Peak capacity and peak capacity per unit time in capillary and microchip zone electrophoresis.

The origins of the peak capacity concept are described and the important contributions to the development of that concept in chromatography and electrophoresis are reviewed. Whereas numerous quantitative expressions have been reported for one- and two-dimensional separations, most are focused on chromatographic separations and few, if any, quantitative unbiased expressions have been developed for capillary or microchip zone electrophoresis. Making the common assumption that longitudinal diffusion is the predominant source of zone broadening in capillary electrophoresis, analytical expressions for the peak capacity are derived, first in terms of migration time, diffusion coefficient, migration distance, and desired resolution, and then in terms of the remaining underlying fundamental parameters (electric field, electroosmotic and electrophoretic mobilities) that determine the migration time. The latter expressions clearly illustrate the direct square root dependence of peak capacity on electric field and migration distance and the inverse square root dependence on solute diffusion coefficient. Conditions that result in a high peak capacity will result in a low peak capacity per unit time and vice-versa. For a given symmetrical range of relative electrophoretic mobilities for co- and counter-electroosmotic species (cations and anions), the peak capacity increases with the square root of the electric field even as the temporal window narrows considerably, resulting in a significant reduction in analysis time. Over a broad relative electrophoretic mobility interval [-0.9, 0.9], an approximately two-fold greater amount of peak capacity can be generated for counter-electroosmotic species although it takes about five-fold longer to do so, consistent with the well-known bias in migration time and resolving power for co- and counter-electroosmotic species. The optimum lower bound of the relative electrophoretic mobility interval [µr,Z, µr,A] that provides the maximum peak capacity per unit time is a simple function of the upper bound, but its direct application is limited to samples with analytes whose electrophoretic mobilities can be varied independently of electroosmotic flow. For samples containing both co- and counter-electroosmotic ions whose electrophoretic mobilities cannot be easily manipulated, comparable levels of peak capacity and peak capacity per unit time for all ions can be obtained by adjusting the EOF to devote the same amount of time to the separation of each class of ions; this corresponds to µr,Z=-0.5.

Calibration-free quantitation in microchip zone electrophoresis with conductivity detection.

The relationship between electrophoretic mobility and molar conductivity has previously led to speculation on achieving quantitation in zone electrophoresis without calibration curves when using conductivity detection. However, little work in this area has been pursued, possibly because of the breakdown of simple sensitivity-mobility relationships when working with partially protonated species. This topic is revisited with the aid of electrophoretic simulation software that produces facile predictions of analyte sensitivity relative to an internal standard. Calibration curve slopes for over 50 analyte/internal standard/BGE combinations were measured with both unbiased and electrokinetically biased injections using microchip electrophoresis with conductivity detection. The results were compared to theoretical expectations as computed with PeakMaster software. Good agreement was observed, with some systems being predicted with quantitative accuracy while others showed significant deviations. Some mechanisms that can lead to deviations from theory are demonstrated, but the causes for some discrepancies are still not understood. Overall, this work exhibits another useful application for simulation software, particularly for disposable devices where device-specific calibration curves cannot be collected. It also serves as quantitative validation for some outputs of PeakMaster simulation software.

Improving chip-to-chip precision in disposable microchip capillary electrophoresis devices with internal standards.

To realize portable systems for routine measurements in point-of-care settings, MCE methods are required to be robust across many single-use chips. While it is well-known internal standards (ISTDs) improve run-to-run precision, a systematic investigation is necessary to determine the significance of chip-to-chip imprecision in MCE and how ISTDs account for it. This paper addresses this question by exploring the reproducibility of Na quantification across six basic, in-house fabricated microchips. A dataset of 900 electrophoerograms was collected from analyzing five concentrations of NaCl with two ISTDs (CsCl and LiCl). While both improved the peak area reproducibility, the Na/Cs ratio was superior to the Na/Li ratio (improving the RSD by a factor of 2-4, depending on the Na concentration). We attribute this to the significant variation in microchannel surface properties, which was accounted for by cesium but not lithium. Microchip dimension and detector variations were only a few percent, and could be improved through commercial fabrication over in-house made microchips. These results demonstrate that ISTDs not only correct for intrachip imprecision, but are also a viable means to correct for chip-to-chip imprecision inherent in disposable, point-of-care MCE devices. However, as expected, the internal standard must be carefully chosen.

Present state of microchip electrophoresis: state of the art and routine applications.

Microchip electrophoresis (MCE) was one of the earliest applications of the micro-total analysis system (µ-TAS) concept, whose aim is to reduce analysis time and reagent and sample consumption while increasing throughput and portability by miniaturizing analytical laboratory procedures onto a microfluidic chip. More than two decades on, electrophoresis remains the most common separation technique used in microfluidic applications. MCE-based instruments have had some commercial success and have found application in many disciplines. This review will consider the present state of MCE including recent advances in technology and both novel and routine applications in the laboratory. We will also attempt to assess the impact of MCE in the scientific community and its prospects for the future.

Variability of microchip capillary electrophoresis with conductivity detection.

Microfluidic CE with conductivity detection platforms could have an impact on the future development of smaller, faster and portable devices. However, for the purpose of reliable identification and quantification, there is a need to understand the degree of irreproducibility associated with the analytical technique. In this study, a protocol was developed to remove baseline drift problems sometimes observed in such devices. The protocol, which consisted of pre-conditioning steps prior to analysis, was used to further assess measurement variability from 24 individual microchips fabricated from six separate batches of glass substrate. Results show acceptable RSD percentage for retention time measurements but large variability in their corresponding peak areas (with some microchips having variability of ∼50%). Sources of variability were not related to substrate batch but possibly to a number of factors such as applied voltage fluctuations or variations in microchannel quality, for example surface roughness that will subsequently affect microchannel dimensions.

Evaluation of a microfluidics-based platform and slab electrophoresis for determination of size, integrity and quantification of in vitro transcribed RNA used as a component in therapeutic drug manufacturing.

Ribonucleic acid (RNA) is gaining utility as a key component of immunotherapeutics to transiently express antigens or to modulate endogenous gene expression for clinical applications. As a key ancillary component of clinical grade products, RNA requires a robust method for quality control. Here we evaluated the microfluidics based platform and slab electrophoresis for determination of integrity, concentration and size of four in vitro-transcribed RNA products with sizes of 1611, 808, 475 and 290 nucleotides (nts). Our data demonstrate that the Bioanalyzer can determine both size and integrity of the RNA, but the analysis suffers from a strong well position effect. For the RNAs tested, the integrity values obtained by the Bioanalyzer demonstrate a reverse correlation with the size of the molecule and are lower than those obtained using slab electrophoresis. Agarose gel electrophoresis produced the information on size of the RNA molecule with good precision, accuracy and reproducibility. We highlight observations which need to be taken into account when developing and qualifying a method of choice for assessment of in vitro-transcribed RNA using either approach.

Microchip electrophoresis, with respect to "profiling of Aß peptides in the cerebrospinal fluid of patients with Alzheimer's disease".

Aggregation of beta amyloid peptides especially Aß1-42 in amyloid plaques is one of the major -neuropathological events in Alzheimer's disease. This event is normally accompanied by a relative reduction of the concentration of Aß1-42 in the cerebrospinal fluid (CSF) of patients developing the signs of Alzheimer's disease. Here, we describe a microchip gel electrophoresis method in a polydimethylsiloxane (PDMS) chip that enables rapid profiling of major Aß peptides. The method was applied to compare the relative concentration of Aß1-42 with other Aß peptides, for example, Aß 1-40 in CSF. In order to increase the sensitivity of detection, Aß peptides in the CSF samples were first captured and concentrated using magnetic beads coated with specific anti-Aß antibodies.

Hydrodynamic injection with pneumatic valving for microchip electrophoresis with total analyte utilization.

A novel hydrodynamic injector that is directly controlled by a pneumatic valve has been developed for reproducible microchip CE separations. The PDMS devices used for the evaluation comprise a separation channel, a side channel for sample introduction, and a pneumatic valve aligned at the intersection of the channels. A low pressure (≤ 3 psi) applied to the sample reservoir is sufficient to drive sample into the separation channel. The rapidly actuated pneumatic valve enables injection of discrete sample plugs as small as ~ 100 pL for CE separation. The injection volume can be easily controlled by adjusting the intersection geometry, the solution back pressure, and the valve actuation time. Sample injection could be reliably operated at different frequencies (< 0.1 Hz to > 2 Hz) with good reproducibility (peak height relative standard deviation ≤ 3.6%) and no sampling biases associated with the conventional electrokinetic injections. The separation channel was dynamically coated with a cationic polymer, and FITC-labeled amino acids were employed to evaluate the CE separation. Highly efficient (≥ 7.0 × 10³ theoretical plates for the ~2.4-cm-long channel) and reproducible CE separations were obtained. The demonstrated method has numerous advantages compared with the conventional techniques, including repeatable and unbiased injections, little sample waste, high duty cycle, controllable injected sample volume, and fewer electrodes with no need for voltage switching. The prospects of implementing this injection method for coupling multidimensional separations for multiplexing CE separations and for sample-limited bioanalyses are discussed.

Fracture mechanism of metal electrode integrated on a chip and fabrication of a poly(ethylene terephthalate) electrophoresis microchip.

Thermal bonding is an important technique to fabricate polymer electrophoresis microchip. However, the metal electrodes deposited on polymer substrate can readily fracture during the thermal bonding. In this paper, poly(ethylene terephthalate) (PET) was exploited to fabricate the electrophoresis microchip with an integrated gold electrode for amperometric detection. The fracture of the gold electrode was studied through FEA (finite element analysis) simulations, the potentially risk positions on the electrode were shown. The calculation results were tested by bonding experiments and were proven to be consistent with the experiments. Besides, an optimal bonding temperature for PET chip was also presented based on FEA simulations and bonding experiments. Considering the low surface properties of PET, oxygen plasma-assisted thermal bonding technique was used to enhance bonding. Upon treated for 150 s, the PET substrates could be thermally bonded at 62 degrees C without electrode fracture. The fabricated PET chips were demonstrated for detection of standard glucose solution. Satisfactory reproducibility was achieved, and the RSD values of peak height and migration time of the PET CE chips were 0.51% and 2.17%, respectively.

Deoxyribonucleic acid modified poly(dimethylsiloxane) microfluidic channels for the enhancement of microchip electrophoresis.

In this paper, deoxyribonucleic acid (DNA) was employed to construct a functional film on the PDMS microfluidic channel surface and apply to perform electrophoresis coupled with electrochemical detection. The functional film was formed by sequentially immobilizing chitosan and DNA to the PDMS microfluidic channel surface using the layer-by-layer assembly. The polysaccharide backbone of chitosan can be strongly adsorbed onto the hydrophobic PDMS surface through electrostatic interaction in the acidic media, meanwhile, chitosan contains one protonatable functional moiety resulting in a strong electrostatic interactions between the surface amine group of chitosan and the charged phosphate backbone of DNA at low pH, which generates a hydrophilic microchannel surface and reveals perfect resistance to nonspecific adsorption of analytes. Aminophenol isomers (p-, o-, and m-aminophenol) served as a separation model to evaluate the effect of the functional PDMS microfluidic chips. The results clearly showed that these analytes were efficiently separated within 60s in a 3.7 cm long separation channel and successfully detected on the modified microchip coupled with in-channel amperometric detection mode at a single carbon fiber electrode. The theoretical plate numbers were 74,021, 92,658 and 60,552 Nm(-1) at the separation voltage of 900 V with the detection limits of 1.6, 4.7 and 2.5 microM (S/N=3) for p-, o-, and m-aminophenol, respectively. In addition, this report offered an effective means for preparing hydrophilic and biocompatible PDMS microchannel surface, which would facilitate the use of microfluidic devices for more widespread applications.

Faster and improved microchip electrophoresis using a capillary bundle.

Joule heating generated in CE microchips is known to affect temperature gradient, electrophoretic mobility, diffusion of analytes, and ultimately the efficiency and reproducibility of the separation. One way of reducing the effect of Joule heating is to decrease the cross-section area of microchannels. Currently, due to the limit of fabrication technique and detection apparatus, the typical dimensions of CE microchannels are in the range of 50-200 microm. In this paper, we propose a novel approach of performing microchip CE in a bundle of extremely narrow channels by using photonic crystal fiber (PCF) as separation column. The PCF was simply encapsulated in a poly(methyl methacrylate) (PMMA) microchannel right after a T-shaped injector. CE was simultaneously but independently carried out in 54 narrow capillaries, each capillary with diameter of 3.7 microm. The capillary bundle could sustain high electric field strength up to 1000 V/cm due to efficient heat dissipation, thus faster and enhanced separation was attained.

Ultra-fast simultaneous analysis of genetically modified organisms in maize by microchip electrophoresis with LIF detector.

This study examined the potential of microchip electrophoresis (ME) with a LIF detector using a programmed field strength gradient (PFSG) in a conventional glass double-T microchip for the ultra-fast detection and simultaneous analysis of genetically modified (GM) maize. The separation efficiency and sensitivity at various sieving gels (poly(ethylene oxide) (PEO, M(r) 8,000,000) and 2-hydroxyethylcellulose (HEC) (M(r) 250,000)) and fluorescent dye concentrations were investigated. The PCR products of both the GM and non-GM maize were analyzed within 30 s under the PFSG (470.6 V/cm for 20 s, 117.6 V/cm for 12 s, and 470.6 V/cm for 30 s) with a 2.5% HEC sieving matrix in the running buffer, 1 x Tris-borate EDTA (TBE) (pH 8.30) and 0.5 ppm ethidium bromide. The five transgenic maize varieties (Event176, MON810, Bt11, GA21, and T25) examined in this study were also clearly differentiated by ME-PFSG within 30 s in a single run without any loss of resolution. The ME-PFSG technique is a powerful tool for the ultra-fast detection and simultaneous analysis of GMOs in a variety of foods including maize.

Rapid analysis of azo-dyes in food by microchip electrophoresis with electrochemical detection.

A method based on microchip electrophoresis with electrochemical (EC) detection has been developed for the simultaneous determination of Yellow AB, Red 2G, Sunset Yellow, New Coccine, and Amaranth which are azo-dyes frequently added to foodstuffs. Factors affecting both separation and detection processes were examined and optimized, with best performance achieved by using a 10 mM phosphate buffer (pH 11) as BGE solution and applying a voltage of 2500 V both in the separation and in the electrokinetic injection (duration 4 s). Under these optimal conditions, the target dye analytes could be separated and detected within 300 s by applying a detection potential of -1.0 V (vs. Ag/AgCl) to the glassy carbon (GC) working electrode. The recorded peaks were characterized by a good repeatability (RSD=1.8-3.2%), high sensitivity, and a wide linear range. Detection limits of 3.8, 3.4, 3.6, 9.1, 15.1 microM were obtained for Yellow AB, Red 2G, Sunset Yellow, New Coccine, and Amaranth, respectively. Fast, sensitive, and selective response makes the new microchip protocol very attractive for the quantitative analysis of commercial soft drinks and candies.

Rapid sample screening method for authenticity controlling vanilla flavors using a CE microchip approach with electrochemical detection.

Five vanilla-related flavors of food significance, vanillic alcohol (VOH), ethyl maltol (EMA), maltol (MAL), ethyl vanillin (EVA) and vanillin (VAN), were separated using CE microchips with electrochemical detection (CE-ED microchips). A +2 kV driving voltage for both injection and separation operation steps, using a borate buffer (pH 9.5, 20 mM) and 1 M nitric acid in the detection reservoir allowed the selective and sensitive detection of the target analytes in less than 200 s with reproducible control of EOF (RSD(migration times)<3%). The analysis in selected real vanilla samples was focusing on VAN and EVA because VAN is a basic fragrance compound of the vanilla aroma, whereas EVA is an unequivocal proof of adulteration of vanilla flavors. Fast detection of all relevant flavors (200 s) with an acceptable resolution (R(s) >1.5) and a high accuracy (recoveries higher than 90%) were obtained with independence of the matrices and samples examined. These results showed the reliability of the method and the potential use of CE microchips in the food control field for fraudulent purposes.

Determination of human blood glucose levels using microchip electrophoresis.

A high-performance monitoring system for human blood glucose levels was developed using microchip electrophoresis with a plastic chip. The combination of reductive amination as glucose labeling with fluorescent 2-aminoacridone (AMAC) and glucose-borate complex formation realized the highly selective detection of glucose even in a complex matrix such as a blood sample. The migration time of a single peak, observed on an electropherogram of AMAC-labeled plasma, closely resembled that of glucose standard solution. The treatment of plasma with hexokinase or glucokinase for glucose phosphorylation resulted in a peak shift from approximately 145 to 70 s, corresponding to glucose and glucose-6-phosphate, respectively. A double-logarithm plot revealed a linear relationship between glucose concentration and fluorescence intensity in the range of 1-300 microM of glucose (r(2) = 0.9963; p <0.01), and the detection limit was 0.92 microM. Furthermore, blood glucose concentrations estimated from the standard curves of three subjects were compared with results obtained by conventional colorimetric analysis using glucose dehydrogenase. Good correlation was observed between methods according to simple linear regression analysis (p <0.05). The reproducibility of the assay was about 6.3-9.1% (RSD) and the within-days and between-days reproducibility were 1.6-8.4 and 5.2-7.2%, respectively. This system enables us to determine blood glucose with high sensitivity and accuracy, and will be applicable to clinical diagnosis.

A fast and reliable route integrating calibration and analysis protocols for water-soluble vitamin determination on microchip-electrochemistry platforms.

A novel analytical route to determine water-soluble vitamins (B group and C) using single channel microchip-electrochemistry platforms is presented. The electrochemical detection protocol was carefully optimized, and it was shown that it was crucial to use 1 M nitric acid in the detector compartment to detect folic acid. A phosphate buffer (pH 6, 10 mM) and a separation voltage of 2 kV gave the complete separation of vitamins in less than 130 s, with good reproducibility (RSDs less than 10%) and accuracy (error less than 9%). In addition, a methodological innovation integrating calibration and analysis of water-soluble vitamins on the chip is also proposed. The strategy consisted in sequentially using both reservoirs (named calibration and analysis reservoirs) as well as a calibration factor (defined as signal/concentration of analyte). The analytical route required 350 s in the overall protocol (employing 130 s in calibration plus 130 s in analysis), an improvement over the times used in both conventional and microchip protocols.

Viability study of HL60 cells in contact with commonly used microchip materials.

This paper presents a study in which different commonly used microchip materials (silicon oxide, borosilicate glass, and PDMS) were analyzed for their effect on human promyelocytic leukemic (HL60) cells. Copper-coated silicon was analyzed for its toxicity and therefore served as a positive control. With quantitative PCR, the expression of the proliferation marker Cyclin D1 and the apoptosis marker tissue transglutaminase were measured. Flow cytometry was used to analyze the distribution through the different phases of the cell cycle (propidium iodide, PI) and the apoptotic cascade (Annexin V in combination with PI). All microchip materials, with the exception of Cu, appeared to be suitable for HL60 cells, showing a ratio apoptosis/proliferation (R(ap)) comparable to materials used in conventional cell culture (polystyrene). These results were confirmed with cell cycle analysis and apoptosis studies. Precoating the microchip material surfaces with serum favor the proliferation, as demonstrated by a lower R(ap) as compared to uncoated surfaces. The Cu-coated surface appeared to be toxic for HL60 cells, showing over 90% decreased viability within 24 h. From these results, it can be concluded that the chosen protocol is suitable for selection of the cell culture material, and that the most commonly used microchip materials are compatible with HL60 culturing.

Numerical model for DNA loading in microdevices: stacking and autogating effects.

Many electrophoresis-based DNA sequencing and genotyping microdevices rely on field-driven effects to load and preconcentrate the sample. A quantitative model is developed for a broad class of electrophoresis-based microfabricated sample injectors. Quantitative predictions of DNA preconcentration are compared with experimental data and are shown to qualitatively reproduce the detailed time-evolving sample distribution in the injector. The model provides practical guidance on device and protocol design, in order to optimize this critical aspect of microfluidic devices.

An optimized microchip electrophoresis system for mutation detection by tandem SSCP and heteroduplex analysis for p53 gene exons 5-9.

With the complete sequencing of the human genome, there is a growing need for rapid, highly sensitive genetic mutation detection methods suitable for clinical implementation. DNA-based diagnostics such as single-strand conformational polymorphism (SSCP) and heteroduplex analysis (HA) are commonly used in research laboratories to screen for mutations, but the slab gel electrophoresis (SGE) format is ill-suited for routine clinical use. The translation of these assays from SGE to microfluidic chips offers significant speed, cost, and sensitivity advantages; however, numerous parameters must be optimized to provide highly sensitive mutation detection. Here we present a methodical study of system parameters including polymer matrix, wall coating, analysis temperature, and electric field strengths on the effectiveness of mutation detection by tandem SSCP/HA for DNA samples from exons 5-9 of the p53 gene. The effects of polymer matrix concentration and average molar mass were studied for linear polyacrylamide (LPA) solutions. We determined that a matrix of 8% w/v 600 kDa LPA provides the most reliable SSCP/HA mutation detection on chips. The inclusion of a small amount of the dynamic wall-coating polymer poly-N-hydroxyethylacrylamide in the matrix substantially improves the resolution of SSCP conformers and extends the coating lifetime. We investigated electrophoresis temperatures between 17 and 35 degrees C and found that the lowest temperature accessible on our chip electrophoresis system gives the best condition for high sensitivity of the tandem SSCP/HA method, especially for the SSCP conformers. Finally, the use of electrical fields between 350 and 450 V/cm provided rapid separations (<10 min) with well-resolved DNA peaks for both SSCP and HA.

Physiochemical properties of various polymer substrates and their effects on microchip electrophoresis performance.

A suite of polymers were evaluated for their suitability as viable substrate materials for microchip electrophoresis applications, which were fabricated via replication technology. The relevant physiochemical properties investigated included the glass transition temperature (T(g)), UV-vis absorption properties, autofluorescence levels, electroosmotic flow (EOF) and hydrophobicity/hydrophilicity as determined by sessile water contact angle measurements. These physiochemical properties were used as a guide to select the proper substrate material for the intended microchip electrophoretic application. The T(g) of these polymers provided a guide for optimizing embossing parameters to minimize replication errors (REs), which were evaluated from surface profilometer traces. RE values ranged from 0.4 to 13.6% for the polymers polycarbonate (PC) and low-density polyethylene (LDPE), respectively. The absorption spectra and autofluorescence levels of the polymers were also measured at several different wavelengths. In terms of optical clarity (low absorption losses and small autofluorescence levels), poly(methyl methacrylate), PMMA (clear acrylic), provided ideal characteristics with autofluorescence levels comparable to glass at excitation wavelengths that ranged from 488-780 nm. Contact angle measurements showed a maximum (i.e., high degree of hydrophobicity) for polypropylene (PP), with an average contact angle of 104 degrees +/-3 degrees and a minimum exhibited by gray acrylic, G-PMMA, with an average contact angle of 27 degrees +/-2 degrees. The EOF was also measured for thermally assembled chips both before and after treatment with bovine serum albumin (BSA). The electrophoretic separation of a mixture of dye-labeled proteins including; carbonic anhydrase, phosphorylase B, beta-galactosidase, and myosin, was performed on four different polymer microchips using laser-induced fluorescence (LIF) excitation at 632.8 nm. A maximum average resolution of 5.04 for several peak pairs was found with an efficiency of 6.68 x 10(4) plates for myosin obtained using a BSA-treated PETG microchip.