Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2014–2016)

Recent advances in microscale separation techniques for glycome analysis

The glycomic analysis holds significant appeal due to the diverse roles that glycans and glycoconjugates play, acting as modulators and mediators in cellular interactions, cell/organism structure, drugs, energy sources, glyconanomaterials, and more. The glycomic analysis relies on liquid-phase separation technologies for molecular purification, separation, and identification. As a miniaturized form of liquid-phase separation technology, microscale separation technologies offer various advantages such as environmental friendliness, high resolution, sensitivity, fast speed, and integration capabilities. For glycan analysis, microscale separation technologies are continuously evolving to address the increasing challenges in their unique manners. This review discusses the fundamentals and applications of microscale separation technologies for glycomic analysis. It covers liquid-phase separation technologies operating at scales generally less than 100 µm, including capillary electrophoresis, nanoflow liquid chromatography, and microchip electrophoresis. We will provide a brief overview of glycomic analysis and describe new strategies in microscale separation and their applications in glycan analysis from 2014 to 2023.

Steady state microdialysis of microliter volumes of body fluids for monitoring of amino acids by capillary electrophoresis with contactless conductivity detection

BACKGROUND

The availability of dialysis membranes in the form of hollow fibres with diameters compatible with the fused silica capillaries used in capillary electrophoresis is very limited. However, haemodialysis bicarbonate cartridges commonly used in human medicine containing polysulfone hollow fibres are available on the market and are used for the fabrication of coaxial microdialysis probes. The miniature probe design ensures that steady-state conditions are achieved during microdialysis of minimal volumes of body fluids.

RESULTS

A coaxial microdialysis probe with a length of 5 cm and an inner diameter of 200 μm is used for microdialysis of 10 μL of body fluid collected into a sampling fused silica capillary with an inner diameter 430 μm. Microdialysis is performed into 0.01 M HCl as a perfusate at stopped flow and 2 μL of the resulting microdialysate are subjected to analysis by capillary electrophoresis with contactless conductivity detection. Microdialysis pre-treatment is verified by analysis of 11 common amino acids at a 100 μM concentration level, resulting in recoveries of 98.3-102.5%. The electrophoretic separation of amino acids is performed in 8.5 M acetic acid at pH 1.37 as a background electrolyte with analysis time up to 4.5 min and LOD in the range of 0.12-0.28 μM. The reproducibility of the developed technique determined for the peak area ranges from 1.2 to 4.5%. Applicability is tested in the quantification of valine and leucine in plasma during fasting and subsequent reconvalescence.

SIGNIFICANCE

The fabrication of a coaxial microdialysis probe for the laboratory preparation of microliter volumes of various types of clinical samples is described, which is coupled off-line with capillary electrophoretic monitoring of amino acids in 2 μL volumes of microdialysate. The developed methodology is suitable for quantification of 20 amino acids in whole human blood, plasma, tears and has potential for analysis of dry blood spots captured on hollow fibre.

Capillary electrophoresis in the analysis of therapeutic peptides-A review

Therapeutic peptides are a growing class of innovative drugs with high efficiency and a low risk of adverse effects. These biomolecules fall within the molecular mass range between that of small molecules and proteins. However, their inherent instability and potential for degradation underscore the importance of reliable and effective analytical methods for pharmaceutical quality control, therapeutic drug monitoring, and compliance testing. Liquid chromatography-mass spectrometry (LC-MS) has long time been the "gold standard" conventional method for peptide analysis, but capillary electrophoresis (CE) is increasingly being recognized as a complementary and, in some cases, superior, highly efficient, green, and cost-effective alternative technique. CE can separate peptides composed of different amino acids owing to differences in their net charge and size, determining their migration behavior in an electric field. This review provides a comprehensive overview of therapeutic peptides that have been used in the clinical environment for the last 25 years. It describes the properties, classification, current trends in development, and clinical use of therapeutic peptides. From the analytical point of view, it discusses the challenges associated with the analysis of therapeutic peptides in pharmaceutical and biological matrices, as well as the evaluation of CE as a whole and the comparison with LC methods. The article also highlights the use of microchip electrophoresis, nonaqueous CE, and nonconventional hydrodynamically closed CE systems and their applications. Overall, the article emphasizes the importance of developing new CE-based analytical methods to ensure the high quality, safety, and efficacy of therapeutic peptides in clinical practice.

Progress in on-line, at-line, and in-line coupling of sample treatment with capillary and microchip electrophoresis over the past 10 years: A review

The review presents an evaluation of the development of on-line, at-line and in-line sample treatment coupled with capillary and microchip electrophoresis over the last 10 years. In the first part, it describes different types of flow-gating interfaces (FGI) such as cross-FGI, coaxial-FGI, sheet-flow-FGI, and air-assisted-FGI and their fabrication using molding into polydimethylsiloxane and commercially available fittings. The second part deals with the coupling of capillary and microchip electrophoresis with microdialysis, solid-phase, liquid-phase, and membrane based extraction techniques. It mainly focuses on modern techniques such as extraction across supported liquid membrane, electroextraction, single drop microextraction, head space microextraction, and microdialysis with high spatial and temporal resolution. Finally, the design of sequential electrophoretic analysers and fabrication of SPE microcartridges with monolithic and molecularly imprinted polymeric sorbents are discussed. Applications include the monitoring of metabolites, neurotransmitters, peptides and proteins in body fluids and tissues to study processes in living organisms, as well as the monitoring of nutrients, minerals and waste compounds in food, natural and wastewater.

In-line sample concentration in capillary electrophoresis by cyclodextrin to admicelle microextraction

Cyclodextrins (CDs) as a pseudophase in pseudophase-to-pseudophase microextraction (P2ME) in capillary zone electrophoresis (CZE) are proposed. In this P2ME mode called CD to admicelle ME, a long plug of dilute analyte solution prepared in cetyltrimethylammonium bromide (CTAB) at the critical micellar concentration was injected into the capillary. This formed CTAB admicelles at the interface between the solution and the negatively charged capillary surface, where the analytes were trapped. The injection of CD solution released the admicelles and the analytes from the capillary surface due to the formation of stable CD/CTAB inclusion complexes. The analytes are concentrated at the CD front during injection and voltage separation. Various neutral CDs were found to be effective for CD to admicelle ME. To implement this in-line sample concentration technique in CZE, CD concentration, sample injection time, and sample:CD solution injection ratio were optimized. The optimized conditions for five model anionic analytes, namely, 4-bromophenol, sulindac, sulfamethizole, 4-vinylbenzoic acid, and succinylsulfathiazole, were 20 mM α-CD in 20 mM sodium tetraborate (pH 9.2) solution, sample injection time of 370 s, and CD:sample injection ratio of 1:2. The sensitivity enhancement factors (SEFs) were between 112 and 168. The SEFs of sulindac and sulfamethizole in particular were similar to previously published off-line microextraction techniques, which are typically time-consuming. The calculated values of LOQ, intra-/inter-day (n = 6/n = 10, 3 days) repeatability, and linearity (R2) of CD to admicelle ME were 0.0125-0.05 µg/mL, 1.5-4.6%, 1.8-4.8%, and ≥0.999, respectively. Finally, the potential of CD to admicelle ME to the analysis of artificial urine samples was demonstrated.

Pseudophase microextraction for in-line sample concentration in micellar electrokinetic chromatography

Pseudophase microextraction (PPME) as a simple in-line sample concentration technique in micellar electrokinetic chromatography (MEKC) is presented. In contrast to popular electric-field driven stacking techniques in MEKC such as sweeping, PPME is pressure-driven. The technique afforded up to 403-2968x improvements in peak heights for fenoprop, dichlorprop, 1- and 2-naphthol compared to typical injection. Under the same MEKC conditions, the improvements in PPME were up to 23-59x better compared to sweeping. Briefly in PPME, the entire capillary was loaded (up to 20 capillary volumes) with the sample prepared in a dilute solution of cetyltrimethylammonium bromide ([CTAB] > critical surface aggregation concentration). The CTAB formed aggregates at the inner capillary walls and these aggregates acted as a stationary chromatographic pseudophase. After clean-up via flushing the capillary with purified water, the MEKC background solution (BGS) with sodium dodecyl sulfate was then introduced by pressure from the outlet end to elute the retained analytes. The analytes concentrate at front of the BGS and the front was moved to the inlet end of the capillary prior to MEKC. Optimization strategies and current limitations in PPME-MEKC are described. The linear ranges using a 4 capillary volume sample load obtained for fenoprop, dichlorprop, 1- and 2-naphthol were between 1 and 160 ng/mL (r²s ≥ 0.996), LOQs = 1-2.5 ng/mL and repeatability %RSDs (n = 6) were ≤5% (intra-day) and ≤7% (inter-day) (using low analyte concentrations 1-5x LOQ). PPME-MEKC with simple dilution of fortified real samples (no off-line sample concentration) was also able to detect low levels of dichlorprop (10 ng/mL, limit set in Australia) and 1- and 2-naphthol (7.5-15 ng/mL) in a drinking water and natural water sample, respectively (% recovery = 84-108%). The concept of PPME may find use in other modes of capillary electrophoresis and other nano-microscale separations.

Improved Sensitivity to Determine Antibiotic Residues in Chicken Meat by In-Line Solid-Phase Extraction Coupled to Capillary Electrophoresis-Tandem Mass Spectrometry

Traditionally, capillary electrophoresis (CE) has been ruled out of many food safety applications, despite its inherent advantages, because its concentration sensitivity has been not high enough, mainly in relation to the monitoring of contaminants and residues, such as pesticides, veterinary medicines, environmental contaminants, toxins, etc. For this reason, researchers have proposed several strategies to overcome this limitation. So far, approaches based on chromatographic principles have been the most successful solutions. These approaches, known as in-line solid phase extraction, consist of the introduction of a small amount of stationary phase in the inlet section of the electrophoretic capillary (analyte concentrator, AC) to retain the analytes before separation takes place. In this chapter, this strategy is applied to CE coupled to tandem mass spectrometry (MS/MS) for the multiresidue detection of quinolone antibiotic residues in chicken meat. A previous sample treatment based on pressurized liquid extraction to obtain an optimum performance is also described.

Porous layer open-tubular column with styrene and itaconic acid-copolymerized polymer as stationary phase for capillary electrochromatography-mass spectrometry

Enhancing the specific surface area of stationary phase is important in chromatographic science, especially in open-tubular column in which the coating only exists on the inner surface. In this work, a porous layer open-tubular (PLOT) column with stationary phase of styrene and itaconic acid-copolymerized polymer was developed. Thermal-initiated polymerization method with strategies like controlling the ratio of reaction reagents to solvents and reaction time, confinement by the narrow inner diameter of capillary were used for preparing the stationary phase with uniform structure and relatively thick layer. Due to the high separation efficiency and capacity, the PLOT column was used for capillary electrochromatography (CEC) separation of multiple groups of analytes like alkylbenzenes, phenyl amines, phenols, vanillins, and sulfonamides with theoretical plates (N) up to 1,54,845 N/m. In addition, due to high permeability of the CEC column and large electroosmotic flow mobility generated by abundant carboxyl groups in the coating material, the PLOT-CEC column was successfully coupled with mass spectrometry (MS) through a sheath flow interface. The developed PLOT-CEC-MS method was used for the analysis of antiseptics like parabens and herbicides like pyridines.

Synthesis, characterization and advanced sustainable applications of titanium dioxide nanoparticles: A review

Nanotechnology is capturing great interest worldwide due to their stirring applications in various fields. Among nanoparticles (NPs), titanium dioxide (TiO₂) NPs have been widely used in daily life and can be synthesized through various physical, chemical, and green methods. Green synthesis is a non-toxic, cost-effective, and eco-friendly route for the synthesis of NPs. Plenty of work has been reported on the green, chemical, physical and biological synthesis of TiO₂ NPs and these NPs can be characterized through high tech. instruments. In the present review, dense data have been presented on the comparative synthesis of TiO₂ NPs with different characteristics and their wide range of applications. Among the TiO₂ NPs synthesis techniques, the green methods have been proven to be efficient than chemical synthesis methods because of the less use of precursors, time-effectiveness, and energy-efficiency during the green synthesis procedures. Moreover, this review describes the types of plants (shrubs, herbs and trees), microorganisms (bacteria, fungi and algae), biological derivatives (proteins, peptides, and starches) employed for the synthesis of TiO₂ NPs. The TiO₂ NPs can be effectively used for the treatment of polluted water and positively affected the plant physiology especially under abiotic stresses but the response varied with types, size, shapes, doses, duration of exposure, metal species along with other factors. This review also highlights the regulating features and future standpoints for the measurable enrichment in TiO₂ NPs product and perspectives of TiO₂ NPs reliable application.

Pretreatment and determination methods for benzimidazoles: An update since 2005

Benzimidazoles, commonly used as pesticides and veterinary drugs, have posed a threat to human health and the environment due to unreasonable use and lack of valid regulation. Therefore, an up-to-date and comprehensive summary of the pretreatment and analytical approaches in different substrates is urgently needed. The present review consequently updates and covers various newly developed pretreatment methods (e.g., cationic micellar precipitation, magnetic-solid phase extraction, hollow fiber liquid phase microextraction, disperse liquid-liquid microextraction-solidified floating organic drop, stir cake sorptive extraction, solid phase microextraction method, QuEChERS, and molecular imprinted polymer-based methods) since 2005. The review also elaborates and discusses different determination methods (e.g., newly developed HPLC and related methods, improved spectrofluorimetry methods, capillary electrophoresis, and the electrochemical sensor). Furthermore, some critical points and prospects are highlighted, to describe the trends in this area.

Online sample preconcentration techniques in nonaqueous capillary and microchip electrophoresis

One of the major drawbacks of electrophoresis in both capillary and microchip is the unsatisfactory sensitivity. Online sample preconcentration techniques can be regarded as the most common and powerful approaches commonly applied to enhance overall detection sensitivity. While the advances of various online preconcentration strategies in capillary and microchip employing aqueous background electrolytes are well-reviewed, there has been limited discussion of the feasible preconcentration techniques specifically developed for capillary and microchip using nonaqueous background electrolytes. This review provides the first consolidated overview of various online preconcentration techniques in nonaqueous capillary and microchip electrophoresis, covering the period of the last two decades. It covers developments in the field of sample stacking, isotachophoresis, and micellar-based stacking. Attention is also given to multi-stacking strategies that have been used for nonaqueous electrophoresis.

Quantification of the bisphosphonate alendronate using capillary electrophoresis mass spectrometry with dynamic pH barrage junction focusing

A quantitative method was developed for the direct identity confirmation and quantification of alendronate using CE-MS combined with a pH-assisted focusing technique, dynamic pH barrage junction focusing. A pH-induced variation in electrophoretic mobility led to online focusing of alendronate at the sample/pH barrage boundary, significantly improving the detection sensitivity. In addition, the use of a flow-through microvial CE electrospray interface and the multiple reaction monitoring mode of MS further improved the specificity and quantification capability of this technology. This quantitative method presented a wide linear dynamic range over 8-2000 ng/mL and an LOD of 2 ng/mL. A 460-fold improvement in sensitivity was obtained when pH barrage junction focusing was applied during the CE process, in comparison to when normal CE was conducted without online sample stacking. The superior detection sensitivity over previously reported methods enables direct analysis of bisphosphonate compounds, eliminating tedious pre-column sample enrichment and derivatization. Validation of alendronate content in a commercial drug tablet further proved the reliability and power of this method. This simple method with no sample derivatization, superior sensitivity, and short run time (<8 min) is a promising alternative for accurate quantification of alendronate and other types of bisphosphonate compounds in both drug formulations and plasma samples.

Determination of amino acids by capillary and microchip electrophoresis with contactless conductivity detection - Theory, instrumentation and applications

This review article summarises aspects of the determination of amino acids using capillary and chip electrophoresis in combination with contactless conductivity detection from their historical beginnings to the present time. Discussion is included of the theory of conductivity detection in electromigration techniques, the design of contactless conductivity cells for detection in capillaries and on microchips, including the use of computer programs for simulation of the conductivity response and the process of the electrophoretic separation of amino acids. Emphasis is placed on optimisation of the background electrolyte composition, chiral separation, multidimensional separation, stacking techniques and the use of multidetection systems. There is also a description of clinical applications, the determination of amino acids in foodstuffs, waters, soils and composts with emphasis on modern techniques of sample treatment, such as microdialysis, liquid membrane extraction and many other techniques.

Electrokinetic stacking of particle zones in confined channels enabling their UV absorbance detection on microchips

In this article, we report a simple approach to stacking micro- and nanoparticle zones by electrokinetically migrating them through moderately confined channels of uniform cross-section. Experiments show the reported pre-concentration process to initiate at the tail end of the zone following its electrokinetic injection, with the stacked region migrating faster than the rest of the sample band. This effect causes the particles traveling in front to merge into the stacked region making it grow both in size and concentration. Because the stacked zone also gradually loses particles from its trailing edge, it eventually disintegrates upon running out of particles at its front end. Nevertheless, enhancements in peak height by over 100-fold were recorded using the reported approach for polystyrene beads with diameters comparable to the channel depth. This enhancement however, exhibited a temporal variation as the particle band migrated through the analysis column reaching a maximum value that depended on the particle diameter, particle concentration, channel depth, electric field strength, electroosmotic mobility, etc. Interestingly, the peak area recorded by the detector remained relatively constant during this particle migration period allowing reliable sample quantitation. Moreover, upon incubating antibody-coated particles against an antigen sample, the peak area for the particle zone was seen to scale linearly with the antigen concentration establishing the utility of the reported focusing phenomenon for chemical/biochemical analysis. The noted stacking technique was further applied to enabling UV absorbance detection of particle zones on microchips which then allowed us to determine the colloidal content in actual natural water samples. .

Dynamic pH barrage junction focusing of amino acids, peptides, and digested monoclonal antibodies in capillary electrophoresis-mass spectrometry

Dynamic pH barrage junction focusing in CE enables effective signal enhancement, quantitative capture efficiencies, and straightforward optimization. The method is a technical variant of dynamic pH junction focusing. CE separation with dynamic pH barrage junction focusing is compatible with both optical and mass spectrometric detection. We developed a CE-MS/MS method using hydrophilic polyethyleneimine-coated capillaries and validated it for the qualitative analysis of amino acids, peptides, and tryptic peptides of digested monoclonal antibodies. The S/N of extracted ion electropherograms of zwitterionic analytes were enhanced by approximately two orders of magnitude with a tradeoff of a shortened separation window. Online focusing improved the MS signal intensity of a diluted antibody digest, enabling more precursor ions to be analyzed with subsequent tandem mass spectrometric identification. It also broadened the concentration range of protein digest samples for which adequate sequence coverage data can be obtained. With only 0.9 ng of digested infliximab sample loaded into the capillary, 76% and 100% sequence coverage was realized for antibody heavy and light chains, respectively, after online focusing. Full coverage was achieved with 9 ng of injected digest.

Electrode-Free Concentration and Recovery of DNA at Physiologically Relevant Ionic Concentrations

Advances in microanalytical and microfluidic technologies have enabled rapid and amplification-free detection of DNA with a high signal-to-noise ratio. The low sample volume, however, poses a limit in the DNA detection sensitivity, which can be challenging for analyzing rare DNA in physiological samples. One way to improve the sensitivity is to concentrate the DNA in the sample prior to the analysis. The most common DNA concentration techniques are based on electrokinetics, which require an external electric field and generally become ineffective in high ionic concentration conditions. In this work, we present a facile method termed high-salt molecular rheotaxis (HiSMRT) to concentrate and recover DNA from samples with physiologically relevant ionic concentrations without any external electric field. HiSMRT requires only pressure-driven flow and ion concentration gradient to induce a stable local electric field and achieve DNA concentration, making it impervious to high ionic concentrations. We demonstrate that HiSMRT performs robustly at ionic concentrations equivalent to 2%-20% of the ionic concentration in blood serum. HiSMRT can concentrate DNA by up to 960-fold and recover an average of 96.4% of the DNA fragments from 2.0 to 23 kbp uniformly. The concentration process using HiSMRT takes as little as 7.5 min. Moreover, we show that this technique can be easily integrated to perform DNA concentration, size separation, and single-molecule detection all in one platform. We anticipate that this technique will be applicable to a wide range of biological samples and will help to improve the sensitivity of nucleic acid detection for low-abundance DNA biomarkers.

Enrichment-free analysis of anionic micropollutants in the sub-ppb range in drinking water by capillary electrophoresis-high resolution mass spectrometry

Reversed-phase liquid chromatography (RPLC) used for water analysis is not ideal for the analysis of highly polar and ionic contaminants because of low retention. Capillary electrophoresis (CE), on the other hand, is perfectly suited for the separation of ionic compounds but rarely applied in environmental analysis due to the weak concentration sensitivity when coupled to mass spectrometry (MS). However, novel interface designs and MS technology strongly improve the sensitivity. Here, a method is presented enabling the screening of anionic micropollutants in drinking water without sample pretreatment by coupling of CE to an Orbitrap mass spectrometer by a nanoflow sheath liquid interface. Targeted analysis of halogenated acetic acids, trifluoromethanesulfonic acid, and perfluorooctanoic and perfluorooctanesulfonic acid was conducted in drinking water samples which were chlorinated for disinfection. A bare fused silica capillary with an optimized background electrolyte (BGE) for separation consisting of 10% acetic acid with 10% isopropanol with large volume sample injection and optimized interface parameters offer limits of quantification in the range of < 0.1 to 0.5 μg/L with good linearity (R² > 0.993) and repeatability (14% standard deviation in area). Concentrations of the target analytes ranged from 0.1 to 6.2 μg/L in the water samples. Masses corresponding to halogenated methanesulfonic acids have been found as suspects and were subsequently verified by standards. Mono-, dichloro-, and bromochloro methanesulfonic acid were quantified in a range of 0.2 to 3.6 μg/L. Furthermore, five sulfonic acids, four organosulfates, and the artificial sweeteners acesulfame and cyclamate as well as inorganics such as halides, halogenates, phosphate, and sulfate could be determined as suspects among more than 300 features in a non-targeted screening. Overall, this approach demonstrates the great potential of CE-nanoESI-MS for the screening of ionic contaminants in environmental samples, complementary to chromatographic approaches.

Advances in the Analysis of Veterinary Drug Residues in Food Matrices by Capillary Electrophoresis Techniques

In the last years, the European Commission has adopted restrictive directives on food quality and safety in order to protect animal and human health. Veterinary drugs represent an important risk and the need to have sensitive and fast analytical techniques to detect and quantify them has become mandatory. Over the years, the availability of different modes, interfaces, and formats has improved the versatility, sensitivity, and speed of capillary electrophoresis (CE) techniques. Thus, CE represents a powerful tool for the analysis of a large variety of food matrices and food-related molecules with important applications in food quality and safety. This review focuses the attention of CE applications over the last decade on the detection of different classes of drugs (used as additives in animal food or present as contaminants in food products) with a potential risk for animal and human health. In addition, considering that the different sample preparation procedures have strongly contributed to CE sensitivity and versatility, the most advanced sample pre-concentration techniques are discussed here.

Highly Sensitive SDS Capillary Gel Electrophoresis with Sample Stacking Requiring Only Nanograms of Adeno-Associated Virus Capsid Proteins

Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) has been the method of choice in the past decades for size-based protein analysis. However, in general it requires the protein concentration in mg/mL level and thus is not practical for trace level protein analysis, not to mention the lengthy labor-intensive procedures. The SDS capillary gel electrophoresis (SDS CGE) method reported herein requires only nanogram-sized proteins loaded onto the autosampler. A sample stacking technique (e.g., head-column field-amplified sample stacking (HC FASS)) was employed, providing three orders of magnitude sensitivity enhancement compared to conventional SDS CGE. This method has been used routinely in purity analysis and characterization of adeno-associated virus (AAV) intermediates and finished gene therapeutics of AAV vectors. The sensitivity achieved is comparable to the currently most sensitive size-based protein assay silver-stained SDS PAGE. The highly sensitive sample stacking SDS CGE can be used for other types of proteins as well.

A critical retrospective and prospective review of designs and materials in in-line solid-phase extraction capillary electrophoresis

Several strategies have been developed to decrease the concentration limits of detection (LODs) in capillary electrophoresis (CE). Nowadays, chromatographic-based preconcentration using a microcartridge integrated in the separation capillary for in-line solid-phase extraction capillary electrophoresis (SPE-CE) is one of the best alternatives for high throughput and reproducible sample clean-up and analyte preconcentration. This review covers different designs (geometrical configurations, with frits or fritless, capillary types, compatibility with commercial instrumentation, etc.) and materials (sorbents, supports, affinity ligands, etc.) applied for almost 30 years to prepare in-line SPE-CE microcartridges (i.e. analyte concentrators), with emphasis on the conventional unidirectional configuration in capillary format. Advantages, disadvantages and future perspectives are analyzed in detail to provide the reader a wide overview about the great potential of this technique to enhance sensitivity and address trace analysis.

Capillary electrophoresis of small ions and molecules in less conventional human body fluid samples: A review

In recent years, advances in sensitive analytical techniques have encouraged the analysis of various compounds in biological fluids. While blood serum, blood plasma and urine still remain the golden standards in clinical, toxicological and forensic science, analyses of other body fluids, such as breast milk, exhaled breath condensate, sweat, saliva, amniotic fluid, cerebrospinal fluid, or capillary blood in form of dried blood spots are becoming more popular. This review article focuses on capillary electrophoresis and microchip electrophoresis of small ions and molecules (e.g. inorganic cations/anions, basic/acidic drugs, small acids/bases, amino acids, peptides and other low molecular weight analytes) in various less conventional human body fluids and hopes to stimulate further interest in the field.

Advances in capillary electrophoresis for the life sciences

Capillary electrophoresis (CE) played an important role in developments in the life sciences. The technique is nowadays used for the analysis of both large and small molecules in applications where it performs better than or is complementary to liquid chromatographic techniques. In this review, principles of different electromigration techniques, especially capillary isoelectric focusing (CIEF), capillary gel (CGE) and capillary zone electrophoresis (CZE), are described and recent developments in instrumentation, with an emphasis on mass spectrometry (MS) coupling and microchip CE, are discussed. The role of CE in the life sciences is shown with applications in which it had a high impact over the past few decades. In this context, current practice for the characterization of biopharmaceuticals (therapeutic proteins) is shown with CIEF, CGE and CZE using different detection techniques, including MS. Subsequently, the application of CGE and CZE, in combination with laser induced fluorescence detection and CZE-MS are demonstrated for the analysis of protein-released glycans in the characterization of biopharmaceuticals and glycan biomarker discovery in biological samples. Special attention is paid to developments in capillary coatings and derivatization strategies for glycans. Finally, routine CE analysis in clinical chemistry and latest developments in metabolomics approaches for the profiling of small molecules in biological samples are discussed. The large number of CE applications published for these topics in recent years clearly demonstrates the established role of CE in life sciences.

On-line sample preconcentration by polarity switching in floating electrode-integrated microchannel

In this study, we found that the polarity switching was effective to enrich and separate fluorescent analytes which have weakly-dissociated groups in a floating platinum electrode (width, 50 µm; thickness, 2.5 µm)-integrated straight-channel in microchip electrophoresis (MCE). In the straight channel filled with an Alexa Flour 488 (AF488) solution, a sharp peak was observed after the polarity inversion with a 530-fold enhancement of the sensitivity relative to the conventional MCE analysis. By using a fluorescent pH indicator, we verified that a sharp high-pH zone was generated nearby the floating electrode and moved toward the anode with maintaining the high pH, which induced the sample enrichment like a dynamic pH junction mechanism. In the floating electrode-embedded channel, the mixture of AF488-labeled proteins was also well concentrated and separated within 100 s.

A separation voltage polarity switching method for higher sample loading capacity and better separation resolution in transient capillary isotachophoresis separation

A separation voltage polarity switching transient capillary isotachophoresis (PS-tCITP) was developed to overcome a major sample loading volume limitation in transient capillary isotachophoresis (tCITP). The fundamental idea of PS-tCITP is to let sample ions move back and forth in a separation capillary during their initial isotachophoresis focusing stage by switching the polarity of the separation voltage, in order to both increase the sample loading volume and improve the separation efficiency as compared to the conventional tCITP method. The experimental evaluation of the novel PS-tCITP method by using two peptide standards at 2 μM concentration showed that the maximum sample loading volume could be increased from 45% of the total separation capillary volume in tCITP to 70% in PS-tCITP, which resulted in a more than 1.5 fold increase in the peptide peak intensity at a given length/volume of the separation capillary. Due to the consecutive focusing of sample volume from each polarity switching of the separation voltage, the separation time window at a given sample loading volume was also increased significantly in PS-tCITP as compared to tCITP. Experiment comparison between tCITP and PS-tCITP at 45% sample loading volume using the same setup showed that the migration time difference between the two peptide peaks increased from 0.3 min in tCITP to 0.363 min in PS-tCITP with similar peak widths and heights, resulting in roughly a 21% improvement in separation resolution. The performance advantages of PS-tCITP separation over tCITP separation were further verified by using a mixture of six peptide standards.

On-Line Immunoaffinity Solid-Phase Extraction Capillary Electrophoresis-Mass Spectrometry for the Analysis of Serum Transthyretin

The analysis of low abundant proteins in biological fluids by capillary electrophoresis (CE) is particularly problematic due to the typically poor concentration limits of detection of microscale separation techniques. Another important issue is sample matrix complexity that requires an appropriate cleanup. Here, we describe an on-line immunoaffinity solid-phase extraction capillary electrophoresis-mass spectrometry (IA-SPE-CE-MS) method for the immunoextraction, preconcentration, separation, detection, and characterization of serum transthyretin (TTR). TTR is a protein biomarker related to diverse types of amyloidosis, such as familial amyloidotic polyneuropathy type I (FAP-I), which is the most common hereditary systemic amyloidosis.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2016-2018)

One of the most cited limitations of capillary and microchip electrophoresis is the poor sensitivity. This review continues to update this series of biannual reviews, first published in Electrophoresis in 2007, on developments in the field of online/in-line concentration methods in capillaries and microchips, covering the period July 2016-June 2018. It includes developments in the field of stacking, covering all methods from field-amplified sample stacking and large-volume sample stacking, through to isotachophoresis, dynamic pH junction, and sweeping. Attention is also given to online or in-line extraction methods that have been used for electrophoresis.

Recent progress of sample stacking in capillary electrophoresis (2016-2018)

Electrophoretic sample stacking comprises a group of capillary electrophoretic techniques where trace analytes from the sample are concentrated into a short zone (stack). This paper is a continuation of our previous reviews on the topic and brings a survey of more than 120 papers published approximately since the second quarter of 2016 till the first quarter of 2018. It is organized according to the particular stacking principles and includes chapters on concentration adjustment (Kohlrausch) stacking, on stacking techniques based on pH changes, on stacking in electrokinetic chromatography and on other stacking techniques. Where available, explicit information is given about the procedure, electrolyte(s) used, detector employed and sensitivity reached. Not reviewed are papers on transient isotachophoresis which are covered by another review in this issue.

A decade of microchip electrophoresis for clinical diagnostics - A review of 2008-2017

A core element in clinical diagnostics is the data interpretation obtained through the analysis of patient samples. To obtain relevant and reliable information, a methodological approach of sample preparation, separation, and detection is required. Traditionally, these steps are performed independently and stepwise. Microchip capillary electrophoresis (MCE) can provide rapid and high-resolution separation with the capability to integrate a streamlined and complete diagnostic workflow suitable for the point-of-care setting. Whilst standard clinical diagnostics methods normally require hours to days to retrieve specific patient data, MCE can reduce the time to minutes, hastening the delivery of treatment options for the patients. This review covers the advances in MCE for disease detection from 2008 to 2017. Miniaturised diagnostic approaches that required an electrophoretic separation step prior to the detection of the biological samples are reviewed. In the two main sections, the discussion is focused on the technical set-up used to suit MCE for disease detection and on the strategies that have been applied to study various diseases. Throughout these discussions MCE is compared to other techniques to create context of the potential and challenges of MCE. A comprehensive table categorised based on the studied disease using MCE is provided. We also comment on future challenges that remain to be addressed.