Evaluation of a methacrylate-bonded cyclodextrins as a monolithic chiral stationary phase for capillary electrochromatography (CEC)-UV and CEC coupled to mass spectrometry

Poly(carboxyethyl acrylate-co-ethylene glycol dimethacrylate) precursor monolith with bonded (S)-(-)-1-(2-naphthyl) ethylamine ligands for use in chiral and achiral separations by capillary electrochromatography

In this research report, the previously developed poly(carboxyethyl acrylate-co-ethylene glycol dimethacrylate) precursor monolith (referred to as carboxy monolith) is further exploited in the preparation of a chiral stationary phase for enantiomeric separations. The carboxy monolith precursor was subjected to post polymerization functionalization (PPF) with the chiral selector (S)-(-)-1-(2-naphthyl) ethylamine (NAS) at room temperature in the presence of N, N´-dicyclohexylcarbodiimide (DCC) in chloroform. The DCC, which is an organic soluble carbodiimide, permits the linkage for the amine functionality of the chiral ligand NAS to the carboxy group of the monolithic surface forming a stable amide linkage. The NAS column thus obtained allowed not only enantiomeric separations in the RP mode via its chiral site but also the separation of nonpolar species via its achiral functionality offering both hydrophobic and π-π interactions for aromatic compounds such toluene derivatives and polyaromatic hydrocarbons. The dual interaction sites (e.g., chiral, and achiral) of the NAS present a convenient column for the separations of slightly polar and nonpolar chiral and achiral solutes in the RP mode.

Capillary Electrophoresis Mass Spectrometry: Developments and Applications for Enantioselective Analysis from 2011–2020

It is now more than 25 years since the first report of enantioselective analysis by capillary electrophoresis-mass spectrometry (CE-MS) appeared. This article reviews the power of chiral CE-MS in resolving issues on the use of chiral selector incompatibility with MS and poor detectability encountered for chiral compounds by UV detection. The review begins with the general principles, requirements, and critical aspects of chiral CE-MS instrumentation. Next, the review provides a survey of MS-compatible chiral selectors (CSs) reported during the past decade, and the key achievements encountered in the time period using these CSs. Within the context of the strategies used to combine CE and MS, special attention is paid to the approaches that feature partial filling technique, counter-migration techniques, and direct use of CS, such as molecular micelles. In particular, the development and application of moving and fixed CS for EKC-MS, MEKC-MS, and CEC-MS demonstrate how various chiral compounds analyses were solved in a simple and elegant way during the 2010–2020 review period. The most noteworthy applications in the determination of chiral compounds are critically examined. The operating analytical conditions are detailed in the Tables, and the authors provide commentary on future trends of chiral separations by CE-MS.

Rapid preparation and characterization of a bridged bis(β-cyclodextrin) functionalized urea–formaldehyde monolithic column by a “one-pot” approach

A new bridged-CD monolith was prepared by “one-pot” method within 30 minutes. The obtained bridged-CD monolith showed satisfactory enantioselectivity compared to a native-CD monolith.

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.

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?

Highly efficient preparation of β-CD-based chiral monolithic column by "one-pot" hydroxymethyl polycondensation for enantioseparation in capillary liquid chromatography

A facile strategy for highly efficient fabrication of chiral monolithic column was carried out by the simple "one-pot" hydroxymethyl polycondensation of native β-cyclodextrin (β-CD) and urea-formaldehyde (UF). In-situ rapid substitution of native β-CD and efficient polycondensation of β-CD products and UF oligomers was proposed and fulfilled in one pot. The feasibility, mechanism and key parameters of polycondensation reaction were discussed. The data on crosslinking polycondensation and reaction kinetics were further evaluated. Characteristics including the morphologies, permeability and structure stability of the resultant monolithic columns were also studied. Under the optimal conditions, the "one-pot" hydroxymethyl polycondensation was accomplished in an aqueous solution within only 10 mins. Satisfactory stability and repeatability were gained, and good enantioseparation of eight model enantiomers was successfully achieved with the resultant β-CD-based monolith. It was simple and highly efficient, and the organic solvents, special CD derivatives and tediously long-time polymerization reaction were successfully avoided, which might light a new access to rapid preparation of CD-chiral monolith for high-performance enantioseparation.

Preparation of a hydroxypropyl-β-cyclodextrin functionalized monolithic column by one-pot sequential reaction and its application for capillary electrochromatographic enantiomer separation

In this study, a hydroxypropyl-β-cyclodextrin (HP-β-CD) functionalized monolithic capillary column was prepared by one-pot sequential reaction for the first time. The preparation of the HP-β-CD functionalized monolithic column involves two sequential reactions in one pot: (1) the ring opening reaction between HP-β-CD and glycidyl methacrylate (GMA) catalyzed by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU); (2) the copolymerization of GMA-HP-β-CD, ethylene dimethacrylate (EDMA) and 2-acrylamido-2-methyl propane sulfonic acid (AMPS). A series of monolithic columns were successfully prepared by varying the temperature of the ring opening reaction or several copolymerization parameters (the type and composition of porogenic solvents, ratio of GMA-HP-β-CD to EDMA and polymerization temperature). Then, the morphologies and structures of the resulting monolithic stationary phases were characterized by optical microscopy, scanning electron microscopy (SEM) and nitrogen adsorption analysis. Raman spectroscopy clearly indicated the successful bonding of HP-β-CD onto the monolith. When the prepared chiral stationary phase (CSP) was applied for the separation of a set of racemic compounds by capillary electrochromatography (CEC), including racemic anticholinergic drugs, β-adrenergic drugs, meptazinol and its intermediates, satisfactory separation selectivities were obtained. Additionally, the column also showed excellent separation abilities towards four flavanone glycosides epimers. Furthermore, the prepared monolithic columns exhibited satisfactory stability and reproducibilities of retention time, resolution and column efficiency. These results demonstrated the potential and usefulness of the developed one-pot sequential strategy in the preparation of other derivatized CD functionalized monolithic columns.

Approaches for enantioselective resolution of pharmaceuticals by miniaturised separation techniques with new chiral phases based on nanoparticles and monolithis

This article discusses new developments in the preparation of nanoparticles and monoliths with emphasis upon their application as the stationary and pseudo-stationary phases for miniaturised liquid phase separation techniques, which have occurred in the last 10 years (from 2006 to the actuality). References included in this review represent current trends and state of the art in the application of these materials to the analysis, by EKC, CEC and miniaturised chromatography, of chiral compounds with environmental interest such as pharmaceuticals. Due to their extraordinary properties, columns prepared with these new chiral stationary or pseudo-stationary phases, based on materials such as gold nanoparticles, metal-organic frameworks, ordered mesoporous silicas, carbonaceous materials, polymeric-based and silica-based monoliths or molecularly imprinted materials, can usually show some improvements in the separation selectivity, column efficiency and chemical stability in comparison with conventional chiral columns available commercially.

Chiral Capillary Electrophoresis-Mass Spectrometry: Developments and Applications in the Period 2010-2015: A Review

The sensitive detection of chiral compounds by capillary electrophoresis (CE) in biological samples remains a significant challenge and is currently considered a bottleneck in many chiral analysis projects. Chiral CE-MS can significantly improve the limit of detection and provide high sensitivity compared with chiral CE-UV. Chiral selectors such as modified cyclodextrins (CDs) and polymeric surfactants (a.k.a. molecular micelles, M_oM_s) in electrokinetic chromatography (EKC), micellar electrokinetic chromatography (MEKC) and capillary electrochromatography (CEC) have been developed to address the need for high sensitivity by CE-MS. However, several problems remain to be investigated to fully understand the potential of these hyphenation modes. This review provides introduction to major chiral CE-MS modes for the novice and highlights the important working principles of each mode of chiral CE-MS. Next, recent practical developments and progress in chiral CE-MS dating from January 2010 to September 2015 are described. The achievements in clinical and biomedical sciences using a variety of chiral selectors such as CDs and M_oM_s in EKC-MS, MEKC-MS and CEC-MS are discussed. Finally, conclusions and future prospects of CE-MS in chiral analysis are drafted.

Monoliths in capillary electrochromatography and capillary liquid chromatography in conjunction with mass spectrometry

Here, we have reviewed separation studies utilizing monolithic capillary columns for separation of compounds preceding MS analysis. The review is divided in two parts according to the used separation method, namely CEC and capillary LC (cLC). Based on our overview, monolithic CEC-MS technique have been more focused on the syntheses of highly specialized and selective separation phase materials for fast and efficient separation of specific types of analytes. In contrast, monolithic cLC-MS is more widely used and is often employed, for instance, in the analysis of oligonucleotides, metabolites, and peptides and proteins in proteomic studies. While poly(styrene-divinylbenzene)-based and silica-based monolithic capillaries found their place in proteomic analyses, the other laboratory-synthesized monoliths still wait for their wider utilization in routine analyses. The development of new monolithic materials will most likely continue due to the demand of more efficient and rapid separation of increasingly complex samples.

Synthesis, characterization, and application of polysodium N-alkylenyl α-d-glucopyranoside surfactants for micellar electrokinetic chromatography-tandem mass spectrometry

Sugar-based ionic surfactants forming micelles are known to suppress ESI of various compounds due to decrease in surface tension upon micelle formation . For the first time, poly (sodium N-undecylenyl-α-d-glucopyranoside 4,6-hydrogen phosphate, (poly-α-d-SUGP) based surfactants with different chain lengths and head groups have been successfully synthesized, characterized, and applied as compatible chiral selector for MEKC-ESI-MS/MS. First, the effect of polymerization concentration of the monomer, α-d-SUGP, was evaluated by enantioseparation of one anionic compound (1,1'-binaphthyl-2,2'diyl-hydrogen phosphate) and one zwitterionic compound (dansylated phenylalanine) in MEKC-UV to find the optimum molar surfactant concentration for polymerization. Next, MEKC-UV and MEKC-MS were compared for the enantioseparation of 1,1'-binaphthyl-2,2'diyl-hydrogen phosphate. The influence of polymeric glucopyranoside based surfactant head groups and carbon chain lengths on chiral Rs was evaluated for two classes of cationic drugs (ephedrine alkaloids and β-blockers). Finally, enantioselective MEKC-MS of ephedrine alkaloids and β-blockers were profiled at their optimum pH 5.0 and 7.0, respectively, using 20 mM NH4 OAc, 25 mM poly-α-d-SUGP at 30 kV and 25°C under optimum spray chamber conditions. The LOD for most of the enantiomers ranges from 10 to 100 ng/mL with S/N of at least ≥3.0.

Supramolecular Peptide/Surface Assembly for Monitoring Proteinase Activity and Cancer Diagnosis

Matrix metalloproteinases (MMP) are a family of proteolytic enzymes, the expression of which in a key step of tumor progression has recently been better defined. The overexpression of one or more MMPs is thus common among malignant tumors. It may characterize tumor progression and help predict its response to chemotherapy. Consequently, the development of a device for measuring MMP activities is an important challenge for diagnosis and prognosis. In this study, we describe an innovative supramolecular peptide/surface assembly for screening MMP activities. This sensor was used to discriminate various MMP activities and to distinguish between invasive and noninvasive cancerous cell suspensions. Our results confirm the proof-of-concept of a powerful tool for the determination of the tumor aggressiveness and a technical building block for future development of MMP lab-on-chip devices.

Chiral capillary electromigration techniques-mass spectrometry-hope and promise

Analytical methods for chiral compounds require a separation step prior to mass spectrometric detection. CE can separate enantiomers by the use of a chiral selector and can be hyphenated with MS. The chiral selector can be either embedded inside the capillary (electrochromatography) or added into the background solution (EKC). This review describes the fundamentals and highlights the recent developments (September 2009-May 2013) of chiral CEC and EKC with detection using MS. There were 20 research and more than 30 review papers during this period. The research efforts were driven by fundamental studies, such as the development of novel chiral selectors in electrochromatography and of advanced partial filling techniques in EKC in order to optimise separation. Other developments were in application studies, such as in food analytics and metabolomics.

Chiral separation-based ligand exchange by open-tubular capillary electrochromatography

Chiral ligand-exchange enantioseparation of aliphatic and aromatic amino acids was successfully performed using a new open-tubular zwitterionic column with tentacle-type polymer stationary phase. The polymeric stationary phase was prepared using 3-chloro-2-hydroxypropyl methacrylate (HPMA-Cl), a new reactive monomer. The preparation procedure of the open-tubular column included silanization, in situ graft polymerization with HPMA-Cl, and L-histidine (L-His) modification. L-His was used as a chiral ligand-exchange selector and copper(II) as a central ion. Successful enantioseparation of six pairs of amino acid enantiomers was achieved with a buffer of 5 mM CuSO₄, 20 mM (NH4)₂SO₄ at pH 3.0.

Comparison of positively and negatively charged achiral co-monomers added to cyclodextrin monolith: improved chiral separations in capillary electrochromatography

Cyclodextrins (CDs) and their derivatives have been one of the most popular and successful chiral additives used in electrokinetic chromatography because of the presence of multiple chiral centers, which leads to multiple chiral interactions. However, there has been relatively less published work on the use of CDs as monolithic media for capillary electrochromatography (CEC). The goal of this study was to show how the addition of achiral co-monomer to a polymerizable CD such as glycidyl methacrylate β-cyclodextrin (GMA/β-CD) can affect the enantioselective separations in monolithic CEC. To achieve this goal, polymeric monoliths columns were prepared by co-polymerizing GMA/β-CD with cationic or anionic achiral co-monomers [(2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and vinyl benzyltrimethyl-ammonium (VBTA)] in the presence of conventional crosslinker (ethylene dimethacrylate) and ternary porogen system including butanediol, propanol and water. A total of 34 negatively charged compounds, 30 positively charged compounds and 33 neutral compounds were screened to compare the enantioresolution capability on the GMA/β-CD, GMA/β-CD-VBTA and GMA/β-CD-AMPS monolithic columns.

A novel positively charged achiral co-monomer for β-cyclodextrin monolithic stationary phase: improved chiral separation of acidic compounds using capillary electrochromatography coupled to mass spectrometry

The work presented here demonstrates the incorporation of vinylbenzyl trimethylammonium (VBTA) as a novel positively charged achiral co-monomer to a glycidyl methacrylate-beta cyclodextrin (GMA/β-CD) based monolith, providing anion exchange sites with reversed electroosmotic flow (EOF) for capillary electrochromatography (CEC). The monolithic phases, GMA/β-CD-VBTA and GMA/β-CD (without co-monomer) were characterized by scanning electron microscopy, optical microscopy, pressure drop/flow-rate curves and nitrogen adsorption analysis. After optimizing the stationary phase and mobile phase parameters, chiral separations of 41 pairs of structurally diverse anionic chiral analytes were compared individually using the GMA/β-CD-VBTA and GMA/β-CD monolithic columns. The GMA/β-CD-VBTA monolith chiral stationary phase separated significantly more acidic compounds compared to the GMA/β-CD column. To-date there has been limited work in the development of chiral monolithic column for CEC-mass spectrometry (MS). Because of good electrodriven flow characteristics, which allow the column to maintain a stable current in the absence of outlet vial, GMA/β-CD-VBTA column was successfully coupled to single quadrupole mass spectrometer for CEC-MS of several chiral test compounds. In addition, the same monolithic CEC column when coupled to a triple quadrupole MS instrument, two orders of magnitude higher sensitivity was observed compared to a single quadrupole MS instrument.