Free-flow zone electrophoresis of peptides and proteins in PDMS microchip for narrow pI range sample prefractionation coupled with mass spectrometry

Assessing Surface Adsorption in Cyclic Olefin Copolymer Microfluidic Devices Using Two-Dimensional Nano Liquid Chromatography-Micro Free Flow Electrophoresis Separations

Surface interactions are a concern in microscale separations, where analyte adsorption can decrease the speed, sensitivity, and resolution otherwise achieved by miniaturization. Here, we functionally characterize the surface adsorption of hot-embossed cyclic olefin copolymer (COC) micro free-flow electrophoresis (μFFE) devices using two-dimensional nLC × μFFE separations, which introduce a 3- to 5 s plug of analyte into the device and measure temporal broadening that arises from surface interactions. COC is an attractive material for microfluidic devices, but little is known about its potential for surface adsorption in applications with continuous fluid flow and temporal measurements. Adsorption was minimal for three small molecule dyes: positively charged rhodamine 123, negatively charged fluorescein, and neutral rhodamine 110. Temporal peak widths for the three dyes ranged from 3 to 7 s and did not change significantly with increasing transit distance. Moderate adsorption was observed for Chromeo P503-labeled myoglobin and cytochrome c with temporal peak widths around 20 s. Overall, the COC surface adsorption was low compared to traditional glass devices, where peak widths are on the order of minutes. Improvements in durability, long-term performance, and ease of fabrication, combined with low overall adsorption, make the COC μFFE devices a practical choice for applications involving time-resolved continuous detection.

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.

Mathematical modeling and process analytical technology for continuous chromatography of biopharmaceutical products

Chromatography is a widely used separation method that is inherently a batch operation. However, the demand for higher productivity and lower cost and labor has prompted industries such as the petrochemical and food industries to transition from batch to continuous chromatography. Growing market competition in the biopharmaceutical industry and the rise of novel biotherapeutics modalities have brought about promising continuous chromatography methods as well as next-generation tools to enable continuous operation in bioprocessing. While these continuous chromatography methods outperform their batch counterpart, their implementation presents challenges due to their greater complexity. This review discusses two key technologies that are essential for the implementation of continuous chromatography: mathematical modeling and novel process analytical technologies. Mechanistic-based models not only aid in process development and optimization but also allow for greater process control and automation.

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.

Reciprocating free-flow isoelectric focusing with online array ultraviolet detector for process monitoring of protein separation

Free-flow isoelectric focusing (FFIEF) is a useful tool for separating and purifying proteins, DNA, cells, and organelles, etc. However, the online monitoring of each fraction during an FFIEF run has not been achieved yet, resulting in a lack of process monitoring of FFIEF. Herein, an online array ultraviolet (UV) detection system was developed for the easy assay of FFE fractions. The detector was integrated with an apparatus of FFIEF with 32 fractions to show the online monitoring, and bovine serum albumin (BSA) and lysozyme were chosen as the model proteins for manifesting the UV detector performance. The experiments revealed that (i) all the fluidic cells had good linearity from 0.03 to 10 mg/mL BSA and fair limits of detection (LODs) of 0.01 mg/mL; (ii) all the cells had good uniformity of UV absorbance; and (iii) the deviations of intra-day and inter-day of UV detector were respectively 3.8% and 5.8%, indicating the fair stability of the UV detector. The UV detector could be well used for the process monitoring of two model proteins through the whole FFIEF run, and the online absorbance assay of proteins at the end of FFIEF. The UV detector herein had the evident potential for rapid and convenient assay of protein fraction in FFIEF as well as other FFE modes.

Microchip Free-Flow Electrophoresis for Bioanalysis, Sensing, and Purification

The separation of complex mixtures is ubiquitous throughout molecular biology, and techniques such as gel-based electrophoresis are common laboratory practice. Such methods are not without their drawbacks, however, which include non-specific interactions between analyte and the separation matrix, poor yields in purification and non-continuous analyte throughput. Microfluidic techniques, which exploit physical phenomena unique to the microscale, promise to improve many aspects of traditional laboratory procedures. These methods offer a quantitative, solution-based alternative to traditional gel electrophoresis, with rapid measurement times enabling the analysis of transient or weak biomolecular interactions that would be challenging to observe with traditional methods. Here, we present a protocol for the lithographic fabrication and operation of microfluidic chips capable of free-flow electrophoretic (FFE) fractionation and analysis of biological analytes. We demonstrate the efficacy of our approach through a protein-sensing methodology based on FFE fractionation of DNA-protein mixtures. In addition, the FFE technique described here can be readily adapted to suit a variety of preparative and analytical applications, providing information on the charge, zeta-potential, and interactions of analytes.

Recent advances in microchip enantioseparation and analysis

This review summarizes recent developments (over the past decade) in the field of microfluidics-based solutions for enantiomeric separation and detection. The progress in various formats of microchip electrodriven separations, such as MCE, microchip electrochromatography, and multidimensional separation techniques, is discussed. Innovations covering chiral stationary phases, surface coatings, and modification strategies to improve resolution, as well as integration with detection systems, are reported. Finally, combinations with other microfluidic functional units are also presented and highlighted.

Microfluidic approaches for the analysis of protein-protein interactions in solution

Exploration and characterisation of the human proteome is a key objective enabling a heightened understanding of biological function, malfunction and pharmaceutical design. Since proteins typically exhibit their behaviour by binding to other proteins, the challenge of probing protein-protein interactions has been the focus of new and improved experimental approaches. Here, we review recently developed microfluidic techniques for the study and quantification of protein-protein interactions. We focus on methodologies that utilise the inherent strength of microfluidics for the control of mass transport on the micron scale, to facilitate surface and membrane-free interrogation and quantification of interacting proteins. Thus, the microfluidic tools described here provide the capability to yield insights on protein-protein interactions under physiological conditions. We first discuss the defining principles of microfluidics, and methods for the analysis of protein-protein interactions that utilise the diffusion-controlled mixing characteristic of fluids at the microscale. We then describe techniques that employ electrophoretic forces to manipulate and fractionate interacting protein systems for their biophysical characterisation, before discussing strategies that use microdroplet compartmentalisation for the analysis of protein interactions. We conclude by highlighting future directions for the field, such as the integration of microfluidic experiments into high-throughput workflows for the investigation of protein interaction networks.

Visualization of Streams of Small Organic Molecules in Continuous-Flow Electrophoresis

Continuous-flow electrophoresis (CFE) separates a stream of a multicomponent mixture into multiple streams of individual components inside a thin rectangular chamber. CFE will be able to benefit flow chemistry when it is both compatible with nonaqueous solvents utilized in organic synthesis and capable of generically detecting streams of small organic molecules. While stable nonaqueous CFE has been demonstrated, generically detecting molecular streams has not been achieved yet. Here we propose a general approach for molecular stream visualization in CFE via analyte-caused obstruction of excitation of a fluorescent layer underneath the separation chamber-fluorescent sublayer-based visualization (FSV). The concept of FSC-based visualization has been adapted from visualization of small organic molecules on fluorescent plates in thin-layer chromatography. We designed and fabricated a CFE device with one side made of quartz and another side made of UV-absorbing visibly fluorescent, chemically inert, machinable plastic. This device was demonstrated to support nonaqueous CFE of small organic molecules and quantitative detection of their streams in real-time with a limit of detection below 100 μM. Thus, CFE may satisfy conditions required for its seamless integration with continuous flow organic synthesis in flow chemistry.

Microfluidic Free-Flow Isoelectric Focusing with Real-Time pI Determination

Free-flow electrophoresis (FFE) may be used for continuous and preparative separation of a wide variety of biomolecules. Isoelectric focusing (IEF) provides for the separation of compounds according to their isoelectric point (pI). Here we describe a microfluidic chip-based protocol for the fabrication, application, and optical monitoring of free-flow isoelectric focusing (FFIEF) of proteins and peptides on the microscale with optical surveillance of the microscopic pH gradient provided by an integrated pH sensing layer. This protocol may be used with modifications also for the FFIEF of other biomolecules and may serve as template for the fabrication of microfluidic chips with integrated fluorescent or luminescent pH sensor layers for FFE and other applications.

On-line microchip electrophoresis-mediated preconcentration of cationic compounds utilizing cationic polyacrylamide gels fabricated by in situ photopolymerization

A simple and efficient method was developed for the fabrication of a cationic sample preconcentrator on a channel of a commercial poly(methyl methacrylate) (PMMA) microchip.

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.

On-chip label-free protein analysis with downstream electrodes for direct removal of electrolysis products

The ability to apply highly controlled electric fields within microfluidic devices is valuable as a basis for preparative and analytical processes. A challenge encountered in the context of such approaches in conductive media, including aqueous buffers, is the generation of electrolysis products at the electrode/liquid interface which can lead to contamination, perturb fluid flows and generally interfere with the measurement process. Here, we address this challenge by designing a single layer microfluidic device architecture where the electric potential is applied outside and downstream of the microfluidic device while the field is propagated back to the chip via the use of a co-flowing highly conductive electrolyte solution that forms a stable interface at the separation region of the device. The co-flowing electrolyte ensures that all the generated electrolysis products, including Joule heat and gaseous products, are flowed away from the chip without coming into contact with the analytes while the single layer fabrication process where all the structures are defined lithographically allows producing the devices in a simple yet highly reproducible manner. We demonstrate that by allowing stable and effective application of electric fields in excess of 100 V cm^-1, the described platform provides the basis for rapid separation of heterogeneous mixtures of proteins and protein complexes directly in their native buffers as well as for the simultaneous quantification of their charge states. We illustrate this by probing the interactions in a mixture of an amyloid forming protein, amyloid-β, and a molecular chaperone, Brichos, known to inhibit the process of amyloid formation. The availability of a platform for applying stable electric fields and its compatibility with single-layer soft-lithography processes opens up the possibility of separating and analysing a wide range of molecules on chip, including those with similar electrophoretic mobilities.

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.

High-Speed, Comprehensive, Two Dimensional Separations of Peptides and Small Molecule Biological Amines Using Capillary Electrophoresis Coupled with Micro Free Flow Electrophoresis

Two-dimensional (2D) separations are able to generate significantly higher peak capacities than their one-dimensional counterparts. Unfortunately, current hyphenated 2D separations are limited by the speed of the second dimension separation and the consequent loss of peak capacity due to under sampling of peaks as they elute from the first dimension separation. Continuous micro free flow electrophoresis (μFFE) separations eliminate under sampling as a limitation when incorporated as the second dimension of a 2D separation. In the current manuscript we describe the first coupling of capillary electrophoresis (CE) with μFFE to perform 2D CE × μFFE separations. The CE separation capillary was directly inserted into the μFFE separation channel using an edge on interface. Analyte peaks streamed directly into the μFFE separation channel as they migrated off the CE capillary. No complicated injection, valving, or voltage changes were necessary to couple the two separation modes. 2D CE × μFFE generated an ideal peak capacity of 2 592 in a 9 min separation of fluorescently labeled peptides (7.6 min separation window, 342 peaks/min). Data points were recorded every 250-500 ms (>8 data points/peak), effectively eliminating under sampling as a source of band broadening. CE × μFFE generated an ideal peak capacity of 1885 in a 2.7 min separation of fluorescently labeled small molecule bioamines (1.8 min separation window, 1053 peaks/min). Peaks in the 2D CE × μFFE separation of peptides covered 30% of the available separation space, resulting in a corrected peak capacity of 778 (102 peaks/min). The fractional coverage of the 2D CE × μFFE separation of small molecule bioamines was 20%, resulting in a corrected peak capacity of 377 (209 peaks/min).

Hyphenation of Production-Scale Free-Flow Electrophoresis to Electrospray Ionization Mass Spectrometry Using a Highly Conductive Background Electrolyte

In this technical note, we demonstrate the hyphenation of production-scale free-flow electrophoresis (FFE) and sheathless electrospray ionization mass spectrometry (ESI-MS). In contrast to previous hyphenation approaches, we used a highly conductive background electrolyte (BGE) required for production-scale FFE. We found that this kind of BGE as well as a production-scale setup leads to significant electric interference between FFE and MS. This interference prevents steady-state FFE operation. We examine this interference in detail and discuss possible solutions to this issue. We demonstrate that the straightforward grounding of the transfer line removes the influence of ESI-MS on FFE, but creates a current leak from the ESI interface, which adversely affects the ESI spray. Furthermore, we show that only the electrical disconnection of the ESI probe from the FFE-MS transfer line suppresses this undesirable current. In order to facilitate the electrical disconnection we used a low conductivity, silica-based ESI probe with withdrawn inner capillary. This approach allowed the interference-free hyphenation of production-scale FFE (using a highly conductive BGE) with ESI-MS.

Continuous on-chip fluorescence labelling, free-flow isoelectric focusing and marker-free isoelectric point determination of proteins and peptides

We present a microfluidic platform that contains a micro flow reactor for on-chip biomolecule labelling that is directly followed by a separation bed for continuous free-flow electrophoresis and has an integrated hydrogel-based near-infrared fluorescent pH sensor layer. Using this assembly, labelling of protein and peptide mixtures, their separation via free-flow isoelectric focusing and the determination of the isoelectric point (pI) of the separated products via the integrated sensor layer could be carried out within typically around 5 minutes. Spatially-resolved immobilization of fluidic and sensing structures was carried out via multistep photolithography. The assembly was characterized and optimized with respect to their fluidic and pH sensing properties and applied in the IEF of model proteins, peptides and a tryptic digest from physalaemine. We have therefore realized continuous sample preparation and preparative separation, analyte detection, process observation and analyte assignment capability based on pI on a single platform the size of a microscope slide.

Label-free microfluidic free-flow isoelectric focusing, pH gradient sensing and near real-time isoelectric point determination of biomolecules and blood plasma fractions

Continuous biomolecular separation and pH gradient observation using UV and NIR fluorescence.

Cation-selective electropreconcentration

A cation-selective microfluidic sample preconcentration system is described. The cation sample was electropreconcentrated using a reversed-direction electroosmotic flow (EOF) and an anion-permselective filter, where an electric double layer (EDL) overlap condition existed. The anion-permselective filter between microchannels was fabricated by three different methods: 1) extending a positively charged, nanoporous, polymer membrane by photopolymerization of poly(diallyldimethylammonium chloride) (PDADMAC); 2) etching a nanochannel and then coating it with a positively-charged monomer, N-[3-(trimethoxysilyl)propyl]-N'-(4-vinylbenzyl)ethylenediamine hydrochloride (TMSVE); and, 3) etching a nanochannel and then coating it with a positively-charged, pre-formed polymer, polyE-323. The EOF direction in the microchannel was reversed by both TMSVE and polyE-323 coatings. The cation-selective preconcentration was investigated using charged fluorescent dyes and tetramethylrhodamine isothiocyanate (TRITC)-tagged peptides/proteins. The preconcentration in the three different systems was compared with respect to efficiency, dependence on buffer concentration and pH, tolerable flow rate, and sample adsorption. Both TMSVE- and polyE-323-coated nanochannels showed robust preconcentration at high flow rates, whereas the PDADMAC membrane maintained anion-permselectivity at higher buffer concentrations. The TMSVE-coated nanochannels showed a more stable preconcentration process, whereas the polyE-323-coated nanochannels showed a lower peptide sample adsorption and robust efficiency under a wide range of buffer pHs. The system described here can potentially be used for the preconcentration of cationic peptides/proteins on microfluidic devices for subsequent analyses.

Tunable membranes for free-flow zone electrophoresis in PDMS microchip using guided self-assembly of silica microbeads

In this paper, we evaluate the strategy of using self-assembled microbeads to build a robust and tunable membrane for free-flow zone electrophoresis in a PDMS microfluidic chip. To fabricate a porous membrane as a salt bridge for free-flow zone electrophoresis, we used silica or polystyrene microbeads between 3-6 μm in diameter and packed them inside a microchannel. After complete evaporation, we infiltrated the porous microbead structure with a positively or negatively charged hydrogel to modify its surface charge polarity. Using this device, we demonstrated binary sorting (separation of positive and negative species at a given pH) of peptides and dyes in standard buffer systems without using sheath flows. The sample loss during sorting could be minimized by using ion selectivity of hydrogel-infiltrated microbead membranes. Our fabrication method enables building a robust membrane for pressure-driven free-flow zone electrophoresis with tunable pore size as well as surface charge polarity.

The potential of electrophoretic sample pretreatment techniques and new instrumentation for bioanalysis, with a focus on peptidomics and metabolomics

This Review highlights the potential of new electromigration-based sample pretreatment techniques for bioanalysis. Sample pretreatment is a challenging part of the analytical workflow, especially in the fields of peptidomics and metabolomics, where the analytes are very diverse, both in physicochemical properties and in endogenous concentration. Electromigration-based techniques have several strengths, such as fast selective analyte concentration and that complementary information on the content of a sample can be obtained when compared with more conventional (chromatography-based) techniques. In the past decade, various new electromigration-based sample pretreatment techniques have been developed, and importantly, new instrumental setups. In this Review, we provide an introduction on electromigration and its strengths. Then, selected examples of electromigration-based sample pretreatment techniques and instrumentation are discussed, namely free-flow electrophoresis, isoelectric focusing, isotachophoresis, electrodialysis, electromembrane extraction and electroextraction. Finally, the promising perspectives of electromigration-based sample pretreatment techniques are outlined.

Label-free real-time imaging in microchip free-flow electrophoresis applying high speed deep UV fluorescence scanning

We report on label-free monitoring of microfluidic free-flow electrophoresis (μFFE) separations in real-time using a custom built high speed deep UV laser scanner. In combination with a novel layout realized in fused silica (FS) FFE chips the setup was successfully applied for continuous separations and detection of unlabeled analytes including native proteins by space-resolved intrinsic deep UV fluorescence scanning.

Liquid-phase-based separation systems for depletion, prefractionation and enrichment of proteins in biological fluids and matrices for in-depth proteomics analysis--an update covering the period 2008-2011

This review article expands on the previous one (Jmeian, Y. and El Rassi, Z. Electrophoresis 2009, 30, 249-261) by reviewing pertinent literature in the period extending from early 2008 to the present. Similar to the previous review article, the present one is concerned with proteomic sample preparation (e.g. depletion of high-abundance proteins, reduction of the protein dynamic concentration range, enrichment of a particular subproteome), and the subsequent chromatographic and/or electrophoretic prefractionation prior to peptide separation and identification by LC-MS/MS. This review article differs from the first version published in Electrophoresis 2009, 30, 249-261 by expanding on capturing/enriching subglycoproteomics by lectin affinity chromatography. Ninety-eight articles published in the period extending from early 2008 to the present have been reviewed. By no means is this review article exhaustive: its aim is to give a concise report on the latest developments in the field.

Microfluidic concentration of bacteria by on-chip electrophoresis

In this contribution, we present a system for efficient preconcentration of pathogens without affecting their viability. Development of miniaturized molecular diagnostic kits requires concentration of the sample, molecule extraction, amplification, and detection. In consequence of low analyte concentrations in real-world samples, preconcentration is a critical step within this workflow. Bacteria and viruses exhibit a negative surface charge and thus can be electrophoretically captured from a continuous flow. The concept of phaseguides was applied to define gel membranes, which enable effective and reversible collection of the target species. E. coli of the strains XL1-blue and K12 were used to evaluate the performance of the device. By suppression of the electroosmotic flow both strains were captured with efficiencies of up to 99%. At a continuous flow of 15 μl/min concentration factors of 50.17 ± 2.23 and 47.36 ± 1.72 were achieved in less than 27 min for XL1-blue and K12, respectively. These results indicate that free flow electrophoresis enables efficient concentration of bacteria and the presented device can contribute to rapid analyses of swab-derived samples.

[Progress in micro free flow electrophoresis]

Micro free flow electrophoresis (micro-FFE) is a continuous micro-separation or preparation technique, which has been applied in the analysis of biomolecules, such as cells, sub cell organics and proteins. In this review, the recent progresses in micro FFE are summarized, with emphasis on the design of microchips, the separation modes and the applications of micro-FFE. Furthermore, the further developments of micro-FFE are prospected.

PDMS free-flow electrophoresis chips with integrated partitioning bars for bubble segregation

In this work, a microfluidic free-flow electrophoresis device with a novel approach for preventing gas bubbles from entering the separation area is presented. This is achieved by integrating partitioning bars to reduce the channel depth between electrode channels and separation chamber in order to obtain electrical contact and simultaneously prevent bubbles from entering the separation area. The three-layer sandwich chip features a reusable carrier plate with integrated ports for fluidic connection combined with a softlithographically cast microfluidic PDMS layer and a sealing glass slide. This design allows for a straightforward and rapid chip prototyping process. The performance of the device is demonstrated by free-flow zone electrophoretic separations of fluorescent dye mixtures as well as by the separation of labeled amines and amino acids with separation voltages up to 297 V.