Continuous free-flow electrophoresis

Electrophoresis, a transport technology that transitioned from moving boundary method to zone method

Gel electrophoresis, a transport technology, is one of the most widely used experimental methods in biochemical and pharmaceutical research and development. Transport technologies are used to determine hydrodynamic or electrophoretic properties of macromolecules. Gel electrophoresis is a zone technology, where a small volume of sample is applied to a large separation gel matrix. In contrast, a seldom-used electrophoresis technology is moving boundary electrophoresis, where the sample is present throughout the separation phase or gel matrix. While the zone method gives peaks of separating macromolecular solutes, the moving boundary method gives a boundary between solute-free and solute-containing phases. We will review electrophoresis as a transport technology of zone and moving boundary methods and describe its principles and applications.

A review of the zone broadening contributions in free-flow electrophoresis

The broadening of analyte streams, as they migrate through a free-flow electrophoresis (FFE) channel, often limits the resolving power of FFE separations. Under laminar flow conditions, such zonal spreading occurs due to analyte diffusion perpendicular to the direction of streamflow and variations in the lateral distance electrokinetically migrated by the analyte molecules. Although some of the factors that give rise to these contributions are inherent to the FFE method, others originate from non-idealities in the system, such as Joule heating, pressure-driven crossflows, and a difference between the electrical conductivities of the sample stream and background electrolyte. The injection process can further increase the stream width in FFE separations but normally influencing all analyte zones to an equal extent. Recently, several experimental and theoretical works have been reported that thoroughly investigate the various contributions to stream variance in an FFE device for better understanding, and potentially minimizing their magnitudes. In this review article, we carefully examine the findings from these studies and discuss areas in which more work is needed to advance our comprehension of the zone broadening contributions in FFE assays.

Investigating peak dispersion in free-flow counterflow gradient focusing due to electroosmotic flow

Free-flow electrophoresis (FFE) has the ability to continuously separate charged solutes from complex biological mixtures. Recently, a free-flow counterflow gradient focusing mechanism has been introduced to FFE, and it offers the potential for improved resolution and versatility. However, further investigation is needed to understand the solute dispersion at the focal position. Therefore, the goal of this work is to model the impact of electroosmotic flow, which is found to produce a pressure-driven backflow to maintain the fixed counterflow inputs. Like the counterflow, this backflow has a parabolic velocity profile that must be considered when predicting the concentration distribution of a given solute. After the model is established, preliminary experimental results are presented for a qualitative comparison. Results demonstrate a reasonable agreement at low applied voltages and provide a strong framework for future experimental validation.

Isolation of native EVs from primary biofluids-Free-flow electrophoresis as a novel approach to purify ascites-derived EVs

Although extracellular vesicles (EVs) have been extensively characterized, efficient purification methods, especially from primary biofluids, remain challenging. Here we introduce free-flow electrophoresis (FFE) as a novel approach for purifying EVs from primary biofluids, in particular from the peritoneal fluid (ascites) of ovarian cancer patients. FFE represents a versatile, fast, matrix-free approach for separating different analytes with inherent differences in charge density and/or isoelectric point (pI). Using a series of buffered media with different pH values allowed us to collect 96 fractions of ascites samples. To characterize the composition of the individual fractions, we used state-of-the-art methods such as nanoflow and imaging flow cytometry (nFCM and iFCM) in addition to classical approaches. Of note, tetraspanin-positive events measured using nFCM were enriched in a small number of distinct fractions. This observation was corroborated by Western blot analysis and electron microscopy, demonstrating only minor contamination with soluble proteins and lipid particles. In addition, these gently purified EVs remain functional. Thus, FFE represents a new, efficient and fast method for separating native and highly purified EVs from complicated primary samples.

Agnostic Life Finder (ALF) for Large-Scale Screening of Martian Life During In Situ Refueling

Before the first humans depart for Mars in the next decade, hundreds of tons of martian water-ice must be harvested to produce propellant for the return vehicle, a process known as in situ resource utilization (ISRU). We describe here an instrument, the Agnostic Life Finder (ALF), that is an inexpensive life-detection add-on to ISRU. ALF exploits a well-supported view that informational genetic biopolymers in life in water must have two structural features: (1) Informational biopolymers must carry a repeating charge; they must be polyelectrolytes. (2) Their building blocks must fit into an aperiodic crystal structure; the building blocks must be size-shape regular. ALF exploits the first structural feature to extract polyelectrolytes from ∼10 cubic meters of mined martian water by applying a voltage gradient perpendicularly to the water's flow. This gradient diverts polyelectrolytes from the flow toward their respective electrodes (polyanions to the anode, polycations to the cathode), where they are captured in cartridges before they encounter the electrodes. There, they can later be released to analyze their building blocks, for example, by mass spectrometry or nanopore. Upstream, martian cells holding martian informational polyelectrolytes are disrupted by ultrasound. To manage the (unknown) conductivity of the water due to the presence of salts, the mined water is preconditioned by electrodialysis using porous membranes. ALF uses only resources and technology that must already be available for ISRU. Thus, life detection is easily and inexpensively integrated into SpaceX or NASA ISRU missions.

Fluorescence Imaging Characterization of the Separation Process in a Monolithic Microfluidic Free-Flow Electrophoresis Device Fabricated Using Low-Temperature Co-Fired Ceramics

A monolithic microfluidic free-flow electrophoresis device, fabricated using low-temperature co-fired ceramic technology, is presented. The device integrates gold electrodes and a 20 µm thick transparent ceramic optical window, suitable for fluorescence imaging, into a multilevel microfluidic chamber design. The microfluidic chamber consists of a 60 µm deep separation chamber and two, 50 µm deep electrode chambers separated by 10 µm deep side channel arrays. Fluorescence imaging was used for in-chip, spatial-temporal characterization of local pH variations in separation conditions as well as to characterize the separation process. The device allowed baseline resolution separation of a sample mixture of Fluorescein, Rhodamine 6G, and 4-Methylumbelliferone at pH 7.0, in only 6 s, using 378 V.s/cm. The results demonstrate the possibility of studying a chemical process using fluorescence imaging within the traditional fields of low-temperature co-fired ceramics technology, such as high-electrical-field applications, while using a simple fabrication procedure suitable for low-cost mass production.

Direct homophilic interaction of LAMP2A with the two-domain architecture revealed by site-directed photo-crosslinks and steric hindrances in mammalian cells

LAMP1 (lysosomal-associated membrane protein 1) and LAMP2 are the most abundant protein components of lysosome membranes. Both LAMPs have common structures consisting of a large lumenal domain composed of two domains (N-domain and C-domain, which are membrane-distal and -proximal, respectively), both with the β-prism fold, a transmembrane domain, and a short cytoplasmic tail. LAMP2 is involved in various aspects of autophagy, and reportedly forms high-molecular weight complexes at the lysosomal membrane. We previously showed that LAMP2 molecules coimmunoprecipitated with each other, but whether the homophilic interaction is direct or indirect has remained to be elucidated. In the present study, we demonstrated the direct homophilic interaction of mouse LAMP2A molecules, using expanded genetic code technologies that generate photo-crosslinking and/or steric hindrance at specified interfaces. Specifically, the results suggested that LAMP2A molecules assemble by facing each other with one side of the β-prism (defined as side A) of the C-domains. The N-domain truncation, which increased the coimmunoprecipitation of LAMP2A molecules in our previous study, permitted the nonspecific involvement of both sides of the β-prism (side A and side B). Thus, the presence of the N-domain restricts the LAMP2A interactions to side A-specific. The truncation of LAMP2A impaired the recruitment of GAPDH (a CMA-substrate) fused to the HaloTag protein to the surface of late endosomes/lysosomes (LE/Lys) and affected a process that generates LE/Lys. The present study revealed that the homophilic interaction of LAMP2A is direct, and the side A-specific, homophilic interaction of LAMP2A is required for the functional aspects of LAMP2A.Abbreviations: Aloc-Lys: Nε-allyloxycarbonyl-l-lysine; CMA: chaperone-mediated autophagy; FFE: free-flow electrophoresis; GAPDH-HT: glyceraldehyde-3-phosphate dehydrogenase fused to HaloTag protein; LAMP1: lysosomal-associated membrane protein 1; LAMP2A: lysosomal-associated membrane protein 2A; LBPA: lysobisphosphatidic acid; LE/Lys: late endosome/lysosomes; MEFs: mouse embryonic fibroblasts; pBpa: p-benzoyl- l-phenylalanine.

Coupling Miniaturized Free-Flow Electrophoresis to Mass Spectrometry via a Multi-Emitter ESI Interface

We present a novel multi-emitter electrospray ionization (ESI) interface for the coupling of microfluidic free-flow electrophoresis (μFFE) with mass spectrometry (MS). The effluents of the μFFE outlets are analyzed in near real-time, allowing a direct optimization of the electrophoretic separation and an online monitoring of qualitative sample compositions. The short measurement time of just a few seconds for all outlets even enables a reasonable time-dependent monitoring. As a proof of concept, we employ the multi-emitter ESI interface for the continuous identification of analytes at 15 μFFE outlets via MS to optimize the μFFE separation of important players of cellular respiration in operando. The results indicate great potential of the presented system in downstream processing control, for example, for the monitoring and purification of products in continuous-flow microreactors.

Purification of low-abundance lysozyme in egg white via free-flow electrophoresis with gel-filtration chromatography

As an effective separation tool, free-flow electrophoresis has not been used for purification of low-abundance protein in complex sample matrix. Herein, lysozyme in complex egg white matrix was chosen as the model protein for demonstrating the purification of low-content peptide via an FFE coupled with gel fitration chromatography (GFC). The crude lysozyme in egg while was first separated via free-flow zone electrophoresis (FFZE). After that, the fractions with lysozyme activity were condensed via lyophilization. Thereafter, the condensed fractions were further purified via a GFC of Sephadex G50. In all of the experiments, a special poly(acrylamide- co-acrylic acid) (P(AM-co-AA)) gel electrophoresis and a mass spectrometry were used for identification of lysozyme. The conditions of FFZE were optimized as follows: 130 μL/min sample flow rate, 4.9 mL/min background buffer of 20 mM pH 5.5 Tris-Acetic acid, 350 V, and 14 °C as well as 2 mg/mL protein content of crude sample. It was found that the purified lysozyme had the purity of 80% and high activity as compared with its crude sample with only 1.4% content and undetectable activity. The recoveries in the first and second separative steps were 65% and 82%, respectively, and the total recovery was about 53.3%. The reasons of low recovery might be induced by diffusion of lysozyme out off P(AM-co-AA) gel and co-removing of high-abundance egg ovalbumin. All these results indicated FFE could be used as alternative tool for purification of target solute with low abundance.

Continuous flow electrophoretic separation - Recent developments and applications to biological sample analysis

Continuous flow electrophoretic separation with continuous sample loading provides the advantage of processing volumes of any sizes, as well as the benefit of a real-time monitoring and optimization of the separation process. In addition, the spatial separation of the sample enables collecting multiple separated components simultaneously and in a continuous manner. The separation is usually performed in mild buffers without organic solvents and detergents (sample biological activity is retained) and it is carried out without usage of a solid support in the separation space preventing the interaction of the sample with it (high sample recovery). The method is used for the separation of proteins/peptides in proteomic applications, and its great applicability is to the separation of the cells, cellular organelles, vesicles, membrane fragments, and DNA. This review focuses on the electrophoretic separation performed in a continuous flow and it describes various electrophoretic modes and instrumental setups. Recent developments in methodology and instrumentation, the integration with other techniques, and the application to the biological sample analysis are discussed as well.

Stream broadening due to fluid shear across the wider transverse dimension of a free-flow zone electrophoresis channel

While the pressure-gradient applied along the length of a free-flow zone electrophoresis (FFZE) chamber is known to produce a parabolic flow profile for the carrier electrolyte across the narrower channel dimension (typically the channel depth), additional fluid shear can arise across the channel width due to a variety of reasons. Most commonly, any variation in the pressure-drop or channel depth across this wider dimension can lead to a gradient in the liquid flow velocity along it, significantly altering the stream broadening and, thereby, the separation performance of the assay. This article assesses the effect of such fluid shear on stream broadening during the FFZE process by describing a mathematical framework for solving the relevant advection-diffusion equation based on the method-of-moments approach. A closed-form expression for the leading order term describing the additional contribution to the spatial stream variance has been derived considering a small linear gradient in the liquid velocity across the wider transverse dimension of the FFZE chamber. The noted analysis predicts this contribution to be governed by two Péclet numbers that are evaluated based on the axial pressure-driven flow and transverse electrophoretic solute velocities. More importantly, this contribution is shown to vary quadratically with the axial distance traversed by the analyte stream as opposed to the classical linear variation known for all other stream broadening contributions in FFZE systems. The results from the analytic theory have been validated with Monte Carlo simulations, which also establish a time and length scale over which the noted analytical results are applicable.

Introduction to Protein Electrophoresis

This chapter briefly discusses the developments in electrophoresis of proteins from Tiselius' moving-boundary electrophoresis to the modern-day two-dimensional polyacrylamide gel electrophoresis. It also touches upon the staining methods used to visualize total proteins post electrophoresis.

A Combined Free-Flow Electrophoresis and DIGE Approach to Compare Proteins in Complex Biological Samples

Free-flow electrophoresis has been applied in numerous studies as a protein separation technique due to its multiple advantages such as fast and efficient sample recovery, high resolving power, high reproducibility and wide applicability to protein classes. As a stand-alone platform, however, its utility in comparative proteomic analysis is limited as protein samples must be run sequentially rather than simultaneously which introduces inherent variability when attempting to perform quantitative analysis. Here we describe an approach combining fluorescent CyDye technology (DIGE) with free-flow electrophoresis to simultaneously separate and identify differentially expressed proteins in a model cell system.

The Craft of Peroxisome Purification-A Technical Survey Through the Decades

Purification technologies are one of the working horses in organelle proteomics studies as they guarantee the separation of organelle-specific proteins from the background contamination by other subcellular compartments. The development of methods for the separation of organelles was a major prerequisite for the initial detection and characterization of peroxisome as a discrete entity of the cell. Since then, isolated peroxisomes fractions have been used in numerous studies in order to characterize organelle-specific enzyme functions, to allocate the peroxisome-specific proteome or to unravel the organellar membrane composition. This review will give an overview of the fractionation methods used for the isolation of peroxisomes from animals, plants and fungi. In addition to "classic" centrifugation-based isolation methods, relying on the different densities of individual organelles, the review will also summarize work on alternative technologies like free-flow-electrophoresis or flow field fractionation which are based on distinct physicochemical parameters. A final chapter will further describe how different separation methods and quantitative mass spectrometry have been used in proteomics studies to assign the proteome of PO.

Micro free flow electrophoresis

Micro free-flow electrophoresis (μFFE) is a continuous separation technique in which analytes are streamed through a perpendicularly applied electric field in a planar separation channel. Analyte streams are deflected laterally based on their electrophoretic mobilities as they flow through the separation channel. A number of μFFE separation modes have been demonstrated, including free zone (FZ), micellar electrokinetic chromatography (MEKC), isoelectric focusing (IEF) and isotachophoresis (ITP). Approximately 60 articles have been published since the first μFFE device was fabricated in 1994. We anticipate that recent advances in device design, detection, and fabrication, will allow μFFE to be applied to a much wider range of applications. Applications particularly well suited for μFFE analysis include continuous, real time monitoring and microscale purifications.

Joule heating induced stream broadening in free-flow zone electrophoresis

The use of an electric field in free-flow zone electrophoresis (FFZE) automatically leads to Joule heating yielding a higher temperature at the center of the separation chamber relative to that around the channel walls. For small amounts of heat generated, this thermal effect introduces a variation in the equilibrium position of the analyte molecules due to the dependence of liquid viscosity and analyte diffusivity on temperature leading to a modification in the position of the analyte stream as well as the zone width. In this article, an analytic theory is presented to quantitate such effects of Joule heating on FFZE assays in the limit of small temperature differentials across the channel gap yielding a closed form expression for the stream position and zone variance under equilibrium conditions. A method-of-moments approach is employed to develop this analytic theory, which is further validated with numerical solutions of the governing equations. Interestingly, the noted analyses predict that Joule heating can drift the location of the analyte stream either way of its equilibrium position realized in the absence of any temperature rise in the system, and also tends to reduce zone dispersion. The extent of these modifications, however, is governed by the electric field induced temperature rise and three Péclet numbers evaluated based on the axial pressure-driven flow, transverse electroosmotic and electrophoretic solute velocities in the separation chamber. Monte Carlo simulations of the FFZE system further establish a time and a length scale over which the results from the analytic theory are valid.

Current advances and challenges in microfluidic free-flow electrophoresis-A critical review

The research field on microfluidic free-flow electrophoresis has developed vast amounts of devices, methods, applications and raised new questions, often in analogy to conventional techniques from which it derives. Most efforts have been employed on device development and a myriad of architectures and fabrication techniques have been reported using simple proof-of-principle separations. As technological aspects reach a quite mature state, researchers' new challenges include the development of protocols for the separation of complex mixtures, as required in the fields of application. The success of this effort is extremely dependent on the capability to transfer the device's fabrication to an industrial setting as well as to ensure interfacing simplicity, namely at the solutions' supply and collection, and actuation such as electric potential application and temperature control. Other advanced applications such as direct interfacing to downstream systems such as mass spectrometry, integration of sensing and feedback controls will require further development in the laboratory. In this review we provide an overview on the field, from basic concepts, through advanced developments both in the theoretical and experimental arenas, and addressing the above details. A comprehensive survey of designs, materials and applications is presented with particular highlights to most recent developments, namely the integration of electrodes, flow control and hyphenation of microfluidic free-flow electrophoresis with other techniques.

Micro free-flow isoelectric focusing with integrated optical pH sensors

Recently, a new observation method for monitoring of pH gradients in microfluidic free-flow electrophoresis has emerged. It is based on the use of chip-integrated fluorescent or luminescent micro sensor layers. These are able to monitor pH gradients in miniaturized separations in real time and spatially resolved; this is particularly useful in isoelectric focusing. Here these multifunctional microdevices that feature continuous separation, monitoring, and in some instances other functionalities, are reviewed. The employed microfabrication procedures to produce these devices are discussed and the different pH sensor matrices that were integrated and their applications in the separation of different types of biomolecules. The procedures for obtaining spatially resolved information about the separated molecules and the pH at the same time and different detection modalities to achieve this such as deep UV fluorescence as well as time-resolved referenced pH sensing and the integration of a precolumn labeling step into these platforms are also highlighted.

Recent advances in protein analysis by capillary and microchip electrophoresis

This review article describes the significant recent advances in the analysis of proteins by capillary and microchip electrophoresis during the period from mid-2014 to early 2017. This review highlights the progressions, new methodologies, innovative instrumental modifications, and challenges for efficient protein analysis in human specimens, animal tissues, and plant samples. The protein analysis fields covered in this review include analysis of native, reduced, and denatured proteins in addition to Western blotting, protein therapeutics and proteomics.

Preliminary results for interval feeding the orthogonal pressurized planar electrochromatography system with sample solution for its preparative separation

The orthogonal pressurized planar electrochromatography (OPPEC) is an example of 2-D separation technique, in which two simultaneous and orthogonal processes of electrophoresis and chromatography are involved in the separation mechanism. In the case of preparative separation of substances characterized by different electrophoretic mobility, such separation system can be constantly fed with the sample solution and the separated components can be constantly collected at its outlet. In the paper, as opposed to the previous studies, we discuss the capabilities of OPPEC technique for preparative separation of substances characterized by the same electrophoretic mobility. According to the proposed solution, the separation system can be periodically fed with the sample solution and separated components can be collected alternately at its outlet. The advantages of this new approach over the column chromatography with regard to the separation of complex mixtures have been signaled.

Direct coupling of a free-flow isotachophoresis (FFITP) device with electrospray ionization mass spectrometry (ESI-MS)

We present the online coupling of a free-flow isotachophoresis (FFITP) device to an electrospray ionization mass spectrometer (ESI-MS) for continuous analysis without extensive sample preparation. Free-flow-electrophoresis techniques are used for continuous electrophoretic separations using an electric field applied perpendicular to the buffer and sample flow, with FFITP using a discontinuous electrolyte system to concurrently focus a target analyte and remove interferences. The online coupling of FFITP to ESI-MS decouples the separation and detection timeframe because the electrophoretic separation takes place perpendicular to the flow direction, which can be beneficial for monitoring (bio)chemical changes and/or extensive MS(n) studies. We demonstrated the coupling of FFITP with ESI-MS for simultaneous concentration of target analytes and sample clean-up. Furthermore, we show hydrodynamic control of the fluidic fraction injected into the MS, allowing for fluidically controlled scanning of the ITP window. Future applications of this approach are expected in monitoring biochemical changes and proteomics.

An analytic description of electrodynamic dispersion in free-flow zone electrophoresis

The present work analyzes the electrodynamic dispersion of sample streams in a free-flow zone electrophoresis (FFZE) chamber resulting due to partial or complete blockage of electroosmotic flow (EOF) across the channel width by the sidewalls of the conduit. This blockage of EOF has been assumed to generate a pressure-driven backflow in the transverse direction for maintaining flow balance in the system. A parallel-plate based FFZE device with the analyte stream located far away from the channel side regions has been considered to simplify the current analysis. Applying a method-of-moments formulation, an analytic expression was derived for the variance of the sample zone at steady state as a function of its position in the separation chamber under these conditions. It has been shown that the increase in stream broadening due to the electrodynamic dispersion phenomenon is additive to the contributions from molecular diffusion and sample injection, and simply modifies the coefficient for the hydrodynamic dispersion term for a fixed lateral migration distance of the sample stream. Moreover, this dispersion mechanism can dominate the overall spatial variance of analyte zones when a significant fraction of the EOF is blocked by the channel sidewalls. The analysis also shows that analyte streams do not undergo any hydrodynamic broadening due to unwanted pressure-driven cross-flows in an FFZE chamber in the absence of a transverse electric field. The noted results have been validated using Monte Carlo simulations which further demonstrate that while the sample concentration profile at the channel outlet approaches a Gaussian distribution only in FFZE chambers substantially longer than the product of the axial pressure-driven velocity and the characteristic diffusion time in the system, the spatial variance of the exiting analyte stream is well described by the Taylor-Aris dispersion limit even in analysis ducts much shorter than this length scale.

Electrophoretic extraction of low molecular weight cationic analytes from sodium dodecyl sulfate containing sample matrices for their direct electrospray ionization mass spectrometry

While the use of sodium dodecyl sulfate (SDS) in separation buffers allows efficient analysis of complex mixtures, its presence in the sample matrix is known to severely interfere with the mass-spectrometric characterization of analyte molecules. In this article, we report a microfluidic device that addresses this analytical challenge by enabling inline electrospray ionization mass spectrometry (ESI-MS) of low molecular weight cationic samples prepared in SDS containing matrices. The functionality of this device relies on the continuous extraction of analyte molecules into an SDS-free solvent stream based on the free-flow zone electrophoresis (FFZE) technique prior to their ESI-MS analysis. The reported extraction was accomplished in our current work in a glass channel with microelectrodes fabricated along its sidewalls to realize the desired electric field. Our experiments show that a key challenge to successfully operating such a device is to suppress the electroosmotically driven fluid circulations generated in its extraction channel that otherwise tend to vigorously mix the liquid streams flowing through this duct. A new coating medium, N-(2-triethoxysilylpropyl) formamide, recently demonstrated by our laboratory to nearly eliminate electroosmotic flow in glass microchannels was employed to address this issue. Applying this surface modifier, we were able to efficiently extract two different peptides, human angiotensin I and MRFA, individually from an SDS containing matrix using the FFZE method and detect them at concentrations down to 3.7 and 6.3 μg/mL, respectively, in samples containing as much as 10 mM SDS. Notice that in addition to greatly reducing the amount of SDS entering the MS instrument, the reported approach allows rapid solvent exchange for facilitating efficient analyte ionization desired in ESI-MS analysis.

Negative-pressure-induced collector for a self-balance free-flow electrophoresis device

Uneven flow in free-flow electrophoresis (FFE) with a gravity-induced fraction collector caused by air bubbles in outlets and/or imbalance of the surface tension of collecting tubes would result in a poor separation. To solve these issues, this work describes a novel collector for FFE. The collector is composed of a self-balance unit, multisoft pipe flow controller, fraction collector, and vacuum pump. A negative pressure induced continuous air flow rapidly flowed through the self-balance unit, taking the background electrolyte and samples into the fraction collector. The developed collector has the following advantages: (i) supplying a stable and harmonious hydrodynamic environment in the separation chamber for FFE separation, (ii) effectively preventing background electrolyte and sample flow-back at the outlet of the chamber and improving the resolution, (iii) increasing the preparative scale of the separation, and (iv) simplifying the operation. In addition, the cost of the FFE device was reduced without using a multichannel peristaltic pump for sample collection. Finally, comparative FFE experiments on dyes, proteins, and cells were carried out. It is evident that the new developed collector could overcome the problems inherent in the previous gravity-induced self-balance collector.

Mathematical and numerical model to study two-dimensional free flow isoelectric focusing

Even though isoelectric focusing (IEF) is a very useful technique for sample concentration and separation, it is challenging to extract separated samples for further processing. Moreover, the continuous sample concentration and separation are not possible in the conventional IEF. To overcome these challenges, free flow IEF (FFIEF) is introduced in which a flow field is applied in the direction perpendicular to the applied electric field. In this study, a mathematical model is developed for FFIEF to understand the roles of flow and electric fields for efficient design of microfluidic chip for continuous separation of proteins from an initial well mixed solution. A finite volume based numerical scheme is implemented to simulate two dimensional FFIEF in a microfluidic chip. Simulation results indicate that a pH gradient forms as samples flow downstream and this pH profile agrees well with experimental results validating our model. In addition, our simulation results predict the experimental behavior of pI markers in a FFIEF microchip. This numerical model is used to predict the separation behavior of two proteins (serum albumin and cardiac troponin I) in a two-dimensional straight microchip. The effect of electric field is investigated for continuous separation of proteins. Moreover, a new channel design is presented to increase the separation resolution by introducing cross-stream flow velocity. Numerical results indicate that the separation resolution can be improved by three folds in this new design compare to the conventional straight channel design.

Ion concentration polarization-based continuous separation device using electrical repulsion in the depletion region

We proposed a novel separation method, which is the first report using ion concentration polarization (ICP) to separate particles continuously. We analyzed the electrical forces that cause the repulsion of particles in the depletion region formed by ICP. Using the electrical repulsion, micro- and nano-sized particles were separated based on their electrophoretic mobilities. Because the separation of particles was performed using a strong electric field in the depletion region without the use of internal electrodes, it offers the advantages of simple, low-cost device fabrication and bubble-free operation compared with conventional continuous electrophoretic separation methods, such as miniaturizing free-flow electrophoresis (μ-FFE). This separation device is expected to be a useful tool for separating various biochemical samples, including cells, proteins, DNAs and even ions.

Free-flow electrophoresis in proteome sample preparation

An aim of proteome research is to identify the entire complement of proteins expressed in defined cell types of humans, animals, plants, and microorganisms. The approach requires searching for low abundant or even rarely expressed proteins in many cell types, as well as the determination of the protein expression levels in subcellular compartments and organelles. In recent years, rather powerful MS technologies have been developed. At this stage of MS device development, it is of highest interest to purify intact cell types or isolate subcellular compartments, where the proteins of interest are originating from, which determine the final composition of a peptide mixture. Free-flow electrophoresis proved to be useful to prepare meaningful peptide mixtures because of its improved capabilities in particle electrophoresis and the enhanced resolution in protein separation. Sample preparation by free-flow electrophoresis mediated particle separation was preferentially performed for purification of either organelles and their subspecies or major protein complexes. Especially, the introduction of isotachophoresis and interval zone electrophoresis improved the purity of the gained analytes of interest. In addition, free-flow IEF proved to be helpful, when proteins of low solubility, obtained, e.g. from cell membranes, were investigated.

Vector separation of particles and cells using an array of slanted open cavities

We present a microfluidic platform for the continuous separation of suspended particles based on their size and settling velocity. The separation method takes advantage of the flow field in the vicinity and inside slanted open cavities. These cavities induce flow along them, which deflects the suspended particles to a different degree depending on the extent to which they penetrate into the cavities. The cumulative deflection in the periodic array ultimately leads to vector chromatography, with the different species in the sample moving in different directions. We demonstrate density and size based separation over a range of flow rates by separating polystyrene and silica particles and show that purities nearing 100% can be achieved for multicomponent mixtures. We also demonstrate the potential of the platform to separate biological cells by fractionating different blood components. We discuss the presence of two regimes, depending on the ratio between the settling velocity and the velocity of the particles across the open cavities. The proposed platform could also integrate additional separative force fields in the direction normal to the plane of the cavities to fractionate specific mixtures based on the distinguishing properties of the component species.

Isolation of monodisperse nanodisc-reconstituted membrane proteins using free flow electrophoresis

Free flow electrophoresis is used for rapid and high-recovery isolation of homogeneous preparations of functionally active membrane proteins inserted into nanodiscs. The approach enables isolation of integral and membrane anchored proteins and is also applicable following introduction of, e.g., fluorescent tags. Preparative separation of membrane protein loaded nanodiscs from empty nanodiscs and protein aggregates results in monodisperse nanodisc preparations ideal for structural and functional characterization using biophysical methods.

Assessing heterogeneity of peroxisomes: isolation of two subpopulations from rat liver

Peroxisomes exhibit a heterogeneous morphological appearance in rat liver tissue. In this respect, the isolation and subsequent biochemical characterization of peroxisome species from different subcellular prefractions should help to solve the question of whether peroxisomes indeed diverge into functionally specialized subgroups in one tissue. As a means to address this question, we provide a detailed separation protocol for the isolation of peroxisomes from both the light (LM-Po) and the heavy (HM-Po) mitochondrial prefraction for their subsequent comparative analysis. Both isolation strategies rely on centrifugation in individually adapted Optiprep gradients. In case of the heavy mitochondrial fraction, free flow electrophoresis is appended as an additional separation step to yield peroxisomes of sufficient purity. In view of their morphology, peroxisomes isolated from both fractions are surrounded by a continuous single membrane and contain a gray-opaque inner matrix. However, beyond this overall similar appearance, HM-Po exhibit a smaller average diameter, float at lower density, and show a more negative average membrane charge when compared to LM-Po.