On-line preconcentration methods for capillary electrophoresis

Photothermal Backscatter Interferometry for Enhanced Detection in Capillary Electrophoresis

Refractive index (RI) detection using backscatter interferometry (BSI) enables universal detection in capillary electrophoresis (CE). BSI detection is a versatile on-capillary approach that is easily integrated with capillary or microfluidic channels, straightforward to miniaturize, and inexpensive. The focused BSI light source can also double as the excitation source for fluorescence, enabling simultaneous universal (BSI) and specific (fluorescence) signals from the same detection volume. To improve BSI detection and expand orthogonal content, we integrate photothermal absorption with BSI detection. Nonradiative relaxation of an excited analyte releases heat into the surroundings, which modifies both the local RI and conductivity (viscosity) of the analyte zone. We recently showed that the BSI signal is sensitive to both RI and conductivity, which makes photothermal absorption a promising route to signal enhancement. Here, we use coaxially delivered BSI and photothermal absorption beams to characterize BSI, photothermal BSI, and fluorescence detection using the separation of test samples. We show that photothermal absorption leads to 3 orders of magnitude improvement in BSI detection limits at the powers studied and provides new opportunities for studying binding interactions with CE.

Transient isotachophoresis-Capillary zone electrophoresis-Mass spectrometry method with off-line microscale solid phase extraction pretreatment for quantitation of intact low molecular mass proteins in various biological fluids

Quantification of proteins is still predominantly done by the traditional bottom-up approach. Targeting of intact proteins in complex biological matrices is connected with multiple challenges during the sample pretreatment, separation, and detection step of the analytical workflow. In this work, we focused on the development of an on-line hyphenated capillary zone electrophoresis-mass spectrometry method employing off-line microscale solid-phase extraction based on hydrophilic lipophilic balance (HLB) sorbent as a sample pretreatment step for the analysis of low molecular mass intact proteins (<20 kDa) spiked in various biological fluids (human serum, plasma, urine, and saliva). A detailed optimization process involved the selection of a suitable capillary surface, background electrolyte (BGE), and comparison of two in-capillary preconcentration methods, namely transient isotachophoresis (tITP) and dynamic pH junction (DPJ), to enhance the sensitivity of the method. Optimum separation of the analytes was achieved using uncoated bare fused silica capillary employing 500 mM formic acid (pH 1.96) + 5 % (v/v) acetonitrile as BGE. tITP was utilized as an optimum preconcentration technique, achieving a 19- to 127-fold increase in the signal intensity when using 200 mM ammonium formate (adjusted to pH 4.00) as the leading electrolyte and BGE as the terminating electrolyte. Off-line microscale solid-phase extraction with various eluate treatment procedures was evaluated to ensure the compatibility of the sample pretreatment method with the selected in-capillary preconcentration, separation, and detection process. Achieved extraction recoveries of spiked proteins were in the range of 76-100 % for urine, 12-54 % for serum, 21-106 % for plasma, and 25-98 % for saliva when the eluate was evaporated and reconstituted in the solution of the leading electrolyte to achieve the tITP process. The optimum method was validated across different biological matrices, offering good linearity, accuracy, and precision, and making it suitable for proteomic studies (e.g., therapeutic drug monitoring, biomarker research) in different biological samples.

A review on hyphenation of droplet microfluidics to separation techniques: From instrumental conception to analytical applications for limited sample volumes

In this study, we review various strategies to couple sample processing in microfluidic droplets with different separation techniques, including liquid chromatography, mass spectrometry, and capillary electrophoresis. Separation techniques interfaced with droplet microfluidics represent an emerging trend in analytical chemistry, in which micro to femtoliter droplets serve as microreactors, a bridge between analytical modules, as well as carriers of target analytes between sample treatment and separation/detection steps. This allows to overcome the hurdles encountered in separation science, notably the low degree of module integration, working volume incompatibility, and cross contamination between different operational stages. For this droplet-separation interfacing purpose, this review covers different instrumental designs from all works on this topic up to May 2023, together with our viewpoints on respective advantages and considerations. Demonstration and performance of droplet-interfaced separation strategies for limited sample volumes are also discussed.

Biological mass spectrometry enables spatiotemporal 'omics: From tissues to cells to organelles

Biological processes unfold across broad spatial and temporal dimensions, and measurement of the underlying molecular world is essential to their understanding. Interdisciplinary efforts advanced mass spectrometry (MS) into a tour de force for assessing virtually all levels of the molecular architecture, some in exquisite detection sensitivity and scalability in space-time. In this review, we offer vignettes of milestones in technology innovations that ushered sample collection and processing, chemical separation, ionization, and 'omics analyses to progressively finer resolutions in the realms of tissue biopsies and limited cell populations, single cells, and subcellular organelles. Also highlighted are methodologies that empowered the acquisition and analysis of multidimensional MS data sets to reveal proteomes, peptidomes, and metabolomes in ever-deepening coverage in these limited and dynamic specimens. In pursuit of richer knowledge of biological processes, we discuss efforts pioneering the integration of orthogonal approaches from molecular and functional studies, both within and beyond MS. With established and emerging community-wide efforts ensuring scientific rigor and reproducibility, spatiotemporal MS emerged as an exciting and powerful resource to study biological systems in space-time.

Enhancing Extracellular Vesicle Analysis by Integration of Large-Volume Sample Stacking in Capillary Electrophoresis with Asymmetrical Flow Field-Flow Fractionation

Extracellular vesicles (EVs) play important roles in cell-cell communication and pathological development. Cargo profiling for the EVs present in clinical specimens can provide valuable insights into their functions and help discover effective EV-based markers for diagnostic and therapeutic purposes. However, the highly abundant and complex matrix components pose significant challenges for specific identification of low-abundance EV cargos. Herein, we combine asymmetrical flow field-flow fractionation (AF4) with large-volume sample stacking and capillary electrophoresis (LVSS/CE), to attain EVs with high purity for downstream protein profiling. This hyphenated system first separates the EVs from the contamination of smaller serum proteins by AF4, and second resolves the EVs from the coeluted, nonvesicular matrix components by CE following LVSS. The optimal LVSS condition permits the injection of 10-fold more EVs into CE compared to the nonstacking condition without compromising separation resolution. Collection and downstream analysis of the highly pure EVs after CE separation were demonstrated in the present work. The high EV purity yields a much-improved labeling efficiency when detected by fluorescent antibodies compared to those collected from the one-dimension separation of AF4, and permits the identification of more EV-specific cargos by LC-MS/MS compared to those isolated by ultracentrifugation (UC), the exoEasy Maxi Kit, and AF4. Our results strongly support that AF4-LVSS/CE can improve EV isolation and cargo analysis, opening up new opportunities for understanding EV functions and their applications in the biomedical fields.

Strategies for capillary electrophoresis: Method development and validation for pharmaceutical and biological applications-Updated and completely revised edition

This review is in support of the development of selective, precise, fast, and validated capillary electrophoresis (CE) methods. It follows up a similar article from 1998, Wätzig H, Degenhardt M, Kunkel A. "Strategies for capillary electrophoresis: method development and validation for pharmaceutical and biological applications," pointing out which fundamentals are still valid and at the same time showing the enormous achievements in the last 25 years. The structures of both reviews are widely similar, in order to facilitate their simultaneous use. Focusing on pharmaceutical and biological applications, the successful use of CE is now demonstrated by more than 600 carefully selected references. Many of those are recent reviews; therefore, a significant overview about the field is provided. There are extra sections about sample pretreatment related to CE and microchip CE, and a completely revised section about method development for protein analytes and biomolecules in general. The general strategies for method development are summed up with regard to selectivity, efficiency, precision, analysis time, limit of detection, sample pretreatment requirements, and validation.

Photoactive hydrogels for pre-concentration, labelling, and controlled release of proteins

We report a novel hydrogel for pre-concentration, fluorescent labelling, and light-triggered release of proteins for detection of low abundance biomarkers. The hydrogel was a co-polymer of acrylamide/bisacrylamide and methacrylamide attached to fluorescein isothiocyanate via a light cleavable bond and a poly(ethylene glycol) spacer arm of molecular weight of 3400 g mol-1. Unlike previous work, proteins were captured by an irreversible chemical reaction rather than by non-covalent affinity binding or physical entrapment. Because the protein-reactive group was attached to fluorescein, which in turn was coupled to the hydrogel by a photocleavable bond, on release the protein was labelled with fluorescein. Our hydrogel offered a pre-concentration factor of up to 236 for a model protein, streptavidin. Each protein molecule was labelled with 85 fluorescein molecules, and 50% of the proteins in the hydrogel were released after UV exposure for ∼100 s. The proteins released from the hydrogel were captured in biotinylated microtitre plates and detected by fluorescence, allowing measurement of at least 0.01 ppm (or ∼166 pM) of protein in sample solutions. The reported hydrogel is promising for detection of low abundance proteins while being less laborious than enzyme-linked immunosorbent assay and less affected by changes in environmental conditions than label-free biosensors.

Offline Coupling of Asymmetrical Flow Field-Flow Fractionation and Capillary Electrophoresis for Separation of Extracellular Vesicles

Extracellular vesicles (EVs) play important roles in cell-to-cell communications and carry high potential as markers targeted in disease diagnosis, prognosis, and therapeutic development. The main obstacles to EV study are their high heterogeneity; low amounts present in samples; and physical similarity to the abundant, interfering matrix components. Multiple rounds of separation and purification are often needed prior to EV characterization and function assessment. Herein, we report the offline coupling of asymmetrical flow field-flow fractionation (AF4) and capillary electrophoresis (CE) for EV analysis. While AF4 provides gentle and fast EV separation by size, CE resolves EVs from contaminants with similar sizes but different surface charges. Employing Western Blotting, ELISA, and SEM, we confirmed that intact EVs were eluted within a stable time window under the optimal AF4 and CE conditions. We also proved that EVs could be resolved from free proteins and high-density lipoproteins by AF4 and be further separated from the low-density lipoproteins co-eluted in AF4. The effectiveness of the coupled AF4-CE system in EV analysis was demonstrated by monitoring the changes in EV secretion from cells and by direct injection of human serum and detection of serum EVs. We believe that coupling AF4 and CE can provide rapid EV quantification in biological samples with much reduced matrix interference and be valuable for the study of total EVs and EV subpopulations produced by cells or present in clinical samples.

Single-Cell Mass Spectrometry of Metabolites and Proteins for Systems and Functional Biology

Molecular composition is intricately intertwined with cellular function, and elucidation of this relationship is essential for understanding life processes and developing next-generational therapeutics. Technological innovations in capillary electrophoresis (CE) and liquid chromatography (LC) mass spectrometry (MS) provide previously unavailable insights into cellular biochemistry by allowing for the unbiased detection and quantification of molecules with high specificity. This chapter presents our validated protocols integrating ultrasensitive MS with classical tools of cell, developmental, and neurobiology to assess the biological function of important biomolecules. We use CE and LC MS to measure hundreds of metabolites and thousands of proteins in single cells or limited populations of tissues in chordate embryos and mammalian neurons, revealing molecular heterogeneity between identified cells. By pairing microinjection and optical microscopy, we demonstrate cell lineage tracing and testing the roles the dysregulated molecules play in the formation and maintenance of cell heterogeneity and tissue specification in frog embryos (Xenopus laevis). Electrophysiology extends our workflows to characterizing neuronal activity in sections of mammalian brain tissues. The information obtained from these studies mutually strengthen chemistry and biology and highlight the importance of interdisciplinary research to advance basic knowledge and translational applications forward.

Electric Field-Induced Specific Preconcentration to Enhance DNA-Based Electrochemical Sensing of Hg2+ via the Synergy of Enrichment and Self-Cleaning

Efficient preconcentration is critical for sensitive and selective electrochemical detection of metal ions, but rapid specific enrichment with depressed absorption of interfering ions at the electrode is challenging. Here, we proposed an electric field-induced specific preconcentration to boost the analytical performance of DNA-based electrochemical sensors for Hg²⁺ detection. As for such preconcentration, a positive external electric field was first used to enrich Hg²⁺ at an electrode assembled with T-rich DNA, thus boosting T-Hg²⁺-T recognitions. The following applied inverse electric field strips the nonspecifically absorbed Hg²⁺ and other interfering ions, thus depressing matrix interferences via self-cleaning. Based on this principle, we designed a portable device to realize programmable control of electric fields; a T-Hg²⁺-T recognition-based electrochemical sensor was thus fabricated as a model platform to assess the feasibility of electric field-induced preconcentration. The experimental results revealed that such a strategy decreased the time of T-Hg²⁺-T-based recognition from 60 to 20 min and led to detection with better reproducibility by depressing the influence of free Hg²⁺ as well as interfering ions. This strategy offered Hg²⁺ detection limits of 0.01 pM─three-fold better than that without preconcentration─within 22 min. The proposed preconcentration strategy offers a new way to enhance the analytical performance of sensing at the solid-liquid interface.

Stereoselective enrichment and determination of citalopram enantiomers by cation-selective exhaustive injection and sweeping coupled with cyclodextrin modified electrokinetic chromatography

An highly sensitive, rapid and enantioselective method was developed for the enantioseparation and determination of citalopram enantiomers by cation selective exhaustive injection-sweeping-cyclodextrin modified electrokinetic chromatography (CSEI-sweeping-CDEKC). The optimized conditions were: 50 mM pH 3.0 phosphate solution with 25 mg·mL^-1 S-β-CD used as background buffer, 50 mbar 300 s hydrodynamical injection of 150 mM pH 3.0 NaH₂PO₄ buffer followed with 5 s water plug, 10 kV 600 s electrokinetic sample injection, -20 kV CDEKC run. Under the optimized conditions, the resolution was Rs=8.04, the enrichment factor as up to 2163 folds, the LOD values were: 3.6 ng·mL^-1 for R-citalopram, 4.1 ng·mL^-1 for S-citalopram, and 3 ng·mL^-1 for both enantiomers in plasma samples. This new method showed good precision, repeatability and stability, which had been successfully applied to the impurity inspection of escitalopram oxalate and the stereoselective pharmacokinetic study of citalopram enantiomers.

Technique development of high-throughput and high-sensitivity sample preparation and separation for proteomics

Sample preparation and separation methods determine the sensitivity and the quantification accuracy of the proteomics analysis. This article covers a comprehensive review of the recent technique development of high-throughput and high-sensitivity sample preparation and separation methods in proteomics research.

CE-MS for Proteomics and Intact Protein Analysis

This chapter aims to explore various parameters involved in achieving high-end capillary electrophoresis hyphenated to mass spectrometry (CE-MS) analysis of proteins, peptides, and their posttranslational modifications. The structure of the topics discussed in this book chapter is conveniently mapped on the scheme of the CE-MS system itself, starting from sample preconcentration and injection techniques and finishing with mass analyzer considerations. After going through the technical considerations, a variety of relevant applications for this analytical approach are presented, including posttranslational modifications analysis, clinical biomarker discovery, and its growing use in the biotechnological industry.

Immunoaffinity Capillary Electrophoresis in the Era of Proteoforms, Liquid Biopsy and Preventive Medicine: A Potential Impact in the Diagnosis and Monitoring of Disease Progression

Over the years, multiple biomarkers have been used to aid in disease screening, diagnosis, prognosis, and response to therapy. As of late, protein biomarkers are gaining strength in their role for early disease diagnosis and prognosis in part due to the advancements in identification and characterization of a distinct functional pool of proteins known as proteoforms. Proteoforms are defined as all of the different molecular forms of a protein derived from a single gene caused by genetic variations, alternative spliced RNA transcripts and post-translational modifications. Monitoring the structural changes of each proteoform of a particular protein is essential to elucidate the complex molecular mechanisms that guide the course of disease. Clinical proteomics therefore holds the potential to offer further insight into disease pathology, progression, and prevention. Nevertheless, more technologically advanced diagnostic methods are needed to improve the reliability and clinical applicability of proteomics in preventive medicine. In this manuscript, we review the use of immunoaffinity capillary electrophoresis (IACE) as an emerging powerful diagnostic tool to isolate, separate, detect and characterize proteoform biomarkers obtained from liquid biopsy. IACE is an affinity capture-separation technology capable of isolating, concentrating and analyzing a wide range of biomarkers present in biological fluids. Isolation and concentration of target analytes is accomplished through binding to one or more biorecognition affinity ligands immobilized to a solid support, while separation and analysis are achieved by high-resolution capillary electrophoresis (CE) coupled to one or more detectors. IACE has the potential to generate rapid results with significant accuracy, leading to reliability and reproducibility in diagnosing and monitoring disease. Additionally, IACE has the capability of monitoring the efficacy of therapeutic agents by quantifying companion and complementary protein biomarkers. With advancements in telemedicine and artificial intelligence, the implementation of proteoform biomarker detection and analysis may significantly improve our capacity to identify medical conditions early and intervene in ways that improve health outcomes for individuals and populations.

Microfluidic device for coupling isotachophoretic sample focusing with nanopore single-molecule sensing

Solid-state nanopores (NPs) are label-free single-molecule sensors, capable of performing highly sensitive assays from a small number of biomolecule translocation events. However, single-molecule sensing is challenging at extremely low analyte concentrations due to the limited flux of analytes to the sensing volume. This leads to a low event rate and increases the overall assay time. In this work, we present a method to enhance the event rate at low analyte concentrations by using isotachophoresis (ITP) to focus and deliver analytes to a nanopore sensor. Central to this method is a device capable of performing ITP focusing directly on a solid-state NP chip, while preventing the focusing electric field from damaging the nanopore membrane. We discuss considerations and trade-offs related to the design of the focusing channel, the ITP electrolyte system and electrical decoupling between the focusing and sensing modes. Finally, we demonstrate an integrated device wherein the concentration enhancement due to ITP focusing leads to an increase in event rate of >300-fold in the ITP-NP device as compared to the NP-only case.

Capillary electrophoresis-mass spectrometry for the direct analysis of glyphosate: method development and application to beer beverages and environmental studies

In this study, we developed and validated a CE-TOF-MS method for the quantification of glyphosate (N-(phosphonomethyl)glycine) and its major degradation product aminomethylphosphonic acid (AMPA) in different samples including beer, media from toxicological analysis with Daphnia magna, and sorption experiments. Using a background electrolyte (BGE) of very low pH, where glyphosate is still negatively charged but many matrix components become neutral or protonated, a very high separation selectivity was reached. The presence of inorganic salts in the sample was advantageous with regard to preconcentration via transient isotachophoresis. The advantages of our new method are the following: no derivatization is needed, high separation selectivity and thus matrix tolerance, speed of analysis, limits of detection suitable for many applications in food and environmental science, negligible disturbance by metal chelation. LODs for glyphosate were < 5 μg/L for both aqueous and beer samples, the linear range in aqueous samples was 5-3000 μg/L, for beer samples 10-3000 μg/L. For AMPA, LODs were 3.3 and 30.6 μg/L, and the linear range 10-3000 μg/L and 50-3000 μg/L, for aqueous and beer samples, respectively. Recoveries in beer samples for glyphosate were 94.3-110.7% and for AMPA 80.2-100.4%. We analyzed 12 German and 2 Danish beer samples. Quantification of glyphosate and AMPA was possible using isotopically labeled standards without enrichment, purification, or dilution, only degassing and filtration were required for sample preparation. Finally, we demonstrate the applicability of the method for other strong acids, relevant in food and environmental sciences such as N-acetyl glyphosate, N-acetyl AMPA (present in some glyphosate resistant crop), trifluoroacetic acid, 2-methyl-4-chlorophenoxyacetic acid, glufosinate and its degradation product 3-(methylphosphinico)propionic acid, oxamic acid, and others.

A Two-Dimensional Affinity Capture and Separation Mini-Platform for the Isolation, Enrichment, and Quantification of Biomarkers and Its Potential Use for Liquid Biopsy

Biomarker detection for disease diagnosis, prognosis, and therapeutic response is becoming increasingly reliable and accessible. Particularly, the identification of circulating cell-free chemical and biochemical substances, cellular and subcellular entities, and extracellular vesicles has demonstrated promising applications in understanding the physiologic and pathologic conditions of an individual. Traditionally, tissue biopsy has been the gold standard for the diagnosis of many diseases, especially cancer. More recently, liquid biopsy for biomarker detection has emerged as a non-invasive or minimally invasive and less costly method for diagnosis of both cancerous and non-cancerous diseases, while also offering information on the progression or improvement of disease. Unfortunately, the standardization of analytical methods to isolate and quantify circulating cells and extracellular vesicles, as well as their extracted biochemical constituents, is still cumbersome, time-consuming, and expensive. To address these limitations, we have developed a prototype of a portable, miniaturized instrument that uses immunoaffinity capillary electrophoresis (IACE) to isolate, concentrate, and analyze cell-free biomarkers and/or tissue or cell extracts present in biological fluids. Isolation and concentration of analytes is accomplished through binding to one or more biorecognition affinity ligands immobilized to a solid support, while separation and analysis are achieved by high-resolution capillary electrophoresis (CE) coupled to one or more detectors. When compared to other existing methods, the process of this affinity capture, enrichment, release, and separation of one or a panel of biomarkers can be carried out on-line with the advantages of being rapid, automated, and cost-effective. Additionally, it has the potential to demonstrate high analytical sensitivity, specificity, and selectivity. As the potential of liquid biopsy grows, so too does the demand for technical advances. In this review, we therefore discuss applications and limitations of liquid biopsy and hope to introduce the idea that our affinity capture-separation device could be used as a form of point-of-care (POC) diagnostic technology to isolate, concentrate, and analyze circulating cells, extracellular vesicles, and viruses.

Enhanced single-cell metabolomics by capillary electrophoresis electrospray ionization-mass spectrometry with field amplified sample injection

Single-cell metabolomics provides information on the biochemical state of an individual cell and its relationship with the surrounding environment. Characterization of metabolic cellular heterogeneity is challenging, in part due to the small amounts of analytes and their wide dynamic concentration ranges within individual cells. CE-ESI-MS is well suited to single-cell assays because of its low sample-volume requirements and low detection limits. While the volume of a cell is in the picoliter range, after isolation, the typical volume of the lysed cell sample is on the order of a microliter; however, only nanoliters are injected into the CE system, with the volume mismatch limiting analytical performance. Here we developed an approach for the detection of intracellular metabolites from a single neuron using field amplified sample injection (FASI) CE-ESI-MS. Through the application of FASI, we achieved 100- to 300-fold detection limit enhancement compared to hydrodynamic injections. We further enhanced the analyte identification and quantification accuracy via introduction of two internal standards. As a result, the relative standard deviations of migration times were reduced to <5%, aiding identification. Finally, we successfully applied FASI CE-ESI-MS to the untargeted profiling of metabolites of Aplysia californica pleural sensory neurons with <50 μm diameter cell somata. As a result, twenty one neurotransmitters and metabolites have been quantified in these neurons.

μLAS technology for DNA isolation coupled to Cas9-assisted targeting for sequencing and assembly of a 30 kb region in plant genome

Cas9-assisted targeting of DNA fragments in complex genomes is viewed as an essential strategy to obtain high-quality and continuous sequence data. However, the purity of target loci selected by pulsed-field gel electrophoresis (PFGE) has so far been insufficient to assemble the sequence in one contig. Here, we describe the μLAS technology to capture and purify high molecular weight DNA. First, the technology is optimized to perform high sensitivity DNA profiling with a limit of detection of 20 fg/μl for 50 kb fragments and an analytical time of 50 min. Then, μLAS is operated to isolate a 31.5 kb locus cleaved by Cas9 in the genome of the plant Medicago truncatula. Target purification is validated on a Bacterial Artificial Chromosome plasmid, and subsequently carried out in whole genome with μLAS, PFGE or by combining these techniques. PacBio sequencing shows an enrichment factor of the target sequence of 84 with PFGE alone versus 892 by association of PFGE with μLAS. These performances allow us to sequence and assemble one contig of 29 441 bp with 99% sequence identity to the reference sequence.

Distribution and Chemical Analysis of Pharmaceuticals and Personal Care Products (PPCPs) in the Environmental Systems: A Review

PPCPs are found almost everywhere in the environment especially at an alarming rate and at very low concentration in the aquatic systems. Many methods-including pressurized hot water extraction (PHWE), pressurized liquid extraction (PLE), ultrasound-assisted extraction (UAE), and micro-assisted extraction (MAE)-have been employed for their extraction from both surface waters and biota. Solid-phase extraction (SPE) proved to be the best extraction method for these polar, non-volatile, and thermally unstable compounds in water. However, ultrasonic extraction works better for their isolation from sediment because it is cheap and consumes less solvent, even though SPE is preferred as a clean-up method for sediment samples. PPCPs are in groups of-acidic (e.g., diclofenac, ibuprofen, naproxen), neutral (e.g., caffeine, carbamazepine, fluoxetine), and basic pharmaceuticals, as well as antibiotics and estrogens amongst others. PPCPs which are present in trace levels (ng/L) are more often determined by liquid chromatography-mass spectrometry (LC-MS), gas chromatography-mass spectrometry (GC-MS), and high-performance liquid chromatography-ultraviolent (HPLC-UV). Of these, LC-MS and LC-MS-MS are mostly employed for the analysis of this class of compounds, though not without a draw-back of matrix effect. GC-MS and GC-MS-MS are considered as alternative cost-effective methods that can also give better results after derivatization.

Quantitative analysis of microcystin variants by capillary electrophoresis mass spectrometry with dynamic pH barrage junction focusing

Dynamic pH junction is an online focusing method in CE based on the electrophoretic mobility difference of analytes in the sample matrix and the background electrolyte. An advantage of this method over the conventional CE is that the sensitivity can be significantly improved. By injecting a long sample plug in the capillary and focusing the analytes at the pH boundary between the background electrolyte and sample matrix, the LOD can be improved by 10-100 folds. The dynamic pH junction method can be easily coupled with ESI-MS. In this work, we used this method for the analysis of microcystins (MCs). The detection limits and dynamic ranges were studied. The separation was optimized by adjusting the injection time, and concentrations and pH values of the background electrolyte. The optimization of analyte focusing leads to enhanced detection response compared to conventional injections, achieving 200-400 fold higher averaged peak heights for four microcystin (MC) variants. More importantly, this method was successfully used for the quantitative analysis of microcystins (MCs) in crude algae samples from natural water bodies, making it promising for practical applications.

Development of a Micellar Electrokinetic Chromatographic Method with Indirect UV Detection for Pregabalin Determination in Serum Samples

Background: A micellar electrokinetic chromatographic (MEKC)/ indirect UV detection method with hydrodynamic and electrokinetic injection has been developed for the determination of pregabalin in the serum samples. Methods: Separation of the drug was achieved on Agilent capillary electrophorese in less than 5 min using a 50 cm × 75 μm i.d. uncoated fused-silica capillary and a background electrolyte (BGE) consisting of 5-aminosalicylic acid (5-ASA, 10 mmol L-1), cetyl trimethylammonium bromide (CTAB, 1 mmol L-1) and tri-sodium citrate (4% w/v). The influence of various parameters on the separation such as separation voltage, injection time, cassette temperature, pH of BGE and organic modifier was investigated. Results: Method validation shown good linearity (R2> 0.999) in the range of 1.5-100 µg mL-1 of pregabalin. A limit of detection (LOD) of 0.8 μg mL-1 and a limit of quantitation (LOQ) of 2.6 μg mL-1 were reported for pregabalin. Conclusion: The proposed method was found to be suitable and accurate for the determination of pregabalin in serum samples.

Fast screening of aflatoxins in dairy cattle feeds with CE-LIF method combined with preconcentration technique of vortex assisted low density solvent-microextraction

Aflatoxin contamination in agricultural products poses a great threat to humans and livestock. The aim of this study was to establish a simple, rapid, highly sensitive, and inexpensive method for the simultaneous detection of aflatoxin B₁ , B₂ , G₁ , and G₂ in agricultural products. We used a vortex assisted low density solvent-microextraction (VALDS-ME) technique for sample preconcentration and sample detection was achieved with a CE-LIF method. Aflatoxins were separated in an uncoated fused-silica capillary with the MEKC mode and were excited by a 355 nm UV laser to produce native fluorescence for detection. The obtained LOD and LOQ for the four aflatoxins were in the range of 0.002-0.075 and 0.007-0.300 μg/L, respectively, and the analysis time was within 6.5 min. Using the established method, aflatoxins were screened in naturally contaminated dairy cattle feed samples including alfalfa, bran, and corn kernel. The result shows that the alfalfa and bran samples were contaminated with aflatoxins to varying degrees. Compared with other analytical techniques for aflatoxin screening in agricultural products, this CE-LIF method combined with VALDS-ME preconcentration technique is simple, rapid, highly efficient, and inexpensive.

Dual-opposite end multiple injection method applied to sequential determination of Na+, K+, Ca+2, Mg+2 ions and free and total glycerol in biodiesel by capillary zone electrophoresis

A novel method for the sequential determination of sodium, potassium, calcium, and magnesium and free and total glycerol in biodiesel by capillary zone electrophoresis is proposed herein. The inorganic cations were separated along an effective length of 43.5 cm. The samples to quantify the free and total glycerol were injected into the opposite capillary end, close to the detection window, with an effective length of 8.5 cm. It was possible to achieve the separation of six analytes within 3 min. The quantification limits for the cations and glycerol ranged from 0.071 to 0.5 mg kg^-1 to and 0.0017% to 0.017% w/w, respectively. Despite the complexity of the injection steps, the values for the instrumental, intraday and interday precision were better than 2.13, 4.49 and 5.68% (RSD), respectively, for the cations and the free and total glycerol. The method has good accuracy and specificity, which was statistically confirmed through an interlaboratory assay, where the method was compared with official methods.

Capillary electrophoresis-tandem mass spectrometry for multiclass analysis of polar marine toxins

Polar marine toxins are more challenging to analyze by mass spectrometry-based methods than lipophilic marine toxins, which are now routinely measured in shellfish by multiclass reversed-phase liquid chromatography-tandem mass spectrometry (MS/MS) methods. Capillary electrophoresis (CE)-MS/MS is a technique that is well suited for the analysis of polar marine toxins, and has the potential of providing very high resolution separation. Here, we present a CE-MS/MS method developed, with use of a custom-built interface, for the sensitive multiclass analysis of paralytic shellfish toxins, tetrodotoxins, and domoic acid in seafood. A novel, highly acidic background electrolyte (5 M formic acid) was designed to maximize protonation of analytes and to allow a high degree of sample stacking to improve the limits of detection. The method was applied to a wide range of regulated and less common toxin analogues, and exhibited a high degree of selectivity between toxin isomers and matrix interference. The limits of detection in mussel tissue were 0.0052 mg/kg for tetrodotoxins, 0.160 mg/kg for domoic acid, and between 0.0018 and 0.120 mg/kg for paralytic shellfish toxins, all of which showed good linearity. Minimal ionization suppression was observed when the response from neat and mussel-matrix-matched standards was corrected with multiple internal standards. Analysis of shellfish matrix reference materials and spiked samples demonstrated good accuracy and precision. Finally, the method was transferred to a commercial CE-MS/MS system to demonstrate its widespread applicability for use in both R & D and routine regulatory settings. The approach of using a highly acidic background electrolyte is of broad interest, and can be considered generally applicable to simultaneous analysis of other classes of small, polar molecules with differing pK_a values. Graphical abstract ᅟ.

Microfluidic ELISA employing an enzyme substrate and product species with similar detection properties

The requirement for an enzyme label to carry out a chemical reaction directly at the signaling region of the enzyme substrate in order to produce a large change in its detectability places a significant constraint on the scope of enzyme-linked immunosorbent assays (ELISAs). In particular, this requirement limits the kinds of enzyme label-substrate couples employable in ELISAs and prevents their independent optimization with respect to the enzyme reaction and the detectability of the enzyme reaction substrate/product. The detection limit and multiplexing capabilities of the assay are consequently restricted in addition to rendering the technique applicable to a narrow range of assay conditions/samples. Attempting to address some of these limitations, the current article describes a microfluidic ELISA method that does not require the enzyme label to act around the signaling region of the substrate molecule. A highly detectable rhodamine based substrate was synthesized to demonstrate the reported assay which upon cleavage by the enzyme label, alkaline phosphatase, transformed from a monoanionic to a monocationic species, both of which had nearly identical fluorescence properties. These species were later separated based on their charge difference using capillary zone electrophoresis in an integrated device yielding a quantitative measure for the analyte (human TNF-α) in our sample. Impressively, the noted approach not only enabled the use of a new kind of enzyme substrate for ELISAs but also allowed the detection of human TNF-α at concentrations over 54-fold lower than that possible on commercial microwell plates primarily due to the better detectability of the rhodamine dye.

Molecular rheotaxis directs DNA migration and concentration against a pressure-driven flow

In-line preconcentration techniques are used to improve the sensitivity of microfluidic DNA analysis platforms. The most common methods are electrokinetic and require an externally applied electric field. Here we describe a microfluidic DNA preconcentration technique that does not require an external field. Instead, pressure-driven flow from a fluid-filled microcapillary into a lower ionic strength DNA sample reservoir induces spontaneous DNA migration against the direction of flow. This migratory phenomenon that we call Molecular Rheotaxis initiates in seconds and results in a concentrated DNA bolus at the capillary orifice. We demonstrate the ease with which this concentration method can be integrated into a microfluidic total analysis system composed of in-line DNA preconcentration, size separation, and single-molecule detection. Paired experimental and numerical simulation results are used to delineate the parameters required to induce Molecular Rheotaxis, elucidate the underlying mechanism, and optimize conditions to achieve DNA concentration factors exceeding 10,000 fold.

Implementing a nucleic acid preconcentration method can improve the sensitivity of microfluidic analysis systems. Here Friedrich et al. concentrate DNA by many orders of magnitude using pressure-driven flow, which could lead to a simple and practical microanalysis platform.

Dialkyl anionic surfactant in field-amplified sample injection and sweeping-micellar electrokinetic chromatography for determination of eight leanness-promoting β-agonists in animal feeds

The beta-adrenergic agonists (β-agonists) working as repartitioning agents that make the carcass leaner and enhance the feeding efficiency in animals have been banned in the European Union, China and Taiwan. Here, traditional anionic surfactants, such as sodium dodecyl sulfate (SDS) were replaced with sodium di-(2-ethylhexyl)-sulfosuccinate (AOT) in field-amplified sample injection and sweeping-micellar electrokinetic chromatography (FASI-sweeping MEKC) for simultaneous analysis of eight β-agonists in animal feeds. The AOT vesicles provided a better resolution of β-agonists than micelles of SDS. The detection limits of the eight β-agonists were above 5ng/mL by using this stacking capillary electrophoresis (CE) method. In comparison of traditional MEKC method (sample injection, 1psi for 5s), the stacking strategy provided 400-2000 fold sensitivity enhancement. After method validation, this method was successfully applied for analyzing four animal feeds, and none β-agonist was detected. This strategy possessing good resolution of eight β-agonists was suitable for serving as a tool for routine analysis of animal feeds.

Recent Advances in Microscale Western Blotting

Western blotting is a ubiquitous tool used extensively in the clinical and research settings to identify proteins and characterize their levels. It has rapidly become a mainstay in research laboratories due to its specificity, low cost, and ease of use. The specificity arises from the orthogonal processes used to identify proteins. Samples are first separated based on size and then probed with antibodies specific for the protein of interest. This confirmatory approach helps avoid pitfalls associated with antibody cross-reactivity and specificity issues. While the technique has evolved since its inception, the last decade has witnessed a paradigm shift in Western blotting technology. The introduction of capillary and microfluidic platforms has significantly decreased time and sample requirements while enabling high-throughput capabilities. These advances have enabled Western analysis down to the single cell level in highly parallel formats, opening vast new opportunities for studying cellular heterogeneity. Recent innovations in microscale Western blotting are surveyed, and the potential for enhancing detection using advances in label-free biosensing is briefly discussed.

Evaluation of the combination of micellar electrokinetic capillary chromatography with sweeping and cation selective exhaustive injection for the determination of 5-nitroimidazoles in egg samples

A methodology is presented for the sensitive determination of nitromidazole residues in egg by means of micellar electrokinetic capillary chromatography in combination with cation selective exhaustive injection and ultraviolet detection. Six compounds have been considered and the separation has been achieved in less than 12min in a 61.5-cm effective length capillary with 50-μm internal diameter. Phosphate buffer 44mM pH 2.5, containing 8% tetrahydrofurane and 123mM sodium dodecyl sulfate was employed as running buffer. Solid phase extraction has been employed for sample clean-up. The methodology has been successfully validated in hen eggs, obtaining method detection limits in the range of 2.1-5.0ng/g. Precision was studied in terms of repeatability and intermediate precision, with relative standard deviations lower than 18.0%. Recoveries were calculated in quail eggs and a commercial pasteurized egg white product, reaching over 70% for most of the considered 5-nitroimidazoles.

Field amplified sample stacking of amyloid beta (1-42) oligomers using capillary electrophoresis

Oligomeric forms of the amyloid beta (Aβ) protein have been indicated to be an important factor in the development of Alzheimer's disease (AD). Since the oligomeric forms of Aβ can vary in size and conformation, it is vital to understand the early stages of Aβ aggregation in order to improve the care and treatment of patients with AD. This is the first study to determine the effect of field amplified sample stacking (FASS) on the separation of oligomeric forms of Aβ1-42 using capillary electrophoresis (CE) with ultraviolet (UV) detection. UV-CE was able to separate two different species of Aβ1-42 oligomers (<7 mers and 7-22 mers). Although FASS required the use of a higher ionic strength buffer, Aβ1-42 oligomers had the same aggregation behavior as under the non-FASS conditions with only small changes in the amounts of oligomers observed. In general, FASS provided smaller peak widths (>75% average reduction) and increased peak heights (>60% average increase) when compared to non-FASS conditions. UV-CE with FASS also provided higher resolution between the Aβ1-42 oligomers for all aggregation time points studied. In addition, Congo red and Orange G inhibition studies were used to help evaluate the conformation of the observed species. This work demonstrates the ability of UV-CE employing FASS to provide higher resolution between oligomeric forms of Aβ1-42 without significantly altering their aggregation.

Capture efficiency of dynamic pH junction focusing in capillary electrophoresis

Dynamic pH junction is one of the techniques used to overcome the issue of poor concentration sensitivity in CE. By introducing a long sample plug in the capillary and focusing the target molecules at the pH boundary between the sample plug and background electrolyte, this focusing technique can achieve a detection limit that is one to two orders of magnitude better than conventional CE. For quantification purposes, the capturing efficiency of the injected molecules should be scrutinized. Focusing of all target molecules inside the sample plug is desired to ensure good linearity across the whole dynamic range. To test the theoretical prediction with a real experiment, nicotine is used as the test molecule for two types of dynamic pH junctions. The first one is with acidic background electrolyte, and can accommodate both optical detection methods and positive-ion mode mass spectrometric detection, while the other is suitable for optical detection only due to the use of basic separation background electrolyte. With a theoretical simulation study, it is demonstrated that, for either of these dynamic pH junctions, focusing of at least 95% of target molecule injected into the capillary was easily achievable. More importantly, a longer sample plug could generate a high percentage of molecules captured by dynamic pH junction focusing. Sharp, symmetrical peaks and good linearity for calibration curve can be obtained. Real samples with complex matrixes were also used to demonstrate that nicotine can be selectively focused and quantified using CE-MS.