Additives In Capillary Electrophoresis Research Articles

Stereoselective separation of psychoactive substances: Multivariate optimization and validation of a capillary electrophoresis method using carboxymethyl-β-CD/deep eutectic solvent dual system

A comprehensive study was performed to determine an optimum enantioseparation method for fluorine-substituted amphetamine and cathinone derivatives (fluor-amphetamine and fluor-cathinone derivatives), using a binary system consisting of carboxymethyl-β-CD (CM-β-CD) and a deep eutectic solvent (DES), namely choline chloride-ethylene glycol (ChCl-EG). Under this framework, the optimization and modeling of the separation conditions in a binary system were performed with the objective of maximizing resolution and minimizing analysis time. This was achieved through the application of response surface methodology. In particular, the effect of chiral selector concentration and percentage of DES on resolution and analysis time were investigated and optimized using a complete experimental design. The optimum enantioseparation conditions were determined to be 13.84 mM CM-β-CD and 0.15% v/v ChCl-EG for fluorine-substituted amphetamine derivatives and 14.36 mM and 0.75% v/v ChCl-EG for fluorine-substituted cathinone derivatives, respectively. This combination resulted in a baseline separation for eight out of the nine analytes studied. Overall, the results demonstrated the synergistic effect of the CM-β-CD/DES dual system and highlighted the significance of DESs as additives in capillary electrophoresis.

Establishment and Molecular Modeling Study of Cyclodextrin-Based Synergistic Enantioseparation Systems with Three New Amino Acid Chiral Ionic Liquids as Additives in Capillary Electrophoresis.

Chiral ionic liquids (CILs) have attracted more and more attention due to their superior performance as chiral additives in capillary electrophoresis. In this work, based on the cyclodextrin (CD) derivatives and three new amino acid CILs (trifluoroacetate-L-Hydroxyproline, nitric acid-L-Hydroxyproline and trifluoroacetate-L-threonine), the new synergistic systems were established for chiral drug separation. In contrast to the traditional single glucosyl-β-CD (Glu-β-CD) separation system, the CIL/Glu-β-CD synergistic systems achieved improved resolution of three model drug racemates. Some experimental variables, such as CIL concentration, Glu-β-CD concentration, buffer pH, applied voltage, and the type and proportion of organic modifier, were optimized in the trifluoroacetate-L-Hydroxyproline/Glu-β-CD synergistic system. In addition, the recognition process in the synergistic system was studied through the molecular modeling method.

Ionic Liquids in Analytical Chemistry: Applications and Recent Trends

Background: Ionic liquids (ILs) are a unique class of compounds consisting exclusively of cations and anions that possess distinctive properties such as low volatility, high thermal stability, miscibility with water and organic solvents, electrolytic conductivity and non-flammability. Ionic liquids have been defined as "design solvents", because it is possible to modify their physical and chemical properties by appropriately choosing cations and anions, in order to meet the specific characteristics based on their potential application. Introduction: Due of their tunable nature and properties, ILs are considered as the perfect candidates for numerous applications in analytical chemistry including sample preparation, stationary phases in liquid or gas chromatography, additives in capillary electrophoresis, or in mass spectrometry for spectral and electrochemical analysis. In the last years, the number of publications regarding ILs has rapidly increased, highlighting the broad applications of these compounds in various fields of analytical chemistry. Results: This review first described the main physico-chemical characteristics of ionic liquids, and subsequently reported the various applications in different subdisciplines of analytical chemistry, including the extraction procedure and separation techniques. Furthermore, in each paragraph the most recent applications of ionic liquids in the food, environmental, biological, etc. fields have been described. Conclusion: Overall, the topic discussed highlights the key role of ionic liquids in analytical chemistry, giving hints for their future applications in chemistry but also in biology and medicine.

Cyclodextrin-induced acidity modification of substituted cathinones studied by capillary electrophoresis supported by density functional theory calculations

This paper shows that the acidity of substituted cathinones can change in a diversified and poorly predictable manner upon supramolecular interaction with cyclodextrins used as buffer additives in capillary electrophoresis. The direction and range of pKa shifts may be noticeably different for the particular cyclodextrins and cathinones, suggesting a strict correlation with structure. The most interesting results were observed for 2-hydroxyethyl-β-cyclodextrin, which is capable for inducing the large negative and enantioselective apparent pKa shifts for α‐pyrrolidinovalerophenone and methylenedioxypyrovalerone, even much above ‐1.0 pH unit. A thermodynamic analysis was performed, to identify the role of enthalpy and entropy in the formation and deprotonation of the respective diastereomeric complexes. The former process turned out to be driven by an energetically favorable increase in entropy, related probably to a hydrophobic effect. Deprotonation enthalpy in the complexed state, in turn, occurred to be more favorable than in the free molecule state, entailing the large drop in pKa after complexation. The DFT calculations allowed us to identify some structural effects that most likely contribute to these phenomena. At last, we have demonstrated that at low cyclodextrin concentration and pH ensuring partial ionization, pKa shifts contribute to chiral separation of the abovementioned cathinones. This analytically important effect may be helpful in anticipating the most efficient chiral separation mechanism of other systems.

Separation of Methylated Histone Peptides via Host-Assisted Capillary Electrophoresis.

Lysine methylation in protein is one important epigenetic mechanism that regulates diverse biological processes but is challenging to study due to the large variability in methylation levels and sites. Here, we show that supramolecular hosts such as calixarenes and cucurbiturils can be applied in the background electrolyte (BGE) of capillary electrophoresis (CE) for highly effective separation of post-translationally methylated histone peptides. The molecular recognition event causes a shift in the electrophoretic mobility of the peptide, allowing affinity measurement for binding between the synthetic receptor and various methylated lysine species. Successful separation of the H3 peptides carrying different methylation levels at the K9 position can be achieved using CX4 and CX6 as the BGE additives in CE, enabling monitoring of the activity of the histone lysine demethylase JMJD2E. This reveals the power of combining high resolution CE with synthetic hosts for study of protein methylation, and the method should be capable of analyzing complex biological samples for better understanding of the functions of histone methylation.

Establishment and molecular modeling study of maltodextrin-based synergistic enantioseparation systems with two new hydroxy acid chiral ionic liquids as additives in capillary electrophoresis

Discovering more superior performance of ionic liquids for the separation science has triggered increasing interest. In this work, two new Hydroxy acid-based chiral ionic liquids (CILs) (tertramethylammonium-d-pantothenate (TMA-d-PAN), tertramethylammonium-d-quinate (TMA-d-QUI)) were designed and first used as additives to establish the maltodextrin-based synergistic systems for enantioseparation in capillary electrophoresis (CE). Compared to traditional single maltodextrin chiral separation system, significantly improved separations of all tested drugs in the CIL/Maltodextrin synergistic systems were obtained. Some parameters (CIL concentration, maltodextrin concentration, buffer pH, and applied voltage) in the TMA-d-PAN/Maltodextrin synergistic system have been examined and optimized for analytes. The molecular docking software AutoDock was applied to simulate the recognition process and surmise feasible resolution mechanism in the Maltodextrin/CILs synergistic systems, which has certain guiding value.

Graphene quantum dots as additives in capillary electrophoresis for separation cinnamic acid and its derivatives

A facile capillary electrophoresis (CE) method for the separation of cinnamic acid and its derivatives (3,4-dimethoxycinnamic acid, 4-methoxycinnamic acid, isoferulic acid, sinapic acid, cinnamic acid, ferulic acid, and trans-4-hydroxycinnamic acid) using graphene quantum dots (GQDs) as additives with direct ultraviolet (UV) detection is reported. GQDs were synthesized by chemical oxidization and further purified by a macroporous resin column to remove salts (Na2SO4 and NaNO3) and other impurities. Transmission electron microscopy (TEM) indicated that GQDs have a relatively uniform particle size (2.3 nm). Taking into account the structural features of GQDs, cinnamic acid and its derivatives were adopted as model compounds to investigate whether GQDs can be used to improve CE separations. The separation performance of GQDs used as additives in CE was studied through variations of pH, concentration of the background electrolyte (BGE), and contents of GQDs. The results indicated that excellent separation can be achieved in less than 18 min, which is mainly attributed to the interaction between the analytes and GQDs, especially isoferulic acid, sinapic acid, and cinnamic acid.

Ionic Liquids in Capillary Electrophoresis.

Recently, a great interest was drawn toward ionic liquids (ILs) in analytical separation techniques. ILs possess many properties making them excellent additives in capillary electrophoresis (CE) background electrolytes (BGE). The most important property is the charge of the dissolved ions in BGE enabling the cations to interact with deprotonated silanol groups on the capillary surface and thereby modifying the electroosmotic flow (EOF). Ionic and/or proton donor-acceptor interactions between analyte and IL are possible interactions facilitating new kinds of separation mechanisms in CE. Further advantages of ILs are the high conductivity, the environmentally friendliness, and the good solubility for organic and inorganic compounds. The most commonly used ILs in capillary electrophoresis are dialkylimidazolium-based ILs, whereas for enantioseparation a lot of innovative chiral cations and anions were investigated.ILs are reported to be additives to a normal CE background electrolyte or the sole electrolyte in CE, nonaqueous CE (NACE), micellar electrokinetic chromatography (MEKC), and in enantioseparation. An overview of applications and separation mechanisms reported in the literature is given here, in addition to the enantioseparation of pseudoephedrine using tetrabutylammonium chloride (TBAC) as IL additive to an ammonium formate buffer containing β-cyclodextrin (β-CD).

Simultaneous separation of acidic and basic proteins using gemini pyrrolidinium surfactants and hexafluoroisopropanol as dynamic coating additives in capillary electrophoresis

The separation of acidic and basic proteins using CE has been limited in part due to the adsorption of proteins onto the capillary wall. In this work, the efficient control of EOF and the simultaneous separation of acidic and basic proteins are achieved by use of C18-4-C18PB as a dynamic coating additive, which is a representative surfactant for 1,1′-(butane-1,s-alkyl)bis(1-alkylpyrrolidinium) bromide (Cn-4-CnPB, n=10, 12, 14, 16 and 18). C18-4-C18PB exhibits a powerful capability in the reversal of EOF, and a low concentration even less than 0.001mM is sufficient to reverse EOF at the tested pH values (3.0–9.0). Baseline separation of eight proteins with sharp peaks and high efficiencies (54,000–297,000 plates/m) is obtained with 30mM NaH2PO4 buffer (pH 5.0) containing 4mM C18-4-C18PB. At the same buffer condition, the Cn-4-CnPB with shorter alkyl chain (n=10, 12, 14, 16) cannot achieve the same effective protein separation as C18-4-C18PB. However, the combined use of small amounts (≤0.5%, v/v) of hexafluoroisopropanol (HFIP) and Cn-4-CnPB (n=10, 12, 14, 16) as additives can completely separate all eight proteins with high efficiencies of 81,000–318,000 plates/m. The RSDs of migration time are less than 0.80% and 5.84% for run-to-run and day-to-day assays (n=5), respectively, and the protein recoveries are larger than 90.15%. To the best of our knowledge, this is the first report on the simultaneous separation of acidic and basic proteins using Cn-4-CnPB surfactants or Cn-4-CnPB surfactants combined with HFIP as dynamic coating additives.

Applications of ionic liquids in chromatographic analysis and determination of ionic liquids by chromatography

As a kind of excellent solvent, ionic liquids have received more attentions. Because of the special physical and chemical properties of ionic liquids, ionic liquids have been more widely used in chromatography. This paper summarizes the applications of ionic liquids in gas chromatography, liquid chromatography and capillary electrophoresis, which include ionic liquids used as stationary phase in gas chromatography, stationary phase and mobile phase additive in liquid chromatography and electrolyte additives in capillary electrophoresis. The determination of ionic liquids by chromatography has also been introduced.

Evaluation of the enantioselectivity of glycogen-based synergistic system with amino acid chiral ionic liquids as additives in capillary electrophoresis

In this paper, two novel amino acid chiral ILs, tetramethylammonium-l-arginine (TMA-l-Arg) and tetramethylammonium-l-aspartic acid (TMA-l-Asp), were applied for the first time in CE enantioseparation to evaluate their potential synergistic effect with glycogen as chiral selector. As observed, significantly improved separation of tested enantiomers were obtained in the chiral ILs/glycogen synergistic systems compared to the single glycogen separation system. Several primary parameters affecting the enantioseparation, such as amino acid ILs (AAILs) concentration, glycogen concentration and buffer pH, were systematically investigated. An achiral tetramethylammonium hydroxide ionic liquid (TMA-OH) modified separation system was also evaluated to validate the superiority of the novel chiral ILs/glycogen synergistic systems. To further optimize the overall synergistic systems, the effect of three other parameters, including buffer concentration, applied voltage and capillary temperature were simultaneously analyzed by a central composite design (CCD), and excellent enantioseparations were achieved with the optimized parameters. The results indicate that the application of chiral ILs/glycogen synergistic systems is a promising way in chiral separation science.

Combined Use of Ionic Liquid and Hydroxypropyl‐β‐Cyclodextrin for the Enantioseparation of Ten Drugs by Capillary Electrophoresis

In the present study, hydroxypropyl-β-cyclodextrin and an ionic liquid (1-ethyl-3-methylimidazolium-l-lactate) were used as additives in capillary electrophoresis for the enantioseparation of 10 analytes, including ofloxacin, propranolol hydrochloride, dioxopromethazine hydrochloride, isoprenaline hydrochloride, chlorpheniramine maleate, liarozole, tropicamide, amlodipine benzenesulfonate, brompheniramine maleate, and homatropine methylbromide. The effects of ionic liquid concentrations, salt effect, cations, and anions of ionic liquids on enantioseparation were investigated and the results proved that there was a synergistic effect between hydroxypropyl-β-cyclodextrin and the ionic liquid, and the cationic part of the ionic liquid played an important role in the increased resolution. With the developed dual system, all the enantiomers of 10 analytes were well separated in resolutions of 5.35, 1.76, 1.85, 2.48, 2.88, 1.43, 5.45, 4.35, 2.76, and 2.98, respectively. In addition, the proposed method was applied to the determination of the enantiomeric purity of S-ofloxacin after validation of the method in terms of selectivity, repeatability, linearity range, accuracy, precision, limit of detection (LOD), and limit of quality (LOQ).

Hydrophobic Interactions of Succinoglycan Dimers Isolated from Sinorhizobium meliloti with Hydrophobic Fluorescence Probes, 8-anilino-1-naphthalenesulfonate and 6-p-toluidino-2-naphthalenesulfonate

Succinoglycan, an acidic exopolysaccharide, is produced by the nitrogen-fixing bacterium, Shinorhizobium meliloti. The main chain consists of a β-1,3, β-1,4, and β-1,6 linked octasaccharide subunit containing one galactose at the reducing end and seven glucose residues. Also, the repeating unit carries pyruvyl, acetyl, and succinyl modifications. The acetyl group is located at the C6 position of the third sugar residue from the reducing galactose, the succinyl group is located at the C6 position of the seventh sugar, and the pyruvyl group is linked to the eighth sugar residue through a 4,6-ketal linkage. The succinoglycan has high molecular weight (HMW) forms consisting of hundreds of octasaccharide units and low molecular weight (LMW) forms that are composed of monomers, dimers, and trimers of the octasaccharide unit. The LMW succinoglycan offers the advantages of the HMW succinoglycan for research because it is not viscous and the structure is simpler than the HMW succinoglycan. Also, the LMW succinoglycan is of particular interest, as some reports have described that it is able to restore the ability of invasion-deficient S. meliloti mutants to invade nodules. In addition to investigation of its biological activity, physico-chemical properties of the LMW succinoglycan have been studied. Monomers of succinoglycan have been isolated and investigated as chiral additives in capillary electrophoresis (CE) for the chiral separation of some flavonoids. The effect of enantioseparation is known to be dependent on the succinate moiety of the monomer structure. Also, flavonoid enantioseparations using succinoglycan monomer show that the monomer may directly interact with flavonoids produced by the host plant. However, there have been no reports on physico-chemical properties of dimers or trimers. Herein, we examine the structural properties of the dimers. The chemical structures of succinoglycan dimers are also shown in Figure 1(a). Fluorescent probes are defined as small molecules that undergo changes in one or more of their fluorescent properties as a result of hydrophobic interaction with environmental macromolecules. As representative fluorescent probes, 8-anilino-1-naphthalene sulfonate (ANS) and 6-ptoluidino-2-naphthalenesulfonate (TNS) have been used to explore the hydrophobicity of biomolecules. The chemical structures of both fluorescent probes are shown in Figure 1(b) and 1(c). In the present study, the fluorescence of ANS and TNS, respectively, in an aqueous solution was investigated under the presence of succinoglycan dimers (D1, D2, D3, and D4), and the hydrophobic character of the dimers was analyzed. Based on double reciprocal plots of the fluorescence increments depending on the dimer concentration, the association constants could be estimated. The results indicate that association constants of ANS and TNS, respectively, with D2 are the largest among the 4 types of

Stereoisomeric separation of some flavanones using highly succinate-substituted α-cyclosophoro-octadecaoses as chiral additives in capillary electrophoresis

α-Cyclosophoro-octadecaoses (α-C18), produced by Rhodobacter sphaeroides, are mostly homogeneous in size with 18 glucose units per ring as the predominant form. α-C18s are linked by β-(1→4)-linkages and one α-(1→6)-linkage and are also known to be highly substituted by acetyl (0–2 per mol) and/or succinoyl groups (1–7 per mol). We isolated and purified α-C18 and successfully used it in capillary electrophoresis (CE) as a chiral additive for the separation of five flavanones and flavanone-7- O-glycosides, including naringenin, hesperetin, eriodictyol, homoeriodictyol, isosakuranetin, and hesperidin. Throughout the CE experiment with unsubstituted α-C18 (uα-C18) obtained after alkaline treatment of the isolated α-C18, we found that successful chiral separation critically depends on the presence of succinate substituents attached to α-C18 in CE, suggesting that succinoylation of α-C18 is decisive for effective stereoisomeric separation.

Application of Crown Ethers as Buffer Additives in Capillary Electrophoresis

This paper provides an overview of the applications of crown ethers as buffer additive in capillary electrophoresis. The applications area can be categorized into the separation of inorganic cations; chiral separation and separation of organic compounds include positional isomers. The use of background electrolytes combining cyclodextrins and crown-ethers for both chiral and non-chiral separation is also presented. Applications published up to the year 2009 from each category are summarized. Keywords: Crown ethers, Capillary electrophoresis, Chiral separation, Inorganic cations analysis

Separation of acidic and basic proteins by capillary electrophoresis using gemini surfactants and gemini-capped nanoparticles as buffer additives

This paper demonstrated simultaneous separation of acidic and basic proteins using cationic gemini surfactants as buffer additives in capillary electrophoresis. We showed that even at a low concentration (0.1 mmol·L−1) of alkanediyl-α,ω-bis(dimethyloctadecylammonium bromide) (18-s-18), the wall adsorption of both acidic and basic proteins could be effectively suppressed under acidic conditions. Smaller micelle size (e.g., s = 5–8) is more effective for the separation of acidic proteins than larger micelle size (e.g., s 10). Varying the spacer length of gemini surfactants can influence the electrophoretic mobility and selectivity of proteins to achieve the desired separation. Under the optimized conditions, RSDs of the migration time were less than 0.8% and 2.2% for run-to-run and day-to-day assays, respectively, and protein recoveries ranged from 79% to 100.4%. Furthermore, we also investigated the use of gemini surfactant-capped gold nanoparticles (gemini@AuNPs) as buffer additives in protein separation. Introduction of AuNPs into the buffer shortened the analysis time and slightly improved the separation efficiencies. Finally, we presented the applications of this method in the analysis of biological samples, including plasma, red blood cells and egg white.

Chiral separation and discrimination of catechin by sinorhizobial octasaccharides in capillary electrophoresis and 13C NMR spectroscopy

Succinoglycan, a sinorhizobial exopolysaccharide produced by Sinorhizobium meliloti, is composed of an octasaccharide subunit. S. meliloti produces both high-molecular-weight and low-molecular-weight ( M r < 10,000) succinoglycans which consist of monomers, dimers, or trimers. Succinoglycan monomers were isolated and further purified in the monomer series (M1, M2, and M3) by the degree of succinylation. We used sinorhizobial octasaccharides (M1, M2, and M3) as chiral additives in capillary electrophoresis (CE) for chiral separation of catechin and also as chiral shift reagents with 13C NMR spectroscopy for chiral discrimination of catechin. Chiral separation of catechin took place when sinorhizobial octasaccharides (M2 and M3) were added to the background electrolyte (BGE) in CE. NMR signal splittings were also observed in the interactions of sinorhizobial octasaccharides with the enantiomers of catechin. Both chiral separation and discrimination of catechin depend on the presence of succinate substituents of the linear monomeric octasaccharide in CE and NMR spectroscopy, suggesting that succinylation of sinorhizobial octasaccharide is decisive for the effective chiral separation and discrimination of catechin.

Application of Nanomaterials in Environmental Analysis and Monitoring

Recent advances in the development and application of nanomaterials in analytical chemistry for environmental analysis and monitoring are reviewed, with focus on sensors, separation and extraction techniques, including the use of nanomaterials as transducer elements for electrochemical sensors, as nanolabels for optical sensors, as stationary phases for liquid chromatography and gas chromatography, as pseudostationary phases or buffer additives in capillary electrophoresis, capillary electrochromatography, and microchip electrophoresis, as well as extraction materials for enrichment prior to chromatographic analysis. Examples of different nanomaterials-based analytical techniques for the detection of major families of environmental pollutants, i.e., organic contaminants, heavy metals and air pollutants, are presented.

Enantioseparation of some chiral flavanones using microbial cyclic β-(1→3),(1→6)-glucans as novel chiral additives in capillary electrophoresis

Cyclic β-(1→3),(1→6)-glucans, microbial cyclooligosaccharides produced by Bradyrhizobium japonicum USDA 110, were used as novel chiral additives for the enantiomeric separation of some flavanones such as eriodictyol, homoeriodictyol, hesperetin, naringenin, and isosakuranetin in capillary electrophoresis (CE). Among the flavanones, eriodictyol was separated with the highest resolution ( R s 5.66) and selectivity factor ( α 1.18) when 20 mM cyclic β-(1→3),(1→6)-glucans were added to the background electrolyte (BGE) at pH 8.3.

The use of neutral cyclodextrins as additives in capillary electrophoresis for the separation and identification of propoxyphene enantiomers

A simple, efficient, and rapid method is described for separation of the enantiomers of propoxyphene by capillary electrophoresis with neutral cyclodextrins as chiral separators. This method has several advantages over the crystallization method employed by some forensic laboratories, including unambiguous results, ease of use, and smaller sample-size requirement. The method enables baseline separation of the propoxyphene enantiomers in approximately six minutes, which is less than one-third of the time required for a previously published method.