From capillaries to microchips, green electrophoretic features for enantiomeric separations: A decade review (2013-2022)

Enantioseparation by the electromigration-based method is well-established and widely discussed in the literature. Electrophoretic strategies have been used to baseline resolve complex enantiomeric mixtures, typically using a selector substance into the background electrolyte (BGE) from capillaries to microchips. Along with developing new materials/substances for enantioseparations, it is the concern about the green analytical chemistry (GAC) principles for method development and application. This review article brings a last decade's update on the publications involving enantioseparation by electrophoresis for capillary and microchip systems. It also brings a critical discussion on GAC principles and new green metrics in the context of developing an enantioseparation method. Chemical and green features of native and modified cyclodextrins are discussed. Still, given the employment of greener substances, ionic liquids and deep-eutectic solvents are highlighted, and some new selectors are proposed. For all the mentioned selectors, green features about their production, application, and disposal are considered. Sample preparation and BGE composition in GAC perspective, as well as greener derivatization possibilities, were also addressed. Therefore, one of the goals of this review is to aid the electrophoretic researchers to look where they have not.

Role of Ionic Liquids in Capillary Electrophoresis

Ionic liquids are a very important class of compounds due to their remarkable properties and wide range of applications. On the other hand, capillary electrophoresis is also gaining importance in separation science because of its fast speed and inexpensive nature. The use of ionic liquids in capillary electrophoresis is gaining importance continuously. The present review article describes the applications of ionic liquids in capillary electrophoresis. This article also describes the general aspects of ionic liquids and capillary electrophoresis. The use of ionic liquids in capillary electrophoresis, optimization of separation, mechanism of separation, and toxicity of ionic liquids, as well as their future perspectives, have also been discussed. It was observed that not much work has been performed in capillary electrophoresis using ionic liquids. It was also realized that the use of ionic liquids in capillary electrophoresis could revolutionize analytical science. Briefly, there is a great need for the use of ionic liquids in capillary electrophoresis for better and more effective separation.

Enantioselectivity Effects in Clinical Metabolomics and Lipidomics

Metabolomics and lipidomics have demonstrated increasing importance in underlying biochemical mechanisms involved in the pathogenesis of diseases to identify novel drug targets and/or biomarkers for establishing therapeutic approaches for human health. Particularly, bioactive metabolites and lipids have biological activity and have been implicated in various biological processes in physiological conditions. Thus, comprehensive metabolites, and lipids profiling are required to obtain further advances in understanding pathophysiological changes that occur in cells and tissues. Chirality is one of the most important phenomena in living organisms and has attracted long-term interest in medical and natural science. Enantioselective separation plays a pivotal role in understanding the distribution and physiological function of a diversity of chiral bioactive molecules. In this context, it has been the goal of method development for targeted and untargeted metabolomics and lipidomic assays. Herein we will highlight the benefits and challenges involved in these stereoselective analyses for clinical samples.

Natural and Artificial Chiral-Based Systems for Separation Applications

Chiral separation has attracted much attention for basic research and industrial applications in analytical chemistry. Generally, chiral separations use natural or artificial chiral-based materials as adsorbents. To improve the precision and efficiency of chiral separation, focus has shifted from natural and synthetic adsorbents to binary combinations of materials. This review specifically summarizes the significant advancements made in natural and artificial chiral adsorbents as promising candidates for diverse drug and biomolecule separation applications as well as the remaining drawbacks and challenges for research on chiral separations. The mechanisms of chiral-based recognition and separation and history and development of natural and artificial chiral-based systems are the focus of this review. Future directions in natural and artificial chiral-based systems for practical separations and other applications are also presented.

Past, present, and future developments in enantioselective analysis using capillary electromigration techniques

Enantioseparation of chiral products has become increasingly important in a large diversity of academic and industrial applications. The separation of chiral compounds is inherently challenging and thus requires a suitable analytical technique that can achieve high resolution and sensitivity. In this context, CE has shown remarkable results so far. Chiral CE offers an orthogonal enantioselectivity and is typically considered less costly than chromatographic techniques, since only minute amounts of chiral selectors are needed. Several CE approaches have been developed for chiral analysis, including chiral EKC and chiral CEC. Enantioseparations by EKC benefit from the wide variety of possible pseudostationary phases that can be employed. Chiral CEC, on the other hand, combines chromatographic separation principles with the bulk fluid movement of CE, benefitting from reduced band broadening as compared to pressure-driven systems. Although UV detection is conventionally used for these approaches, MS can also be considered. CE-MS represents a promising alternative due to the increased sensitivity and selectivity, enabling the chiral analysis of complex samples. The potential contamination of the MS ion source in EKC-MS can be overcome using partial-filling and counter-migration techniques. However, chiral analysis using monolithic and open-tubular CEC-MS awaits additional method validation and a dedicated commercial interface. Further efforts in chiral CE are expected toward the improvement of existing techniques, the development of novel pseudostationary phases, and establishing the use of chiral ionic liquids, molecular imprinted polymers, and metal-organic frameworks. These developments will certainly foster the adoption of CE(-MS) as a well-established technique in routine chiral analysis.

Ionic liquids in electrokinetic chromatography

There is an interest in the application of ionic liquids as additives into the separation media to improve achiral and chiral separations in electrokinetic chromatography (EKC). This review will critically discuss the developments on the use of ionic liquids in the different modes of EKC during the last five years (2015-mid 2020). A healthy number of 48 research articles searched through Scopus were categorised into two: ionic liquids as sole pseudophase (micelles, microemulsions, ligand exchange pseudophase or molecular pseudophase) and ionic liquids with pseudophase (achiral or chiral). More than half of the papers dealt with chiral separations that were mostly facilitated by another additive or pseudophase. The role of ionic liquids for improvement of separations were analysed, and we provided some recommendations for further investigations. Finally, the use of ionic liquids in different on-line sample concentration or stacking methods (i.e., field enhancement and sweeping) was briefly discussed.

Recent advances in the enantioseparation promoted by ionic liquids and their resolution mechanisms

More and more various chemical media are being applied in enantioseparation; among them, ionic liquids (ILs) have attracted the long-term attention in this decade as green designable solvents. This paper provides comprehensive overview for the applications of ILs in chiral extraction, gas chromatography, liquid chromatography, capillary electrophoresis and other techniques for enantioseparation. Additionally, the important resolution mechanisms based on ILs have also been summarized and discussed. This review focuses on the latest development of enantioseparation methods by using ILs in various modes, leading to meaningful and valuable information to related fields and thus promotes further research and application of reported methods.

The Influence of Ionic Liquids on the Effectiveness of Analytical Methods Used in the Monitoring of Human and Veterinary Pharmaceuticals in Biological and Environmental Samples-Trends and Perspectives

Recent years have seen the increased utilization of ionic liquids (ILs) in the development and optimization of analytical methods. Their unique and eco-friendly properties and the ability to modify their structure allows them to be useful both at the sample preparation stage and at the separation stage of the analytes. The use of ILs for the analysis of pharmaceuticals seems particularly interesting because of their systematic delivery to the environment. Nowadays, they are commonly detected in many countries at very low concentration levels. However, due to their specific physiological activity, pharmaceuticals are responsible for bioaccumulation and toxic effects in aquatic and terrestrial ecosystems as well as possibly upsetting the body's equilibrium, leading to the dangerous phenomenon of drug resistance. This review will provide a comprehensive summary of the use of ILs in various sample preparation procedures and separation methods for the determination of pharmaceuticals in environmental and biological matrices based on liquid-based chromatography (LC, SFC, TLC), gas chromatography (GC) and electromigration techniques (e.g., capillary electrophoresis (CE)). Moreover, the advantages and disadvantages of ILs, which can appear during extraction and separation, will be presented and attention will be given to the criteria to be followed during the selection of ILs for specific applications.

Amino acid chiral ionic liquids combined with hydroxypropyl-β-cyclodextrin for drug enantioseparation by capillary electrophoresis

Four amino acid chiral ionic liquids were evaluated in dual systems with hydroxypropyl-β-cyclodextrin to investigate the enantioseparation by CE of a group of seven drugs as model compounds (duloxetine, verapamil, terbutaline, econazole, sulconazole, metoprolol, and nadolol). The use of two of these chiral ionic liquids (tetramethylammonium L-Lysine ([TMA][L-Lys]) and tetramethylammonium L-glutamic acid ([TMA][L-Glu])) as modifiers in CE is reported for the first time in this work whereas tetrabutylammonium L-lysine ([TBA][L-Lys]) and tetrabutylammonium L-glutamic acid ([TBA][L-Glu]) were employed previously in CE although very scarcely. The effect of the nature and the concentration of each ionic liquid added to the separation buffer containing the neutral cyclodextrin on the enantiomeric resolution and the migration time obtained for each drug, was investigated. A synergistic effect was observed when combining each chiral ionic liquid with hydroxypropyl-β-cyclodextrin in the case of the five compounds for which the cyclodextrin showed enantiomeric discrimination power when used as sole chiral selector (duloxetine, verapamil, terbutaline, econazole, sulconazole). Buffer concentration and pH, temperature and separation voltage were varied in order to optimize the enantiomeric separation of these five compounds using dual systems giving rise to resolutions ranging from 1.1 to 6.6.

Enantioseparation by Capillary Electrophoresis Using Ionic Liquids as Chiral Selectors

Capillary electrophoresis (CE) is one of the most widely employed analytical techniques to achieve enantiomeric separations. In spite of the fact that there are many chiral selectors commercially available to perform enantioseparations by CE, one of the most relevant topics in this field is the search for new selectors capable of providing high enantiomeric resolutions. Chiral ionic liquids (CILs) have interesting characteristics conferring them a high potential in chiral separations although only some of them are commercially available. The aim of this article is to review all the works published on the use of CILs as chiral selectors in the development of enantioselective methodologies by CE, covering the period from 2006 (when the first research work on this topic was published) to 2017. The use of CILs as sole chiral selectors, as chiral selectors in dual systems or as chiral ligands will be considered. This review also provides detailed analytical information on the experimental conditions used to carry out enantioseparations in different fields as well as on the separation mechanism involved.

Analysis of antibiotics by CE and CEC and their use as chiral selectors: An update

Natural, synthetic or semisynthetic antibiotics are highly used to prevent or treat diseases in humans and animals, and to promote animal growth. This fact makes that antibiotics residues or their transformation products may be present in food or in the environment after human or animal excretion. For this reason, it is imperative to develop reliable and sensitive analytical methodologies for their analysis. The main aim of this work is to present and discuss the most recent applications of capillary electromigration methods for the analysis of antibiotics, including the developments and applications of their use as chiral selectors in CE. The literature published from June 2015 to June 2017 is included following the previous review by Domínguez-Vega et al. (Electrophoresis, 2016, 37, 189-211). Information about the use of different detection systems, off-line and on-line strategies to improve sensitivity, and microchip devices for the analysis of antibiotics is provided and properly discussed.