shortened the analysis time up to 50% but with a significant loss of enantiomeric resolution (approximately 60%). On the other hand, the sulfonated CDMPC CSP (sulfur content 1.76%, w/w) not only provided fast throughput but also maintained excellent resolving power. In addition, its synthesis is much more straightforward than the sulfated one. Furthermore, we studied several stationary phase parameters (CSP loading and silica gel pore size) and mobile phase parameters (including type of mobile phase and its composition) to evaluate the throughput and enantioselectivity. Using the optimized conditions, a chiral pool containing 66 analytes was screened to evaluate the enantioselectivity under three different mobile phase modes (i.e., NP, polar organic phase (POP) and reversed-phase (RP) eluents). Among these mobile phase modes, the RP mode showed the highest success rate, whereas some degree of complementary enantioselectivity was observed with NP and POP. Finally, the feasibility of applying this CSP for CEC-MS enantioseparation using internal tapered column was evaluated with NP, POP and RP eluents. In particular, the NP-CEC-MS provided significantly enhanced sensitivity when methanol was replaced with isopropanol in the sheath liquid. Using aminoglutethimide as model chiral analyte, all three modes of CEC-MS demonstrated excellent durability as well as excellent reproducibility of retention time and enantioselectivity.

Enantioselective separation of the novel antidepressant mirtazapine and its main metabolites by CEC

In this work, the simultaneous enantioseparation of the second-generation antidepressant drug mirtazapine and its main metabolites 8-hydroxymirtazapine and N-desmethylmirtazapine by chiral CEC is reported. The separation of all enantiomers under study was achieved employing a capillary column packed with a vancomycin-modified diol stationary phase. With the aim to optimize the separation of the three pairs of enantiomers in the same run, different experimental parameters were studied including the mobile phase composition (buffer concentration and pH, organic modifier type and ratio, and water content), stationary phase composition, and capillary temperature. A capillary column packed with vancomycin mixed with silica particles in the ratio (3:1) and a mobile phase composed of 100 mM ammonium acetate buffer (pH 6)/H(2)O/MeOH/ACN (5:15:30:50, by vol.) allowed the complete enantioresolution of each pair of enantiomers but not the simultaneous separation of all the studied compounds. For this purpose, a packing bed composed of vancomycin-CSP only was tested and the baseline resolution of the three couples of enantiomers was achieved in a single run in less than 30 min, setting the applied voltage and temperature at 25 kV and 20 degrees C, respectively. In order to show the potential applicability of the developed CEC method to biomedical analysis, a study concerning precision, sensitivity, and linearity was performed. The method was then applied to the separation of the enantiomers in a human urine sample spiked with the studied compounds after suitable SPE procedure with strong cation-exchange (SCX) cartridges.

Recent accomplishments in the field of enantiomer separation by CEC

The present review intends to summarize recent developments in the field of enantioselective separations and analysis by CEC. It covers studies published in English language in common peer-reviewed journals within the period between 2003 and 2006. Both, methods making use of chiral mobile phase additives as well as chiral stationary phases for electrochromatographic enantiomer separations, are reviewed. Achievements that have been made on the various column technologies, such as open-tubular, particle-packed, inorganic, organic and particle-fixed (hybrid-type) monolithic as well as molecularly imprinted polymer phases, are discussed.

Recent applications in nanoliquid chromatography

Since its first introduction by Karlsson and Novotny in 1988 nano-LC has emerged as a complementary and/or competitive separation method to conventional HPLC, offering several advantages such as higher efficiency, ability to work with minute sample sizes and lower consumption of mobile phases, and better compatibility with MS, etc. Although its use was not so extended initially, in the last years new and interesting applications have appeared which deserve to be carefully considered. The aim of this review is therefore to provide an updated and critical survey of different nano-LC applications in analytical chemistry.

Enantiomeric separation of some demethylated analogues of clofibric acid by capillary zone electrophoresis and nano-liquid chromatography

The enantiomeric separation of some demethylated analogues of clofibric acid, namely 2-(6-chloro-benzothiazol-2-ylsulfanyl)-, 2-(6-methoxy-benzothiazol-2-ylsulfanyl)-, 2-(quinolin-2-yloxy)-, 2-(6-chloro-quinolin-2-yloxy)-, 2-(7-chloro-quinolin-4-yloxy)-propionic acid (compounds A-E, respectively), has been studied by CZE and nano-LC using for the first technique two beta-CD derivatives and vancomycin added to the BGE and vancomycin-modified silica particles for the second one, with the aim to find the optimum experimental conditions for the baseline resolution. The type and the concentration of the chiral selector added to the BGE, the buffer pH, the type of organic modifier and its concentration, the capillary temperature and the applied voltage played a very important role in the enantioresolution of the analysed compounds. The use of 6-monodeoxy-6-monoamino-beta-CD allowed to achieve baseline resolution of four of five clofibric acid derivatives in less than 10 min while heptakis-(2,3,6-tri-O-methyl)-beta-CD partially resolved the same compounds in their enantiomers. Employing vancomycin as the chiral selector in CZE, the counter-current partial filling method was chosen achieving baseline resolution of four analytes. All the studied compounds were enantioresolved employing a capillary column packed with vancomycin stationary phase by nano-LC, and the resolution was strongly influenced by the concentration of the organic modifier and by the pH of the mobile phase. The best results were achieved at pH 4.5 in presence of 60% of methanol (MeOH). However, longer analysis times were observed in the experiments carried out by nano-LC.

Use of teicoplanin stationary phase for the enantiomeric resolution of atenolol in human urine by nano-liquid chromatography–mass spectrometry

Nano-liquid chromatography (nano-LC) was used for the enantiomeric resolution of atenolol employing a teicoplanin modified silica stationary phase prepared in our laboratory. Experiments were carried out in a fused silica capillary of 75 microm i.d. packed with chiral modified silica particles of 5 microm diameter. Separated enantiomers were revealed by on-line UV detector at 205 nm or electrospray-ion-trap mass spectrometer (ESI-MS). Atenolol enantiomers were eluted utilizing a mobile phase with the following composition: 500 mM ammonium acetate pH 4.5/methanol/acetonitrile 1:60:39 (v/v/v) allowing to achieve good enantioresolution in a reasonable analysis time (about 8 min) with a flow rate of about 900 nL/min. After comparing the sensitivity of the nano-LC method using a conventional UV detector for capillary electrophoresis, a zeta cell (3 nL volume) employed in nano-LC and the ion-trap MS the method was validated with the MS detector offering the highest sensitivity (limit of detection (LOD)=50 ng/mL; limit of quantification (LOQ)=400 ng/mL for each atenolol enantiomer). (-)-Psi-Nor-ephedrine was used as the internal standard. The method was successfully applied to the analysis of atenolol enantiomers present in human urine samples of a patient under atenolol therapy.

Enantiomeric separation of mirtazapine and its metabolites by nano-liquid chromatography with UV-absorption and mass spectrometric detection

Mirtazapine (MIR) and two of its main metabolites, namely, 8-hydroxymirtazapine and N-desmethylmirtazapine, were separated in totheir enantiomers by nanoLC in a laboratory-made fused-silica capillary column (75 microm ID) packed with a vancomycin-modified silica stationary phase. The simultaneous separation of the three couples of the studied enantiomers was achieved in less than 33 min, using an experimentally optimized mobile phase delivered in the isocratic mode. Optimization of the mobile-phase composition was achieved by testing the influence of the buffer pH and concentration, the water concentration, the organic modifier type and concentration, and on the retention and resolution of the analytes. The optimum mobile-phase composition contained 500 mM ammonium acetate pH 4.5/water/MeOH/MeCN, 1:14:40:45 v/v/v/v. Using a UV detector at 205 nm, the method was validated studying several experimental parameters such as LOD and LOQ, intraday and interday repeatability, and linearity. Good results were achieved: LOD and LOQ were in the range 5-15 and 10-40 microg/mL, respectively (the highest value was obtained for the DEMIR enantiomers); correlation coefficients, 0.9993-0.9999; the intraday and interday precision was acceptable (RSD < 2%) using an internal standard. The method was tested for the separation of the studied enantiomers in an extracted (solid-phase) serum sample spiked with standard racemic mixture of MIR and its two metabolites. Finally, the nanoLC system was connected to a mass spectrometer through a nanoelectrospray interface and the MS, MS2, and MS3 spectra were acquired showing the potential of the system used for characterization and identification of the separated analytes.

Capillary electrochromatographic chiral separations with potential for pharmaceutical analysis

The use of capillary electrochromatography as a chiral separation technique for pharmaceutical applications is reviewed. Publications of the past 10 years that provide a potential practical application in pharmaceutical analysis are considered. Method development or validation, separation strategies, and potential routine analysis by the methods/applications cited are the main subjects on which we focused our attention. The indirect chiral separation method was only used once in CEC mode. In the direct chiral separations, the use of chiral stationary phases was obviously preferred over the use of chiral mobile phases with non-chiral stationary phases. Amongst the chiral stationary phases, those based on macrocyclic antibiotics and polysaccharide selectors were the most frequently used. Monolithic stationary phases also have several applications, but not so extended as those with packed capillary electrochromatography. The considered papers not only describe the applicability of the technique for relatively large sets of chiral analytes, they also showed that various types of stationary phases can be produced in-house in a simple manner. However, to survive as a mature separation technique, considerable time and effort are still needed to solve some disadvantages currently characterizing capillary electrochromatography.

Analysis of ketorolac and its related impurities by capillary electrochromatography

Capillary electrochromatography (CEC) was employed for the assay of ketorolac (KT) and its known related impurities [1-hydroxy analog of ketorolac (HK), 1-keto analog of ketorolac (KK), ketorolac decarboxylated (DK)] in both drug substance and coated tablets. Detection was made at 323 nm and flufenamic acid was selected as internal standard. The experiments were performed in a 100 microm i.d. capillary packed with RP-18 silica particles (33.0, 24.5, 23.0 cm total, effective and packed lengths, respectively). The composition of the mobile phase was optimised by changing pH of the buffer and acetonitrile (ACN) content and by addition of other organic modifiers (methanol, ethanol, isopropanol, n-propanol) in order to evaluate the effect of these factors on the method performance (efficiency, retention and resolution). The optimum mobile phase consisted of a mixture of 50 mM ammonium formate buffer pH 3.5-water-acetonitrile (10:20:70, v/v/v), while voltage and temperature were set at 30 kV and 20 degrees C, respectively. Applying these conditions, all peaks were baseline resolved and the analysis was performed in less than 9 min. Selectivity, repeatability of retention time and peak area, detection and quantitation limits, linearity and range, precision and accuracy were also investigated. R.S.D. and bias values obtained for all the analytes were below 5% and sensitivity was satisfactory, thus the method was deemed suitable for pharmaceutical quality control. Applying the method to coated tablets, a recovery of 98.5+/-0.8% and an R.S.D. of 0.5% were found.

Comparison of the miniaturised techniques capillary electrochromatography and capillary liquid chromatography for the chiral separation of chlorthalidone

The aim of this study was to compare the miniaturised techniques, capillary electrochromatography (CEC) and capillary liquid chromatography (CLC), for the chiral separation of chlorthalidone. In both cases, hydroxypropyl-beta-cyclodextrin was used as a chiral selector in the mobile phase, while an achiral stationary phase was used. Earlier, this separation was already optimised in CEC. Now, the separation was optimised in CLC. The influence of the organic modifier content and the cyclodextrin concentration on the separation was studied by means of a central composite design. Optimal separation conditions were determined, after response modelling, from the response surface contour plots. When these conditions were compared with those of the CEC optimisation, we can see the potential of using CLC as a chiral separation technique since less chiral selector was used, faster separations were obtained and better repeatability was observed in comparison with its electrical-driven counterpart.

Separation of tocopherols by nano-liquid chromatography

Nanoliquid chromatography (nano-LC) was used for the separation of tocopherols (delta-, gamma-, alpha-TOH), alpha-tocopherol acetate (alpha-TOH-Ac) and an antioxidant compound, namely butylated hydroxytoluene (BHT) used to prevent TOHs autoxidation. The separation was carried out in a fused silica capillary of 100 microm I.D. and 375 microm O.D. packed in our laboratory with RP18 silica stationary phase of either 5- or 3-microm diameter (23-cm long). The mobile phase was composed by mixtures of methanol (MeOH), acetonitrile (MeCN) and water. Typical analyses time for the separation of all the five components of the mixture were 6-9 min depending on the composition of the mobile phase. Efficiency and resolution were strongly influenced by the particle diameter and the highest Rs and N/m values were observed using 3-microm RP18 particles. Experiments performed with capillaries packed with 3-microm RP18 particles provided good limit of detection (LOD) and limit of quantification (LOQ) (for delta-, gamma-TOH, alpha-TOH-Ac were 4 and 8 microg/ml, while for alpha-TOH were 6 and 10 microg/ml, respectively). The optimized method was applied to extracts of serum and pharmaceutical preparation containing alpha-TOH and alpha-TOH-Ac.