Sample Injection for Capillary Electrophoresis on a Micro Fabricated Device/On Chip CE Injection

Recent Advances and Applications of Microfluidic Capillary Electrophoresis: A Comprehensive Review (2017-Mid 2019)

Microfluidic capillary electrophoresis (MCE) is the novel technique resulted from the CE mininaturization as planar separation and analysis device. This review presents and discusses various application fields of this advanced technology published in the period 2017 till mid-2019 in eight different sections including clinical, biological, single cell analysis, environmental, pharmaceuticals, food analysis, forensic and ion analysis. The need for miniaturization of CE and the consequence advantages achieved are also discussed including high-throughput, miniaturized detection, effective separation, portability and the need for micro- or even nano-volume of samples. Comprehensive tables for the MCE applications in the different studied fields are provided. Also, figure comparing the number of the published papers applying MCE in the eight discussed fields within the studied period is included. The future investigation should put into consideration the possibility of replacing conventional CE with the MCE after proper validation. Suitable validation parameters with their suitable accepted ranges should be tailored for analysis methods utilizing such unique technique (MCE).

Novel volumetric method for highly repeatable injection in microchip electrophoresis

A novel injector for microchip electrophoresis (MCE) has been designed and evaluated that achieves very high repeatability of injection volume suitable for quantitative analysis. It eliminates the injection biases in electrokinetic injection and the dependence on pressure and sample properties in hydrodynamic injection. The microfluidic injector, made of poly(dimethylsiloxane) (PDMS), operates similarly to an HPLC injection valve. It contains a channel segment (chamber) with a well-defined volume that serves as an "injection loop". Using on-chip microvalves, the chamber can be connected to the sample source during the "loading" step, and to the CE separation channel during the "injection" step. Once the valves are opened in the second state, electrophoretic potential is applied to separate the sample. For evaluation and demonstration purposes, the microinjector was connected to a 75 μm ID capillary and UV absorbance detector. For single compounds, a relative standard deviation (RSD) of peak area as low as 1.04% (n = 11) was obtained, and for compound mixtures, RSD as low as 0.40% (n = 4) was observed. Using the same microchip, the performance of this new injection technique was compared to hydrodynamic injection and found to have improved repeatability and less dependence on sample viscosity. Furthermore, a non-radioactive version of the positron-emission tomography (PET) imaging probe, FLT, was successfully separated from its known 3 structurally-similar byproducts with baseline resolution, demonstrating the potential for rapid, quantitative analysis of impurities to ensure the safety of batches of short-lived radiotracers. Both the separation efficiency and injection repeatability were found to be substantially higher when using the novel volumetric injection approach compared to electrokinetic injection (performed in the same chip). This novel microinjector provides a straightforward way to improve the performance of hydrodynamic injection and enables extremely repeatable sample volume injection in MCE. It could be used in any MCE application where volume repeatability is needed, including the quantitation of impurities in pharmaceutical or radiopharmaceutical samples.

Acupuncture Injection Combined with Electrokinetic Injection for Polydimethylsiloxane Microfluidic Devices

We recently reported acupuncture sample injection that leads to reproducible injection of nL-scale sample segments into a polydimethylsiloxane (PDMS) microchannel for microchip capillary electrophoresis. The advantages of the acupuncture injection in microchip capillary electrophoresis include capability of minimizing sample loss and voltage control hardware and capability of introducing sample plugs into any desired position of a microchannel. However, the challenge in the previous study was to achieve reproducible, pL-scale sample injections into PDMS microchannels. In the present study, we introduce an acupuncture injection technique combined with electrokinetic injection (AICEI) technique to inject pL-scale sample segments for microchip capillary electrophoresis. We carried out the capillary zone electrophoresis (CZE) separation of FITC and fluorescein, and the mixture of 10 μM FITC and 10 μM fluorescein was separated completely by using the AICEI method.

Instrumentation design for hydrodynamic sample injection in microchip electrophoresis: a review

Reproducible and representative sample injection in microchip electrophoresis has been a bottleneck for quantitative analytical applications. Electrokinetic sample injection is the most used because it is easy to perform. However, this injection method is usually affected by sample composition and the bias effect. On the other hand, these drawbacks are overcome by the hydrodynamic (HD) sample injection, although this injection mode requires HD flow control. This review gives an overview of the basic principles, the instrumentation designs, and the performance of HD sample injection systems for microchip electrophoresis.

Experimental and numerical analysis of high-resolution injection technique for capillary electrophoresis microchip

This study presents an experimental and numerical investigation on the use of high-resolution injection techniques to deliver sample plugs within a capillary electrophoresis (CE) microchip. The CE microfluidic device was integrated into a U-shaped injection system and an expansion chamber located at the inlet of the separation channel, which can miniize the sample leakage effect and deliver a high-quality sample plug into the separation channel so that the detection performance of the device is enhanced. The proposed 45° U-shaped injection system was investigated using a sample of Rhodamine B dye. Meanwhile, the analysis of the current CE microfluidic chip was studied by considering the separation of Hae III digested ϕx-174 DNA samples. The experimental and numerical results indicate that the included 45° U-shaped injector completely eliminates the sample leakage and an expansion separation channel with an expansion ratio of 2.5 delivers a sample plug with a perfect detection shape and highest concentration intensity, hence enabling an optimal injection and separation performance.

Integration of a precolumn fluorogenic reaction, separation, and detection of reduced glutathione

Reduced glutathione (GSH) has been determined by fluorescence detection after derivatization together with a variety of separations. The reactions between GSH and fluorescent reagents usually are carried out during the sample pretreatment and require minutes to hours for complete reactions. For continuous monitoring of GSH, it would be very convenient to have an integrated microdevice that could perform online precolumn derivatization, separation, and detection. Heretofore, thiol-specific fluorogenic reagents require fairly long reaction times, preventing effective online precolumn derivatization. We demonstrate here that the fluorogenic, thiol-specific reagent, ThioGlo-1, reacts rapidly enough for efficient precolumn derivatization. The second order rate constant for the reaction of GSH and reagent (pH 7.5, room temperature) is 2.1 x 10(4) M(-1)s(-1). The microchip integrates this precolumn derivatization, continuous flow gated sampling, separation, and detection on a single device. We have validated this device for monitoring GSH concentration continuously by studying the kinetics of glutathione reductase (EC 1.8.1.7), an enzyme that catalyzes the reduction of oxidized glutathione (GSSG) to GSH in the presence of beta-NADPH (beta-nicotinamide adenine dinucleotide phosphate, reduced form) as a reducing cofactor. During the experiment, GSH being generated in the enzymatic reaction was labeled with ThioGlo-1 as it passed through a mixing channel on the microfluidic chip. Derivatization reaction products were introduced into the analysis channel every 10 s using flow gated injections of 0.1 s. Baseline separation of the internal standard, ThioGlo-1, and the fluorescently labeled GSH was successfully achieved within 4.5 s in a 9 mm separation channel. Relative standard deviations of the peak area, peak height, and full width at half-maximum (fwhm) for the internal standard were 2.5%, 2.0%, and 1.0%, respectively, with migration time reproducibility for the internal standard of less than 0.1% RSD in any experiment. The GSH concentration and mass detection limit were 4.2 nM and approximately 10(-18) mol, respectively. The Michaelis constants (K(m)) for GSSG and beta-NADPH were found to be 40 +/- 11 and 4.4 +/- 0.6 muM, respectively, comparable with those obtained from UV/vis spectrophotometric measurements. These results show that this system is capable of integrating derivatization, injection, separation, and detection for continuous GSH determinations.

An accessible micro-capillary electrophoresis device using surface-tension-driven flow

We present a rapidly fabricated micro-capillary electrophoresis chip that utilizes surface-tension-driven flow for sample injection and extraction of DNA. Surface-tension-driven flow (i.e. passive pumping) [G. M. Walker et al., Lab. Chip. 2002, 2, 131-134] injects a fixed volume of sample that can be predicted mathematically. Passive pumping eliminates the need for tubing, valves, syringe pumps, and other equipment typically needed for interfacing with microelectrophoresis chips. This method requires a standard micropipette to load samples before separation, and remove the resulting bands after analysis. The device was made using liquid phase photopolymerization to rapidly fabricate the chip without the need of special equipment typically associated with the construction of microelectrophoresis chips (e.g. cleanroom) [A. K. Agarwal et al., J. Micromech. Microeng. 2006, 16, 332-340; S. K. Mohanty et al., Electrophoresis 2006, 27, 3772-3778]. Batch fabrication time for the device presented here was 1.5 h including channel coating time to suppress electroosmotic flow. Devices were constructed out of poly-isobornyl acrylate and glass. A standard microscope with a UV source was used for sample detection. Separations were demonstrated using Promega BenchTop 100 bp ladder in hydroxyl ethyl cellulose (HEC) and oligonucleotides of 91 and 118 bp were used to characterize sample injection and extraction of DNA bands. The end result was an inexpensive micro-capillary electrophoresis device that uses tools (e.g. micropipette, electrophoretic power supplies, and microscopes) already present in most labs for sample manipulation and detection, making it more accessible for potential end users.

A low-leakage sample plug injection scheme for crossform microfluidic capillary electrophoresis devices incorporating a restricted cross-channel intersection

This study develops a crossform CE microfluidic device in which a single-circular barrier or a double-circular barrier is introduced at the cross-channel intersection. Utilizing a conventional crossform injection scheme, it is shown that these barriers reduce sample leakage and deliver a compact sample band into the separation channel, thereby ensuring an enhanced detection performance. A series of numerical and experimental investigations are performed to investigate the effects of the barrier type and the barrier ratio on the flow streamlines within the microchannel and to clarify their respective effects on the sample leakage ratio and sample plug variance during the injection process. The results indicate that a single-circular barrier injector with a barrier ratio greater than 20% and a double-circular barrier injector with a barrier ratio greater than 40% minimize the sample leakage ratio and produce a compact sample plug. As a result, both injectors have an excellent potential for use in high-quality, high-throughput chemical analysis procedures and in many other applications throughout the micro-total analysis systems field.

Quantitative analysis by microchip capillary electrophoresis: current limitations and problem-solving strategies

Obstacles and possible solutions for the application of microchip capillary electrophoresis in quantitative analysis are described and critically discussed. Differences between the phenomena occurring during conventional capillary electrophoresis and microchip-based capillary electrophoresis are pointed out, with particular focus on electrolysis, bubble formation, clogging, surface interactions, injection and aspects related to the power supply. Current drawbacks are specified and improvements for successful quantitative microchip capillary electrophoresis are suggested.

Recent trends in CE of inorganic ions: From individual to multiple elemental species analysis

The major methodological developments in CE related to inorganic analysis are overviewed. This is an update to a previous review article by the author (Timerbaev, A. R., Electrophoresis 2004, 25, 4008-4031) and it covers the review work and innovative research papers published between January 2004 and the first part of 2006. As was underlined in that review, a growing interest of analytical community in providing elemental speciation information found a sound response of the CE method developers. Presently, almost every second research paper in the field of interest deals with element species analysis, the use of inductively coupled plasma MS detection and biochemical applications being the topics of utmost research efforts. On the other hand, advances in general methodology traditionally centered on a CE system modernization for improvements in sensitivity and separation selectivity have attracted less attention over the review period. While there is no indication that inorganic ion applications would surpass by the developmental rate the more matured analysis of organic analytes, CE can now be seen as an analytical technique to be before long customary in a number of inorganic analysis arenas.

High performance microfluidic capillary electrophoresis devices

This paper presents a novel microfluidic capillary electrophoresis (CE) device featuring a double-T-form injection system and an expansion chamber located at the inlet of the separation channel. This study addresses the principal material transport mechanisms depending on parameters such as the expansion ratio, the expansion length, the fluid flow. Its design utilizes a double-L injection technique and combines the expansion chamber to minimize the sample leakage effect and to deliver a high-quality sample plug into the separation channel so that the detection performance of the device is enhanced. Experimental and numerical testing of the proposed microfluidic device that integrates an expansion chamber located at the inlet of the separation channel confirms its ability to increase the separation efficiency by improving the sample plug shape and orientation. The novel microfluidic capillary electrophoresis device presented in this paper has demonstrated a sound potential for future use in high-quality, high-throughput chemical analysis applications and throughout the micro-total-analysis systems field.