Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips

Nanoplastic Sample Cleanup by Micro-Electromembrane Extraction across Free Liquid Membranes

Sample preparation techniques enabling the separation and cleanup of nanoplastics removing other components present in complex sample matrices are scarce. Herein, micro-electromembrane extraction (μ-EME) has been explored for this purpose based on the extraction of nanoplastic particles across a free liquid membrane (FLM). The extraction unit is based on a perfluoroalkoxy tube sequentially filled with the acceptor solution (20 μL 5 mM phosphate buffer, pH 10.7), FLM (10 μL 1-pentanol), and donor solution (20 μL sample/standard solution). Sulfonated polystyrene beads (200 nm particle size) were selected as a model mimicking negatively charged nanoplastics. At 700 V, nanoplastics transferred from the donor solution into the FLM before moving across the FLM into the acceptor solution. Quantitative nanoplastic measurements after μ-EME were performed by injecting the acceptor solution into a capillary electrophoresis system with diode array detection. μ-EME allowed the rapid nanoplastic sample cleanup, requiring an extraction time of just 90 s and obtaining a nanoplastic transfer yield through the FLM of 60% with RSD values below 9%. The μ-EME technique enabled the efficient sample matrix cleanup of nanoplastics spiked in different tea matrices. Nanoplastic transfer yield through the FLM for black tea and flavored tea matrices were 56% and 47%, respectively, with complete sample matrix removal of UV-absorbing compounds.

Isotachophoresis: Theory and Microfluidic Applications

Isotachophoresis (ITP) is a versatile electrophoretic technique that can be used for sample preconcentration, separation, purification, and mixing, and to control and accelerate chemical reactions. Although the basic technique is nearly a century old and widely used, there is a persistent need for an easily approachable, succinct, and rigorous review of ITP theory and analysis. This is important because the interest and adoption of the technique has grown over the last two decades, especially with its implementation in microfluidics and integration with on-chip chemical and biochemical assays. We here provide a review of ITP theory starting from physicochemical first-principles, including conservation of species, conservation of current, approximation of charge neutrality, pH equilibrium of weak electrolytes, and so-called regulating functions that govern transport dynamics, with a strong emphasis on steady and unsteady transport. We combine these generally applicable (to all types of ITP) theoretical discussions with applications of ITP in the field of microfluidic systems, particularly on-chip biochemical analyses. Our discussion includes principles that govern the ITP focusing of weak and strong electrolytes; ITP dynamics in peak and plateau modes; a review of simulation tools, experimental tools, and detection methods; applications of ITP for on-chip separations and trace analyte manipulation; and design considerations and challenges for microfluidic ITP systems. We conclude with remarks on possible future research directions. The intent of this review is to help make ITP analysis and design principles more accessible to the scientific and engineering communities and to provide a rigorous basis for the increased adoption of ITP in microfluidics.

Out-of-plane faradaic ion concentration polarization: stable focusing of charged analytes at a three-dimensional porous electrode

Ion concentration polarization (ICP) accomplishes preconcentration for bioanalysis by localized depletion of electrolyte ions, thereby generating a gradient in electric field strength that facilitates electrokinetic focusing of charged analytes by their electromigration against opposing fluid flow. Such ICP focusing has been shown to accomplish up to a million-fold enrichment of nucleic acids and proteins in single-stage preconcentrators. However, the rate at which the sample volume is swept is limited, requiring several hours to achieve these high enrichment factors. This limitation is caused by two factors. First, an ion depleted zone (IDZ) formed at a planar membrane or electrode may not extend across the full channel cross section under the flow rate employed for focusing, thereby allowing the analyte to "leak" past the IDZ. Second, within the IDZ, large fluid vortices lead to mixing, which decreases the efficiency of analyte enrichment and worsens with increased channel dimensions. Here, we address these challenges with faradaic ICP (fICP) at a three-dimensional (3D) electrode comprising metallic microbeads. This 3D-electrode distributes the IDZ, and therefore, the electric field gradient utilized for counter-flow focusing across the full height of the fluidic channel, and its large area, microstructured surface supports smaller vortices. An additional bed of insulating microbeads restricts flow patterns and supplies a large area for surface conduction of ions through the IDZ. Finally, the resistance of this secondary bed enhances focusing by locally strengthening sequestering forces. This easy-to-build platform lays a foundation for the integration of enrichment with user-defined packed bed and electrode materials.

Bottom-Up Analysis of Proteins by Peptide Mass Fingerprinting with tCITP-CZE-ESI-TOF MS After Tryptic Digest

The identification of proteins in samples of moderate to complex composition is primarily done by bottom-up approaches. Therefore, proteins are enzymatically digested, mostly by trypsin, and the resulting peptides are then separated prior to their transfer to a mass spectrometer. The following protocol portrays a bottom-up method, which was optimized for the application of CZE-ESI-TOF MS. Protein denaturation is achieved by addition of 2,2,2-trifluoroethanol (TFE) and heat treatment. Afterwards, disulfide bonds are reduced with tris-(2-carboxyethyl)phosphine (TCEP) and subsequently alkylated with iodoacetamide (IAA). The tryptic digest is performed in an ammonium bicarbonate buffer at pH 8.0. The digested protein sample is then concentrated in-capillary by transient capillary isotachophoresis (tCITP) with subsequent CZE separation of tryptic peptides in an acidic background electrolyte. Hyphenation to a time-of-flight (TOF) mass spectrometer is carried out by a triple-tube coaxial sheath flow interface, which uses electrospray ionization (ESI). Peptide identification is done by peptide mass fingerprinting (PMF). The protocol is outlined exemplarily for a model protein, i.e., bovine β-lactoglobulin A.

Assessment of submicron particle zeta potential in simple electrokinetic microdevices

The present communication illustrates the use of simple electrokinetic devices for the assessment of the zeta potential of submicron polystyrene particles. A combination of manual and automatic particle tracking was employed. This approach allows for characterizing particles in the same conditions and devices in which they can be separated, e.g. dielectrophoretic separations; making the resulting data readily applicable.

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.

Microchip electrophoresis separation of a panel of preterm birth biomarkers

Preterm birth (PTB) is responsible for over one million infant deaths annually worldwide. Often, the first and only indication of PTB risk is the onset of early labor. Thus, there is an urgent need for an early PTB risk diagnostic that is inexpensive, reliable, and robust. Here, we describe the development of a microchip electrophoresis (μCE) method for separating a mixture of six PTB protein and peptide biomarkers present in maternal blood serum. μCE devices were photografted with a poly(ethylene glycol) diacrylate surface coating to regulate EOF and reduce nonspecific analyte adsorption. Separation conditions including buffer pH, buffer concentration, and applied electric field were varied to improve biomarker peak resolution while minimizing deleterious effects like Joule heating. In this way, it was possible to separate six PTB biomarkers, the first μCE separation of this biomarker panel. LODs were also measured for each of the six PTB biomarkers. In the future, this μCE separation can be integrated with upstream maternal blood serum sample preparation steps to yield a complete PTB risk diagnosis microdevice.

Enzyme catalysis-electrophoresis titration for multiplex enzymatic assay via moving reaction boundary chip

In this work, we developed the concept of enzyme catalysis-electrophoresis titration (EC-ET) under ideal conditions, the theory of EC-ET for multiplex enzymatic assay (MEA), and a related method based on a moving reaction boundary (MRB) chip with a collateral channel and cell phone imaging. As a proof of principle, the model enzymes horseradish peroxidase (HRP), laccase and myeloperoxidase (MPO) were chosen for the tests of the EC-ET model. The experiments revealed that the EC-ET model could be achieved via coupling EC with ET within a MRB chip; particularly the MEA analyses of catalysis rate, maximum rate, activity, Km and Kcat could be conducted via a single run of the EC-ET chip, systemically demonstrating the validity of the EC-ET theory. Moreover, the developed method had these merits: (i) two orders of magnitude higher sensitivity than a fluorescence microplate reader, (ii) simplicity and low cost, and (iii) fairly rapid (30 min incubation, 20 s imaging) analysis, fair stability (<5.0% RSD) and accuracy, thus validating the EC-ET method. Finally, the developed EC-ET method was used for the clinical assay of MPO activity in blood samples; the values of MPO activity detected via the EC-ET chip were in agreement with those obtained by a traditional fluorescence microplate reader, indicating the applicability of the EC-ET method. The work opens a window for the development of enzymatic research, enzyme assay, immunoassay, and point-of-care testing as well as titration, one of the oldest methods of analysis, based on a simple chip.

Telmisartan

Telmisartan is an angiotensin-II receptor antagonist (ARB) used in the treatment of hypertension. Generally, angiotensin-II receptor blockers such as telmisartan bind to the angiotensin-II type 1 receptors with high affinity, causing inhibition of the action of angiotensin II on vascular smooth muscle, ultimately leading to a reduction in arterial blood pressure. The present study gives a comprehensive profile of telmisartan, including detailed nomenclature, formulae, elemental analysis, and appearance of the drug are mentioned. The uses and applications and the several methods described for its preparation of the drug are outlined. The profile contains the physicochemical properties including: pKa value, solubility, X-ray powder diffraction, melting point, and methods of analysis (including compendial, electrochemical, spectroscopic, and chromatographic methods of analysis). Developed validated stability-indicating (HPLC and biodiffusion assay methods under accelerated acidic, alkaline, and oxidative conditions, in addition to effect of different types of light, temperature, and pH. Detailed Pharmacology also presented (Pharmacological actions, Therapeutic uses and Dosing, Interactions, and adverse effects and precautions). More than 80 references were given as a proof of the above-mentioned studies.

Low-picomolar analysis of peptides by on-line coupling of fritless solid-phase extraction to sheathless capillary electrophoresis-mass spectrometry

A novel fritless solid-phase extraction (SPE) microcartridge was designed for combination with sheathless capillary electrophoresis-mass spectrometry (sheathless CE-MS) employing a prototype porous-tip capillary for nanoelectrospray ionization (nanoESI). The inlet of the separation capillary (30μm inner diameter (id), 150μm outer diameter (od)) was inserted in a 4mm long SPE microcartridge (150μm id, 365μm od) packed with a C18 sorbent of 55-105μm particle size. Performance of the SPE-CE-MS system was evaluated using diluted solutions of the three opioid peptides dynorphin A (1-7) (DynA), endomorphin 1 (End1) and met-enkephalin (Met). Sample volumes of 1.5μL were loaded on the SPE microcartridge and the retained peptides were eluted with 22nL of an acidic methanol/water (60:40, v/v) solution. Using a pressure of 50mbar during separation to speed up the analysis, good peptide resolution was obtained with acceptable plate numbers (between 53,000 and 92,000). Intraday relative standard deviations (% RSD) for peptide migration times and peak areas were below 4% and 9%, respectively. The SPE-CE-MS method showed good linearity in the 0.05-5ngmL(-1) range and limits of detection (LODs) were 10pgmL(-1). However, loading a larger volume of sample (8μL), LODs could be decreased down to 2pgmL(-1) (2.2-3.5pM). This represents an improvement of up to 5000-fold with respect to the LODs achieved by sheathless CE-MS without on-line preconcentration demonstrating the potential of on-line SPE for further enhancing sensitivity.

Electrical field-induced extraction and separation techniques: promising trends in analytical chemistry--a review

Sample preparation is an important issue in analytical chemistry, and is often a bottleneck in chemical analysis. So, the major incentive for the recent research has been to attain faster, simpler, less expensive, and more environmentally friendly sample preparation methods. The use of auxiliary energies, such as heat, ultrasound, and microwave, is one of the strategies that have been employed in sample preparation to reach the above purposes. Application of electrical driving force is the current state-of-the-art, which presents new possibilities for simplifying and shortening the sample preparation process as well as enhancing its selectivity. The electrical driving force has scarcely been utilized in comparison with other auxiliary energies. In this review, the different roles of electrical driving force (as a powerful auxiliary energy) in various extraction techniques, including liquid-, solid-, and membrane-based methods, have been taken into consideration. Also, the references have been made available, relevant to the developments in separation techniques and Lab-on-a-Chip (LOC) systems. All aspects of electrical driving force in extraction and separation methods are too specific to be treated in this contribution. However, the main aim of this review is to provide a brief knowledge about the different fields of analytical chemistry, with an emphasis on the latest efforts put into the electrically assisted membrane-based sample preparation systems. The advantages and disadvantages of these approaches as well as the new achievements in these areas have been discussed, which might be helpful for further progress in the future.

Effect of a low-conductivity zone on field-amplified sample stacking in microchip micellar electrokinetic chromatography

The effects of a low-conductivity (low-C) zone of an on-line sample preconcentration based on field-amplified sample stacking (FASS) under an acidic condition was investigated in microchip micellar electrokinetic chromatography (MCMEKC). By employing originally fabricated microchips with T- and cross-channel injectors on a single device, partial injection of the low-C zone and large-volume injection of a sample solution were demonstrated. As a typical result, the observed peak intensities of the fluorescent dyes were 1.5 - 2.0-fold enhanced by partial injection of the low-conductivity zone, as compared to those in FASS-MCMEKC without injecting the low-C zone; also, 30 - 40-fold enhancements of the detectability were achieved as compared to the conventional MCMEKC analyses. The resolutions of the hydrophobic analytes were also improved by introducing the low-C zone. The difference in the retention factors of model analytes in the high-C background and low-C zones was also estimated in order to clarify the effect of the low-C zone on the resolution and detectability in FASS-MCMEKC.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2010-2012)

CE has been alive for over two decades now, yet its sensitivity is still regarded as being inferior to that of more traditional methods of separation such as HPLC. As such, it is unsurprising that overcoming this issue still generates much scientific interest. This review continues to update this series of reviews, first published in Electrophoresis in 2007, with updates published in 2009 and 2011 and covers material published through to June 2012. 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 inline extraction methods that have been used for electrophoresis.

Approaches to enhancing the sensitivity of carbohydrate separations in capillary electrophoresis

Electrophoresis in both capillaries (CE) and microchips (ME) is an extremely powerful liquid phase-separation technique that is indispensable for the separation of carbohydrates. It is capable of separating both small mono- and disaccharides, through to more complex oligo- and polysaccharides, with high resolution, but as with all CE and ME separations, the detection limits are often inferior to those that can be achieved with liquid chromatographic methods. One avenue to address this is to use an on-line concentration strategy. Various approaches have been developed over the past 20 years, and this chapter will highlight their application to improve the sensitivity of carbohydrate separations in both CE and ME.