Effects of carbon dioxide on peak mode isotachophoresis: simultaneous preconcentration and separation

Detecting Isotachophoresis Zones Hidden in Noise Using Signal Processing

Lowering the limit of detection in chemical or biochemical analysis is key to extending the application scope of sensing schemes. Usually, this is related to an increased instrumentation effort, which in turn precludes many commercial applications. We demonstrate that the signal-to-noise ratio of isotachophoresis-based microfluidic sensing schemes can be substantially increased merely by postprocessing of recorded signals. This becomes possible by exploiting knowledge about the physics of the underlying measurement process. The implementation of our method is based on microfluidic isotachophoresis and fluorescence detection, for which we take advantage of the physics of electrophoretic sample transport and the structure of noise in the imaging process. We demonstrate that by processing only 200 images, the detectable concentration, compared to the detection from a single image, is already lowered by 2 orders of magnitude without any additional instrumentation effort. Furthermore, we show that the signal-to-noise ratio is proportional to the square root of the number of fluorescence images, which leaves room for further lowering of the detection limit. In the future, our results could be relevant for various applications where the detection of minute sample amounts plays a role.

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.

Potential of microchip electrophoresis in pharmaceutical analysis: Development of a universal method for frequently prescribed nonsteroidal anti-inflammatory drugs

A novel microchip electrophoresis method with conductivity detection for the determination of nonsteroidal anti-inflammatory drugs (NSAIDs) in several pharmaceutical formulations was developed. The three frequently used NSAIDs - acetylsalicylic acid, diclofenac and ibuprofen were baseline separated on a poly(methyl methacrylate) microchip with coupled separation channels. Elimination of matrix components such as excipients, was realized through online combination of isotachophoresis (ITP) with zone electrophoresis (ZE). ITP-ZE hyphenation can also facilitate preconcentration of target analytes. ITP was carried out in the first separation channel at pH 6.5, while the second channel of the microchip was used for ZE separation and detection of the analytes at pH 7.0. The developed ITP-ZE method was demonstrated to be applicable for direct and reliable determination of NSAIDs in eleven pharmaceutical formulations. The noticeable advantage of this approach is that only simple sample pretreatment (filtration and dilution) is necessary. The method performance parameters, such as linearity (20-250% of nominal concentration of studied NSAIDs in the test samples), accuracy (98-102%) and precision (less than 2% RSD) were obtained. This universal approach is suitable for the determination of frequently used NSAIDs in a single analysis in less than 15 min. In addition to simple sample pretreatment, low running costs and minimal environmental impact could make this method attractive for the analysis of pharmaceutical preparations.

Instabilities of the pH gradient in carrier ampholyte-based isoelectric focusing: Elucidation of the contributing electrokinetic processes by computer simulation

Electrokinetic processes that lead to pH gradient instabilities in carrier ampholyte-based IEF are reviewed. In addition to electroosmosis, there are four of electrophoretic nature, namely (i) the stabilizing phase with the plateau phenomenon, (ii) the gradual isotachophoretic loss of carrier ampholytes at the two column ends in presence of electrode solutions, (iii) the inequality of the mobilities of positively and negatively charged species of ampholytes, and (iv) the continuous penetration of carbonate from the catholyte into the focusing column. The impact of these factors to cathodic and anodic drifts was analyzed by simulation of carrier ampholyte-based focusing in closed and open columns. Focusing under realistic conditions within a 5 cm long capillary in which three amphoteric low molecular mass dyes were focused in a pH 3-10 gradient formed by 140 carrier ampholytes was investigated. In open columns, electroosmosis displaces the entire gradient toward the cathode or anode whereas the electrophoretic processes act bidirectionally with a transition around pH 4 (drifts for pI > 4 and pI < 4 typically toward the cathode and anode, respectively). The data illustrate that focused zones of carrier ampholytes have an electrophoretic flux and that dynamic simulation can be effectively used to assess the magnitude of each of the electrokinetic destabilizing factors and the resulting drift for a combination of these effects. Predicted drifts of focused marker dyes are compared to those observed experimentally in a setup with coated capillary and whole column optical imaging.

Simultaneous Preconcentration and Separation of Native Protein Variants Using Thermal Gel Electrophoresis

Proteins must maintain proper folding conformations and express the correct post-translational modifications (PTMs) to exhibit appropriate biological activity. However, assessing protein folding and PTMs is difficult because routine polyacrylamide gel electrophoresis (PAGE) methods lack the separation resolution necessary to identify variants of a single protein. Additionally, standard PAGE denatures proteins prior to analysis precluding determinations of folding states or PTMs. To overcome these limitations, a microfluidic thermal gel electrophoresis platform was developed to provide high-sensitivity, high-resolution analyses of native protein variants. A thermally reversible gel was utilized as a separation matrix while in its solid state (30 °C). This thermal gel provided sufficient separation resolution to identify three variants of a fluorescently labeled model protein. To increase detection sensitivity, analyte preconcentration was conducted in parallel with the separation. Continuous analyte enrichment afforded detection limits of 500 fg of protein (250 pM) while simultaneous baseline separation resolution was achieved between variants. The effects of temperature on thermal gel electrophoresis were also characterized. The unique temperature-dependent outcomes illustrated how method performance can be tuned through a thermal dimension. Ultimately, the high detection sensitivity and separation resolution provided by thermal gel electrophoresis enabled rapid screening of native protein variants.

Multimaterial 3D Printed Fluidic Device for Measuring Pharmaceuticals in Biological Fluids

Multimaterial 3D printing provides a unique capability for the creation of highly complex integrated devices where complementary functionality is realized using differences in material properties. Using a single and automated print process, microfluidic devices were fabricated containing (i) an optically transparent structure for fluorescence detection, (ii) electrodes for electrokinetic transport, (iii) a primary membrane to remove particulates and macromolecules including proteins, and (iv) a secondary membrane to concentrate small molecule targets. The device was used for the simultaneous extraction and concentration of small molecule pharmaceuticals from urine, which was followed by an on-chip electrophoretic separation of the concentrated targets for quantitative analysis. Owing to the high level of functional integration inside the device, manual handling was minimal and restricted to the introduction of the sample and buffer solutions. The 3D printed sample-in/answer-out device allowed the direct quantification of ampicillin-a small molecule pharmaceutical-in untreated urine within 3 min, down to 2 ppm. These results demonstrate the potential of 3D printing for on-demand fabrication of disposable, functionally integrated devices for low-cost point-of-collection (POC) diagnostics.

Semiquantitative Nucleic Acid Test with Simultaneous Isotachophoretic Extraction and Amplification

Nucleic acid amplification tests (NAATs) provide high diagnostic accuracy for infectious diseases and quantitative results for monitoring viral infections. The majority of NAATs require complex equipment, cold chain dependent reagents, and skilled technicians to perform the tests. This largely confines NAATs to centralized laboratories and can significantly delay appropriate patient care. Low-cost, point-of-care (POC) NAATs are especially needed in low-resource settings to provide patients with diagnosis and treatment planning in a single visit to improve patient care. In this work, we present a rapid POC NAAT with integrated sample preparation and amplification using electrokinetics and paper substrates. We use simultaneous isotachophoresis (ITP) and recombinase polymerase amplification (RPA) to rapidly extract, amplify, and detect target nucleic acids from serum and whole blood in a paper-based format. We demonstrate simultaneous ITP and RPA can consistently detect 5 copies per reaction in buffer and 10 000 copies per milliliter of human serum with no intermediate user steps. We also show preliminary extraction and amplification of DNA from whole blood samples. Our test is rapid (results in less than 20 min) and made from low-cost materials, indicating its potential for detecting infectious diseases and monitoring viral infections at the POC in low resource settings.

Focusing analytes from 50 μL into 500 pL: On-chip focusing from large sample volumes using isotachophoresis

The use of on-chip isotachophoresis assays for diagnostic applications is often limited by the small volumes of standard microfluidic channels. Overcoming this limitation is particularly important for detection of 'discrete' biological targets (such as bacteria) at low concentrations, where the volume of processed liquid in a standard microchannel might not contain any targets. We present a novel microfluidic chip that enables ITP focusing of target analytes from initial sample volumes of 50 μL into a concentrated zone with a volume of 500 pL, corresponding to a 100,000-fold increase in mean concentration, and a 300,000-fold increase in peak concentration. We present design considerations for limiting sample dispersion in such large-volume focusing (LVF) chips and discuss the trade-off between assay time and Joule heating, which ultimately governs the scalability of LVF designs. Finally, we demonstrate a 100-fold improvement of ITP focusing performance in the LVF chip as compared to conventional microchannels, and apply this enhancement to achieve highly sensitive detection of both molecular targets (DNA, down to 10 fM) and whole bacteria (down to 100 cfu/mL).

Charge-dependent transport switching of single molecular ions in a weak polyelectrolyte multilayer

The tunable nature of weak polyelectrolyte multilayers makes them ideal candidates for drug loading and delivery, water filtration, and separations, yet the lateral transport of charged molecules in these systems remains largely unexplored at the single molecule level. We report the direct measurement of the charge-dependent, pH-tunable, multimodal interaction of single charged molecules with a weak polyelectrolyte multilayer thin film, a 10 bilayer film of poly(acrylic acid) and poly(allylamine hydrochloride) PAA/PAH. Using fluorescence microscopy and single-molecule tracking, two modes of interaction were detected: (1) adsorption, characterized by the molecule remaining immobilized in a subresolution region and (2) diffusion trajectories characteristic of hopping (D ∼ 10(-9) cm(2)/s). Radius of gyration evolution analysis and comparison with simulated trajectories confirmed the coexistence of the two transport modes in the same single molecule trajectories. A mechanistic explanation for the probe and condition mediated dynamics is proposed based on a combination of electrostatics and a reversible, pH-induced alteration of the nanoscopic structure of the film. Our results are in good agreement with ensemble studies conducted on similar films, confirm a previously-unobserved hopping mechanism for charged molecules in polyelectrolyte multilayers, and demonstrate that single molecule spectroscopy can offer mechanistic insight into the role of electrostatics and nanoscale tunability of transport in weak polyelectrolyte multilayers.

Isotachophoresis with emulsions

An experimental study on isotachophoresis (ITP) in which an emulsion is used as leading electrolyte (LE) is reported. The study aims at giving an overview about the transport and flow phenomena occurring in that context. Generally, it is observed that the oil droplets initially dispersed in the LE are collected at the ITP transition zone and advected along with it. The detailed behavior at the transition zone depends on whether or not surfactants (polyvinylpyrrolidon, PVP) are added to the electrolytes. In a system without surfactants, coalescence is observed between the droplets collected at the ITP transition zone. After having achieved a certain size, the droplets merge with the channel walls, leaving an oil film behind. In systems with PVP, coalescence is largely suppressed and no merging of droplets with the channel walls is observed. Instead, at the ITP transition zone, a droplet agglomerate of increasing size is formed. In the initial stages of the ITP experiments, two counter rotating vortices are formed inside the terminating electrolyte. The vortex formation is qualitatively explained based on a hydrodynamic instability triggered by fluctuations of the number density of oil droplets.

A method for non-invasive full-field imaging and quantification of chemical species

We present a novel method for full-field scalar visualization and quantification of species concentration fields. We term this method species-altered fluorescence imaging (SAFI). The method employs electrically neutral fluorescent dyes whose quantum yields are selectively quenched or enhanced by species of interest. SAFI enables simultaneous imaging of material interfaces and provides non-invasive, scalar-field quantitation of two-dimensional species concentration fields. We describe criteria for choosing SAFI dyes and tabulate 35 promising SAFI dyes and their relevant properties. Next, we describe species concentration quantification with SAFI via Stern-Volmer quenching and discuss the sensitivity and resolution of our method. We demonstrate this method with two dyes, 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 10-(3-sulfopropyl)acridinium betaine (SAB). We demonstrate our method in full-field visualization of several challenging electrokinetic flows: isotachophoresis (ITP) in both cationic and anionic modes, and in a convective electrokinetic instability (EKI) flow. Through these experiments we collectively quantify ion concentration shock velocities, simultaneously measure concentrations of five species, and quantify the development of an unsteady, chaotic, 2D flow.

Dual-channel bipolar electrode focusing: simultaneous separation and enrichment of both anions and cations

In this paper we show that a microelectrochemical cell comprising two parallel microchannels spanned by a single bipolar electrode can be used to simultaneously enrich and separate both anions and cations within a single microchannel. This is possible because reduction and oxidation of water at the cathodic and anodic poles of the bipolar electrode, respectively, lead to ion depletion zones. Specifically, TrisH(+) is neutralized by OH(-) at the cathodic pole, while acetate buffer is neutralized by H(+) at the anodic pole. This action creates a local electric field gradient having both positive and negative components, and hence positive and negative ions follow their respective field gradients leading to separation. In the presence of an opposing counter-flow (pressure driven flow in this case), enrichment also occurs. In addition to separation and enrichment in a single channel, it is also possible to simultaneously enrich cations in one microchannel and anions in the other. Enrichment is achieved by controlling experimental parameters, including the type of buffer and the direction and magnitude of the opposing counter-flow.

Limits of miniaturization: assessing ITP performance in sub-micron and nanochannels

The feasibility of isotachophoresis in channels of sub micrometer and nanometer dimension is investigated. A sample injection volume of 0.4 pL is focused and separated in a 330 nm deep channel. The sample consists of a biomatrix containing the fluorescently-labeled amino acids glutamate and phenylalanine, 20 attomoles of each. Isotachophoretic focusing is successfully demonstrated in a 50 nm deep channel. Separation of the two amino acids in the 50 nm deep channel however, could not be performed as the maximum applicable voltage was insufficient. This limit is imposed by bubble formation that we contribute to cavitation as a result of the mismatch in electro-osmotic flow, so called electrocavitation. This represents an unexpected limit on the miniaturization of ITP. Nonetheless, we report the smallest isotachophoretic separation and focusing experiment to date, both in terms of controlled sample injection volume and channel height.

Rapid hybridization of nucleic acids using isotachophoresis

We use isotachophoresis (ITP) to control and increase the rate of nucleic acid hybridization reactions in free solution. We present a new physical model, validation experiments, and demonstrations of this assay. We studied the coupled physicochemical processes of preconcentration, mixing, and chemical reaction kinetics under ITP. Our experimentally validated model enables a closed form solution for ITP-aided reaction kinetics, and reveals a new characteristic time scale which correctly predicts order 10,000-fold speed-up of chemical reaction rate for order 100 pM reactants, and greater enhancement at lower concentrations. At 500 pM concentration, we measured a reaction time which is 14,000-fold lower than that predicted for standard second-order hybridization. The model and method are generally applicable to acceleration of reactions involving nucleic acids, and may be applicable to a wide range of reactions involving ionic reactants.

Development of a magnetic immunosorbent for on-chip preconcentration of amyloid β isoforms: Representatives of Alzheimer's disease biomarkers

Determination of amyloid β (Aβ) isoforms and in particular the proportion of the Aβ 1-42 isoform in cerebrospinal fluid (CSF) of patients suspected of Alzheimer's disease might help in early diagnosis and treatment of that illness. Due to the low concentration of Aβ peptides in biological fluids, a preconcentration step prior to the detection step is often necessary. This study utilized on-chip immunoprecipitation, known as micro-immunoprecipitation (μIP). The technique uses an immunosorbent (IS) consisting of magnetic beads coated with specific anti-Aβ antibodies organized into an affinity microcolumn by a magnetic field. Our goal was to thoroughly describe the critical steps in developing the IS, such as selecting the proper beads and anti-Aβ antibodies, as well as optimizing the immobilization technique and μIP protocol. The latter includes selecting optimal elution conditions. Furthermore, we demonstrate the efficiency of anti-Aβ IS for μIP and specific capture of 5 Aβ peptides under optimized conditions using various subsequent analytical methods, including matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), capillary electrophoresis, microchip electrophoresis, and immunoblotting. Synthetic Aβ peptides samples prepared in buffer and spiked in human CSF were analyzed. Finally, on-chip immunoprecipitation of Aβ peptides in human CSF sample was performed.

Electrospun drug-eluting sutures for local anesthesia

We have developed a local anesthetic-eluting suture system which would combine the function and ubiquity of the suture for surgical repair with the controlled release properties of a biodegradable polymeric matrix. Drug-free and drug-loaded poly(lactic-co-glycolic acid) (PLGA) sutures were fabricated by electrospinning, with or without the local anesthetic bupivacaine. The tensile strength of the electrospun sutures decreased as drug content increased, but strains remained relatively similar across all groups. Sutures released their entire drug payload over the course of 12 days and maintained approximately 12% of their initial tensile strength after 14 days of incubation in vitro. In a rat skin wound model, local analgesia was achieved 1 day after surgery and lasted approximately 1 week in 90% of treated animals (n=10, p<0.05), and all wounds were able to heal normally without the need for further reinforcement. The sutures caused tissue reaction in vivo that was comparable to that seen with a commercially available suture composed of PLGA. Such sutures may enhance perioperative analgesia and mitigate the need for standard postoperative opioid analgesics.

A new fabrication technique to form complex polymethylmethacrylate microchannel for bioseparation

Recent studies show that reduction in cross-sectional area can be used to improve the concentration factor in microscale bioseparations. Due to simplicity in fabrication process, a step reduction in cross-sectional area is generally implemented in microchip to increase the concentration factor. But the sudden change in cross-sectional area can introduce significant band dispersion and distortion. This paper reports a new fabrication technique to form a gradual reduction in cross-sectional area in polymethylmethacrylate (PMMA) microchannel for both anionic and cationic isotachophoresis (ITP). The fabrication technique is based on hot embossing and surface modification assisted bonding method. Both one-dimensional and two-dimensional gradual reduction in cross-sectional area microchannels were formed on PMMA with high fidelity using proposed techniques. ITP experiments were conducted to separate and preconcentrate fluorescent proteins in these microchips. Thousand fold and ten thousand fold increase in concentrations were obtained when 10 × and 100 × gradual reduction in cross-sectional area microchannels were used for ITP.

Rapid detection of urinary tract infections using isotachophoresis and molecular beacons

We present a novel assay for rapid detection and identification of bacterial urinary tract infections using isotachophoresis (ITP) and molecular beacons. We applied on-chip ITP to extract and focus 16S rRNA directly from bacterial lysate and used molecular beacons to achieve detection of bacteria specific sequences. We demonstrated detection of E. coli in bacteria cultures as well as in patient urine samples in the clinically relevant range 1E6-1E8 cfu/mL. For bacterial cultures we further demonstrate quantification in this range. The assay requires minimal sample preparation (a single centrifugation and dilution), and can be completed, from beginning of lysing to detection, in under 15 min. We believe that the principles presented here can be used for design of other rapid diagnostics or detection methods for pathogenic diseases.

Transport and separation of micron sized particles at isotachophoretic transition zones

Conventionally, isotachophoresis (ITP) is used for separation of ionic samples according to their electrophoretic mobilities. We demonstrate that the scope of ITP applications may be extended toward particle concentration and separation. Owing to the distributions of electrolyte concentration and electric field inside a transition zone between two electrolytes, a number of different forces act on a small particle. As far as possible, we provide estimates for the order of magnitude of these forces and analyze their scaling with the particle size and the electric-field strength. Furthermore, we experimentally demonstrate that polymer beads of 5 μm diameter dispersed in a high mobility "leading" electrolyte are picked up and carried along by an ITP transition zone which is formed with a low mobility "trailing" electrolyte. By studying the particle positions and trajectories, we show that impurities in the electrolytes play a significant role in the experiments. Additionally, it is experimentally shown that different types of beads can be separated at an ITP transition zone. In particular, beads of 1 μm diameter are not carried along with the transition zone, in contrast to the 5 μm beads. The presented technique thus adds to the portfolio of electrokinetic transport, concentration, and separation methods in microfluidics.

10,000-fold concentration increase of the biomarker cardiac troponin I in a reducing union microfluidic chip using cationic isotachophoresis

This paper describes the preconcentration of the biomarker cardiac troponin I (cTnI) and a fluorescent protein (R-phycoerythrin) using cationic isotachophoresis (ITP) in a 3.9 cm long poly(methyl methacrylate) (PMMA) microfluidic chip. The microfluidic chip includes a channel with a 5× reduction in depth and a 10× reduction in width. Thus, the overall cross-sectional area decreases by 50× from inlet (anode) to outlet (cathode). The concentration is inversely proportional to the cross-sectional area so that as proteins migrate through the reductions, the concentrations increase proportionally. In addition, the proteins gain additional concentration by ITP. We observe that by performing ITP in a cross-sectional area reducing microfluidic chip we can attain concentration factors greater than 10,000. The starting concentration of cTnI was 2.3 μg mL⁻¹ and the final concentration after ITP concentration in the microfluidic chip was 25.52 ± 1.25 mg mL⁻¹. To the author's knowledge this is the first attempt at concentrating the cardiac biomarker cTnI by ITP. This experimental approach could be coupled to an immunoassay based technique and has the potential to lower limits of detection, increase sensitivity, and quantify different isolated cTnI phosphorylation states.

10,000-fold concentration increase in proteins in a cascade microchip using anionic ITP by a 3-D numerical simulation with experimental results

This paper describes both the experimental application and 3-D numerical simulation of isotachophoresis (ITP) in a 3.2 cm long "cascade" poly(methyl methacrylate) (PMMA) microfluidic chip. The microchip includes 10 × reductions in both the width and depth of the microchannel, which decreases the overall cross-sectional area by a factor of 100 between the inlet (cathode) and outlet (anode). A 3-D numerical simulation of ITP is outlined and is a first example of an ITP simulation in three dimensions. The 3-D numerical simulation uses COMSOL Multiphysics v4.0a to concentrate two generic proteins and monitor protein migration through the microchannel. In performing an ITP simulation on this microchip platform, we observe an increase in concentration by over a factor of more than 10,000 due to the combination of ITP stacking and the reduction in cross-sectional area. Two fluorescent proteins, green fluorescent protein and R-phycoerythrin, were used to experimentally visualize ITP through the fabricated microfluidic chip. The initial concentration of each protein in the sample was 1.995 μg/mL and, after preconcentration by ITP, the final concentrations of the two fluorescent proteins were 32.57 ± 3.63 and 22.81 ± 4.61 mg/mL, respectively. Thus, experimentally the two fluorescent proteins were concentrated by over a factor of 10,000 and show good qualitative agreement with our simulation results.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2008-2010)

Capillary electrophoresis 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 an update published in 2009 and covers material published through to June 2010. It includes developments in the fields of stacking, covering all methods from field-amplified sample stacking and large volume sample stacking, through to ITP, dynamic pH junction and sweeping. Attention is also given to on-line or in-line extraction methods that have been used for electrophoresis.

Dye/water separation through supported liquid membrane extraction

The separation of synthetic dye Rhodamine 6G (R6G) and water was investigated using blended organic liquids in a supported liquid membrane (SLM) extraction system. Liquid membrane (LM) components include octyl alcohol (OcOH) as the dye extractant and a polysiloxane liquid as the stabilizing agent. Initial permeation results revealed the suitability of poly (phenyl methyl) siloxane (PPMS) over poly (octyl methyl) siloxane as the blending agent. The most acceptable condition for dye extraction was determined at feed solution pH congruent with 1, wherein highest distribution coefficient, K(D) (OcOH/H(2)O)=18, was attained. Though permeability decreased at optimal blending condition of 1:1 (w/w) OcOH/PPMS, SLM longevity was exhibited with>98% LM retention after 15 h operation in contrast to pure OcOH SLM system (>60% LM loss). Equilibrium experiments reveal that dye extraction followed Langmuir adsorption principle. The dye transport was elucidated using mass transfer analysis wherein it showed a decrease in overall coefficient (k(o)) at increasing feed concentrations. This was a direct consequence of K(D) decline, which becomes more apparent at higher concentrations when SLM saturation point is approached. At varied hydrodynamic conditions, improved k(o) values were observed up to Re(omega)=10,000 when minimal variation in film resistance is attained. Beyond this condition, k(o) becomes independent from stirring rate effect nonetheless SLM stability is compromised due to shear-induced LM losses.

Development of a membrane-less dynamic field gradient focusing device for the separation of low-molecular-weight molecules

Dynamic field gradient focusing uses an electric field gradient generated by controlling the voltage profile of an electrode array to separate and concentrate charged analytes according to their individual electrophoretic mobilities. This study describes a new instrument in which the electrodes have been placed within the separation channel. The major challenge faced with this device is that when applied voltages to the electrodes are larger than the redox potential of water, electrolysis will occur, producing hydrogen ions (H+) plus oxygen gas on the anodes and hydroxide (OH(-)) plus hydrogen gas on the cathodes. The resulting gas bubbles and pH excursions can cause problems with system performance and reproducibility. An on-column, degassing system that can remove gas bubbles "on-the-fly" is described. In addition, the use of a high capacity, low-conductivity buffer to address the problem of the pH shift that occurs due to the production of H+ on the anodes is illustrated. Finally, the successful separation of three, low-molecular-weight dyes (amaranth, bromophenol blue and methyl red) is described.

Basic principles of electrolyte chemistry for microfluidic electrokinetics. Part I: Acid-base equilibria and pH buffers

We review fundamental and applied acid-base equilibrium chemistry useful to microfluidic electrokinetics. We present elements of acid-base equilibrium reactions and derive rules for pH calculation for simple buffers. We also present a general formulation to calculate pH of more complex, arbitrary mixtures of electrolytes, and discuss the effects of ionic strength and temperature on pH calculation. More practically, we offer advice on buffer preparation and on buffer reporting. We also discuss "real world" buffers and likely contamination sources. In particular, we discuss the effects of atmospheric carbon dioxide on buffer systems, namely, the increase in ionic strength and acidification of typical electrokinetic device buffers. In Part II of this two-paper series, we discuss the coupling of acid-base equilibria with electrolyte dynamics and electrochemistry in typical microfluidic electrokinetic systems.