Background theory and applications of microemulsion electrokinetic chromatography

Droplet Electrophoresis with Internal Free Ions: Effect of Permittivity Changes in the Electric Double Layer

Droplet electrophoresis (EP) is of interest in biological systems, microfluidics, and separation techniques. We investigate EP of an oil droplet that contains free ions and is stabilized in an electrolyte solution through an amphoteric surfactant. The presence of mobile ions within the droplet leads to the creation of a distinct nonzero space charge density inside the droplet and consequently, formation of an inner EDL inside the droplet in addition to the traditionally considered outside EDL. While we assume the permittivity inside the inner EDL to remain constant, we consider both the case of constant and variable permittivity in the outer EDL. Our findings demonstrate a change in the droplet direction of motion in the electric field when transitioning from acidic to alkaline pH, regardless of permittivity and ionic strength in both oil and electrolyte. We further find a significant reduction in the magnitude of droplet velocity in the case of a variable permittivity due to reduction of the local space charge density within the EDL surrounding the droplet. When decreasing the viscosity ratio of the oil to the electrolyte, in all cases we find a reduction in droplet velocity. This decline is attributed mostly to the formation and strength of a vortex around the droplet. We finally demonstrate that with constant permittivity in the outer EDL, the variation in κaouter has a more significant effect on the droplet's EP velocity than altering κainner. However, in cases where the body forces inside of the droplet dominate, minor changes in the outer electrolyte concentration have no influence on the droplet motion, which is relevant for biological colloids that can contain significant free internal charges. Our results are important for the manipulation of biological colloids, water and waste treatment such as lubricant removal from processing streams.

Separation and determination of phenolic compounds using novel deep eutectic solvent-in-water microemulsion electrokinetic chromatography

BACKGROUND

Microemulsion electrokinetic chromatography (MEEKC) is a mode of capillary electrophoresis with a wide range of applications in which microemulsion is utilized as background electrolyte to achieve the separation of analytes. Microemulsions are composed of oil droplets, aqueous buffer, surfactant, and co-surfactant. Currently, conventional organic reagents act as the most commonly used oil phase in microemulsions, which are unfriendly to the environment. Recently, deep eutectic solvent (DES) has become a new type of eco-friendly solvent due to its non-toxicity. Therefore, it is of great value to establish a new MEEKC method by replacing conventional organic reagents as the oil phase with DES.

RESULTS

The novel DES/W MEEKC method was established for phenolic compounds in Senecio scandens samples. Single-factor experiments and response surface methodology were performed to systematically optimize the crucial parameters for the method, including the type and content of the oil phase, surfactant content, concentration of borax buffer, and pH of the background solution. Under the optimized conditions, satisfactory regression curves were established for all standard analytes with correlation coefficients ≥0.9990. The method featured high sensitivity and favorable accuracy, with the instrumental detection limit in the range of 0.22-1.04 μg/mL, and intraday and interday precision for migration time expressed as relative standard deviations of 0.18-0.82% and 1.25-2.50%, respectively. The DES/W MEEKC method was successfully applied to Senecio scandens with good recoveries of 87.72-106.99%. In conclusion, the newly established DES/W MEEKC method is highly efficient, green and environmentally friendly.

SIGNIFICANCE

DES is considered a green and efficient solvent. The DES/W MEEKC method is highly efficient and environmentally friendly. Actually, the method provides a novel and effective analytical tool for the simultaneous separation and determination of multiple phenolic compounds, especially in complex plant matrices. In the future, the DES/W MEEKC method still has the prospect of being widely used in the separation of other complex phytochemicals.

pH-Tunable electrokinetic movement of droplets

Manipulation and control of droplet motion in an electric field is of interest in biological systems, microfluidics and electrokinetic (EK) separation techniques. In this work, we show that the electrokinetic motion of oil-in-water (O/W) emulsions stabilized by an amphoteric surfactant can be controlled by changing the pH. Amphoteric surfactants carry both positive and negative head groups and change charge under the influence of changing pH, which allows them to impact the surface charge of droplets as a function of pH, and in extension their direction of motion in an electric field. Using a microfluidic system, we evaluate the effect of pH, surfactant concentration and droplet size on the EK velocity of droplets, which is a combination of electrophoresis (EP) and electro-osmotic flow (EOF). We show that by changing the pH from acidic to alkali, the direction of droplet motion in an external electric field changes. The magnitude of the EK velocity at acidic and neutral pH is not significantly altered as a result of the competition of the EP and EOF in the system, which generally have opposite directions. Our results are in good agreement with theoretical predictions for the droplet EP mobility and can thus serve as a verification of the theoretical descriptions. In addition to the pH, the surfactant concentration affects droplet EK velocity, most specifically at pH of 7 which is close to the isoelectric point of the surfactant monomers. At this pH, changing the surfactant concentration changes the direction of droplet motion due the competing effect of the EP and EOF at different surfactant concentrations. By increasing the droplet size, the magnitude of the EK velocity increases because of the larger local ζ-potential of the larger droplets as well as the wall-enhanced effect in the system. The results from this work can be applied to design on-chip droplet separation strategies based on pH variations and are relevant for systems in which pH gradients naturally occur, such as the human body.

Nanoemulsion supported microemulsion electrokinetic chromatography coupled with selected preconcentration techniques as an approach for analysis of highly hydrophobic compounds

In this paper, an oil-in-water (O/W) nanoemulsion (NE) prepared by water cold dilution of an O/W microemulsion (ME) was introduced as a sample matrix in microemulsion electrokinetic capillary chromatography (MEEKC) for the highly hydrophobic compounds analysis. Several model compounds with log P_O/W values in the 4.1-10.9 range, from different chemical groups, including retinol, α-tocopherol, cholecalciferol, phylloquinone, menaquinone-7, dichlorodiphenyltrichloroethane, ivermectin have been tested. As a proof of the concept of NE formation, a dynamic light scattering technique was employed to determine the size distribution profile of NE particles. Moreover, due to relatively low conductivity of the NE matrix (50-100 times lower in comparison to the separation buffer) and a negative electric charge provided to hydrophobic compounds through NE dispersed phase, NE matrices have been combined with preconcentration techniques based on electrokinetic dosing, namely field amplified sample injection (FASI) and pressure assisted electrokinetic injection (PAEKI). The detection limits for vitamin K₁ and K₂-MK7 in the NE matrix in combination with FASI (NE-MEEKC-FASI) as well as PAEKI (NE-MEEKC-PAEKI) were up to 42.9 and 12.1 ng mL^-1, respectively. In comparison to standard hydrodynamic injection for microemulsion sample matrix NE-MEEKC-PAEKI grant 45-fold improvement in signal sensitivity. The study presents an innovative approach, as it enables the use of preconcentration techniques for highly hydrophobic compounds (log P_O/W > 4), which was not previously possible for implementation in the electromigration techniques. Likewise, the use of organic solvents has been reduced by using ME as a solvent for stock solutions and diluting with water prior to the analysis. The application to real samples was investigated using a dietary supplement containing vitamin K₂-MK7 obtained from the fermentation product of soybeans.

Mechanistic studies of droplet electrophoresis: A review

Electrophoresis (EP) of droplets is an intriguing phenomenon that has applications in biological systems, separation strategies, and reactor engineering. Droplet EP is significantly different from the classic particle EP because of droplet characteristics such as a mobile surface charge and the nonrigidity of the interface. Also, the liquid-liquid system, where there is an interplay between the hydrodynamic and electrokinetic forces in both phases, adds to the complexity of electrophoretic motion. Due to the vast amount of potential applications of droplet EP, a mechanistic understanding of the droplet motion in the presence of an external electric field is crucial. This review provides a background on the mechanism of droplet EP and summarizes the intrinsic interplay between the different relevant forces in these systems. The review also describes the key differences between droplet EP and particle EP, and the impact of these differences on droplet mobility. Additionally, we schematically summarize the effects of key parameters on droplet EP mobility, such as electric double layer polarization, the development of internal flow inside a droplet and boundary effects.

Quantitative analysis of sesquiterpenes and comparison of three Curcuma wenyujin herbal medicines by micro matrix solid phase dispersion coupled with MEEKC

A simple, efficient and environmental friendly method was proposed for determining five sesquiterpenoids of Curcuma wenyujin by MSPD extraction coupled with MEEKC separation. Molecular sieve was applied as a solid support for extraction of sesquiterpenoids for the first time. Various parameters affecting extraction and separation efficiency were investigated. The optimized conditions involved dispersing sample (200 mg) with 200 mg of TS-1 for 150 s and using 1000 μL of methanol to elute five target analytes. Finally, they were well separated by using a running buffer containing 1.3% SDS, 5.0% 1-butanol, 0.5% ethyl acetate and 10% acetonitrile in 10 mM borate buffer at pH 9.0. Consequently, the developed method was fully validated and successfully applied to determine the five sesquiterpenoids including curdine, curcumenol, germacrone, furanodiene and β-elemene in Curcuma wenyujin origin's Chinese herbal medicines. Furthermore, hierarchical cluster analysis was performed based on the contents of target compounds for distinguishing steamed and non-steamed drugs. The present study provided a promising method for fast investigation and discrimination of chemical difference in steam & non-steamed Chinese medicines from Curcuma wenyujin origin.

Phospholipid bilayer affinities and solvation characteristics by electrokinetic chromatography with a nanodisc pseudostationary phase

Phospholipid bilayer nanodiscs composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and synthetic maleic acid-styrene copolymer belts have been introduced as a pseudostationary phase (PSP) in electrokinetic chromatography and demonstrated good performance. The nanodiscs provide a suitable migration range and high theoretical plate counts. Using this nanodisc pseudostationary phase, the affinity of the bilayer structure for probe solutes was determined and characterized. Good correlation is observed between retention factors and octanol water partition coefficients for particular categories of solutes, but the general correlation is weak primarily because the nanodiscs show stronger affinity than octanol for hydrogen bond donors. This suggests that a more appropriate application of this technology is to measure and characterize interactions between solutes and lipid bilayers directly. Linear solvation energy relationship analysis of the nanodisc-solute interactions in this study demonstrates that the nanodiscs provide a solvation environment with low cohesivity and weak hydrogen bond donating ability, and provide relatively strong hydrogen bond acceptor strength.

Surfactant-free microemulsion electrokinetic chromatography (SF-MEEKC) with UV and MS detection - a novel approach for the separation and ESI-MS detection of neutral compounds

Microemulsion electrokinetic chromatography (MEEKC) is a powerful tool to separate neutral species based on differences in their hydrophobic and hydrophilic properties. However, as a major drawback the conventionally used SDS based microemulsions are not compatible with electrospray ionization mass spectrometry (ESI-MS). In this work, a surfactant-free microemulsion (SFME) consisting of water, ethanol, and 1-octanol is used for surfactant-free microemulsion electrokinetic chromatography (SF-MEEKC). Ammonium acetate was added to the SFME enabling electrophoretic separations. The stability of SFMEs containing ammonium acetate was investigated using small-angle X-ray scattering and dynamic light scattering. A method for the separation of a model system of hydrophobic and hydrophilic neutral vitamins, namely the vitamins B₂ and D₃, and the cationic vitamin B₁ was developed using UV/VIS detection. The influence of the ammonium acetate concentration on the separation performance was studied in detail. The method was characterized concerning reproducibility of migration times and peak areas and concerning the linearity of the calibration data. Furthermore, SF-MEEKC was coupled to ESI-MS investigating the compatibility between SFMEs and the ESI process. The signal intensities of ESI-MS measurements of the model analytes were comparable for SFMEs and aqueous systems. Finally, the vitamin D₃ content of a drug treating vitamin D₃ deficiency was determined by SF-MEEKC coupled to ESI-MS using 25-hydroxycholecalciferol as an internal standard. Graphical abstract The concept of surfactant-free microemulsion electrokinetic chromatography coupled to electrospray ionization mass spectrometry.

Microemulsion Electrokinetic Chromatography

Microemulsion electrokinetic chromatography (MEEKC) is a special mode of capillary electrophoresis employing a microemulsion as carrier electrolyte. Analytes may partition between the aqueous phase of the microemulsion and its oil droplets which act as a pseudostationary phase. The technique is well suited for the separation of neutral species, in which case charged oil droplets (obtained by addition of an anionic or cationic surfactant) are present. A single set of separation parameters may be sufficient for separation of a wide range of analytes belonging to quite different chemical classes. Fine-tuning of resolution and analysis time may be achieved by addition of organic solvents, by changes in the nature of the surfactants (and cosurfactants) used to stabilize the microemulsion, or by various additives that may undergo some additional interactions with the analytes. Besides the separation of neutral analytes (which may be the most important application area of MEEKC), it can also be employed for cationic and/or anionic species. In this chapter, MEEKC conditions are summarized that have proven their reliability for routine analysis. Furthermore, the mechanisms encountered in MEEKC allow an efficient on-capillary preconcentration of analytes, so that the problem of poor concentration sensitivity of ultraviolet absorbance detection is circumvented.

Recent advances in CE analysis of antibiotics and its use as chiral selectors

Antibiotics are a class of therapeutic molecules widely employed in both human and veterinary medicine. This article reviews the most recent advances in the analysis of antibiotics by CE in pharmaceutical, environmental, food, and biomedical fields. Emphasis is placed on the strategies to increase sensitivity as diverse off-line, in-line, and on-line preconcentration approaches and the use of different detection systems. The use of CE in the microchip format for the analysis of antibiotics is also reviewed in this article. Moreover, since the use of antibiotics as chiral selectors in CE has grown in the last years, a new section devoted to this aspect has been included. This review constitutes an update of previous published reviews and covers the literature published from June 2011 until June 2013.

Quantification of the level of fat-soluble vitamins in feed based on the novel microemulsion electrokinetic chromatography (MEEKC) method

BACKGROUND

Simultaneous quantification of liposoluble vitamins is not a new area of interest, since these compounds co-determine the nutritional quality of food and feed, a field widely explored in the human and animal diet. However, the development of appropriate methods is still a matter of concern, especially when the vitamin composition is highly complex, as is the case with feed designated for laboratory animals, representing a higher health and microbiological status.

RESULTS

A method combining microemulsion electrokinetic chromatography (MEEKC) with liquid-liquid extraction was developed for the determination of four fat-soluble vitamins in animal feed. A separation medium consisting of 25 mmol L⁻¹ phosphate buffer (pH 2.5), 2-propanol, 1-butanol, sodium dodecyl sulfate and octane allowed the simultaneous determination of vitamins A, D, E and K within a reasonable time of 25 min. The polarity of the separation voltage was reversed in view of the strongly suppressed electro-osmotic flow, and the applied voltage was set at 12 kV. The fat-soluble vitamins were separated in the order of decreasing hydrophobicity.

CONCLUSION

It was proved that the proposed MEEKC method was sufficiently specific and sensitive for screening fat-soluble vitamins in animal feed samples after their sterilization.

A review of developments in the methodology and application of microemulsion electrokinetic chromatography

MEEKC is a mode of CE, which utilizes microemulsion (ME) as the BGE to achieve separation of a diverse range of analytes. MEs are composed of nanometer-sized oil droplets suspended in aqueous buffer which are stabilized by the presence of a surfactant and co-surfactant. These MEs are commonly referred to as oil-in-water MEs and their application in MEEKC has been extensively examined. This review details advances in the theory, methodology, and application of MEEKC during the period 2010-2012. Areas covered include online sample concentration, advances in chiral separations, use of coated capillaries, chemometric approaches, and the use of novel additives to the ME system. This review also provides the reader with an introduction to MEEKC and a presentation of recent applications.

Simultaneous determination of α-, β- and γ-asarone in Acorus tatarinowii by microemulsion electrokinetic chromatography with [BMIM]PF6 as oil phase

In the present study, a rapid and repeatable microemulsion electrokinetic chromatography (MEEKC) method was developed for the simultaneous determination of three isomers (α-, β- and γ-asarone) in Acorus tatarinowii by using ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF(6)) as oil phase. Experimental parameters including the microemulsion compositions (concentrations of surfactant, co-surfactant and oil phase), pH, concentration of borate buffer, capillary temperature and voltage were intensively investigated. Finally, the main compounds in the methanol extract of A. tatarinowii were well separated within 11 min using a running buffer composed of 40 mmol/L sodium dodecyl sulfonate (SDS), 2.0 mol/L n-propanol, 8 mmol/L [BMIM]PF(6) in 10 mmol/L borate buffer of pH 9.5. The developed method was applied to determine the contents of α-, β- and γ-asarone in A. tatarinowii from five different producing areas in China (Anhui, Hebei, Sichuan, Zhejiang and Chongqing). The results indicated that the contents of three asarones are quite different in the investigated A. tatarinowii samples. On the other hand, the MEEKC with ionic liquid as oil phase should be a promising method for the analysis of volatile components especially isomers in medicinal herbs.

Lipidomics era: accomplishments and challenges

Lipid mediators participate in signal transduction pathways, proliferation, apoptosis, and membrane trafficking in the cell. Lipids are highly complex and diverse owing to the various combinations of polar headgroups, fatty acyl chains, and backbone structures. This structural diversity continues to pose a challenge for lipid analysis. Here we review the current state of the art in lipidomics research and discuss the challenges facing this field. The latest technological developments in mass spectrometry, the role of bioinformatics, and the applications of lipidomics in lipid metabolism and cellular physiology and pathology are also discussed.

Determination of aromatic amines in environmental water sample by hollow fiber-liquid phase microextraction and microemulsion electrokinetic chromatography

A novel method of microemulsion electrokinetic chromatography (MEEKC) coupled with hollow fiber-liquid phase microextraction (HF-LPME) was developed for determination of six aromatic amines including 4-methylaniline, 3-nitroaniline, 2,4-dimethylaniline, 4-chloroaniline, 3,4-dichloraniline and 4-aminobiphenyl. Baseline separation of six aromatic amines was achieved within 8 min by using the microemulsion buffer containing a 10 mM borate buffer at pH 9.0, 0.8% (v/v) ethyl acetate as oil droplets, 60 mM sodium cholate as surfactant, 5.0% (v/v) 1-butanol as co-surfactant. The influence factors relevant to the HF-LPME process were systemically investigated. The obtained enrichment factors were ranged between 70 and 157 in a 30 min extraction time, and the limits of detection ranged between 0.0021 and 0.0048 microg/mL. This purposed method was successfully applied for the analysis of aromatic amines in water sample and the recoveries were ranged from 87.2% to 99.8%.

Recent advances in the methodology, optimisation and application of MEEKC

MEEKC is an electrodriven separation technique. Oil-in-water microemulsions (MEs) and to a lesser extent water-in-oil MEs have been used in MEEKC as BGEs to achieve separation of a diverse range of solutes. The more common (oil-in-water) MEs are composed of nanometre-sized droplets of oil suspended in an aqueous buffer. Interfacial tension between the oil and aqueous phase is reduced close to zero by the presence of a surfactant and a co-surfactant. MEEKC is capable of providing fast and efficient separations for a wide range of acidic, basic and neutral, water-soluble and -insoluble compounds. This review details the advances in MEEKC-based separations from the period 2006 to 2008. Areas covered include online sample concentration, chiral separation, suppressed electroosmosis MEEKC, MEEKC-MS, and the use of MEEKC in predicting migration behaviour and solute characteristics. A fundamental introduction to MEEKC, along with the presentation and discussion of recent applications is also included.

On-line concentration of neutral analytes by complexation and acetonitrile sweeping in nonionic microemulsion electrokinetic chromatography with direct ultraviolet detection

To separate and detect neutral solutes in nonionic microemulsion electrokinetic chromatography (MEEKC), a novel method was developed, combining complex formation and acetonitrile (ACN) sweeping. In this report, dynamic borate complexation and on-line sweeping occurred simultaneously during a run. The operating parameters which affected the performance of analyte sweeping in nonionic MEEKC were examined in terms of borate complexation, ACN content, Brij-35 concentration and sample plug length. In addition, the validation of the method included tests of the limit of detection, reproducibility and sensitivity enhancement. 60-110-Fold of magnitude improvement in detection sensitivity for model compounds (ginsenoside Rf, ginsenoside Rb2, ginsenoside Re) using Brij-35 microemulsion was demonstrated. Furthermore, the method was applied to the determination of glucosides in the plant extract.

Capillary separation: micellar electrokinetic chromatography

Micellar electrokinetic chromatography (MEKC), a separation mode of capillary electrophoresis (CE), has enabled the separation of electrically neutral analytes. MEKC can be performed by adding an ionic micelle to the running solution of CE without modifying the instrument. Its separation principle is based on the differential migration of the ionic micelles and the bulk running buffer under electrophoresis conditions and on the interaction between the analyte and the micelle. Hence, MEKC's separation principle is similar to that of chromatography. MEKC is a useful technique particularly for the separation of small molecules, both neutral and charged, and yields high-efficiency separation in a short time with minimum amounts of sample and reagents. To improve the concentration sensitivity of detection, several on-line sample preconcentration techniques such as sweeping have been developed.

Capillary electrophoresis-based methods for the determination of lipids--a review

Capillary electrophoresis (CE) is a high-resolution technique for the separation of complex biological and chemical mixtures. CE continues to emerge as a powerful tool in the determination of lipids. Here we review the analytical potential of CE for the determination of a wide range of lipids. The different classes of lipids are introduced, and the different modes of CE and optimization methods for the separation of lipids are described. The advantages and disadvantages of the different modes of CE compared to traditional methods like gas chromatography (GC) and liquid chromatography (LC) in the determination of lipids are discussed. Finally, the potential of CE in the determination of lipids in the future is illustrated.