Novel variable volume injector for performing sample introduction in a miniaturised isotachophoresis device

Reduction in sample injection bias using pressure gradients generated on chip

Sample injection in microchip-based capillary zone electrophoresis (CZE) frequently rely on the use of electric fields which can introduce differences in the injected volume for the various analytes depending on their electrophoretic mobilities and molecular diffusivities. While such injection biases may be minimized by employing hydrodynamic flows during the injection process, this approach typically requires excellent dynamic control over the pressure gradients applied within a microfluidic network. The current article describes a microchip device that offers this needed control by generating pressure gradients on-chip via electrokinetic means to minimize the dead volume in the system. In order to realize the desired pressure-generation capability, an electric field was applied across two channel segments of different depths to produce a mismatch in the electroosmotic flow rate at their junction. The resulting pressure-driven flow was then utilized to introduce sample zones into a CZE channel with minimal injection bias. The reported injection strategy allowed the introduction of narrow sample plugs with spatial standard deviations down to about 45 μm. This injection technique was later integrated to a capillary zone electrophoresis process for analyzing amino acid samples yielding separation resolutions of about 4-6 for the analyte peaks in a 3 cm long analysis channel.

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.

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.

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.

Experimental and numerical investigation into leakage effect in injectors of microfluidic devices

This paper performs an experimental and numerical investigation into low-leakage injectors designed for electrophoresis microchips. The principal material transport mechanisms of electrokinetic migration, fluid flow, and diffusion are considered in developing a mathematical model of the electrophoresis process. Low-leakage injectors designed with injection channels orientated at various included angles are designed and tested. The numerical and experimental results indicate that the injector with a 30 degrees included angle successfully minimizes sample leakage and has an exciting potential for use in high-quality, high-throughput chemical analysis procedures and in many other applications in the field of micro-total analysis systems.

Recent progress in capillary ITP

ITP has been attracting constant attention for many years due to its principal capability to concentrate trace analytes by several orders of magnitude. In the current capillary format, it is able to concentrate trace analytes diluted to several microliters of an original sample into concentrated zones having volumes in the range of picoliters. Due to this reason, ITP holds an important position in many current multistage and multidimensional separation schemes. This article links up previous reviews on the topic and summarizes the progress of analytical capillary ITP since 2002. Almost 100 papers are reviewed that include methodological novelties, instrumental aspects, and analytical applications. Papers using ITP and/or isotachophoretic principles as part of multistage and/or multidimensional separation schemes are also included.

Free flow isotachophoresis in an injection moulded miniaturised separation chamber with integrated electrodes

An injection moulded free flow isotachophoresis (FFITP) microdevice with integrated carbon fibre loaded electrodes with a separation chamber of 36.4mm wide, 28.7 mm long and 100 microm deep is presented. The microdevice was completely fabricated by injection moulding in carbon fibre loaded polystyrene for the electrodes and crystal polystyrene for the remainder of the chip and was bonded together using ultrasonic welding. Two injection moulded electrode designs were compared, one with the electrode surface level with the separation chamber and one with a recessed electrode. Separations of two anionic dyes, 0.2mM each of amaranth and acid green and separations of 0.2mM each of amaranth, bromophenol blue and glutamate were performed on the microdevice. Flow rates of 1.25 ml min(-1) for the leading and terminating electrolytes were used and a flow rate of 0.63 ml min(-1) for the sample. Electric fields of up to 370 V cm(-1) were applied across the separation chamber. Joule heating was not found to be significant although out-gassing was observed at drive currents greater than 3 mA.

Improvements on the electrokinetic injection technique for microfluidic chips

This paper presents a T-form electrokinetic injection system for the discrete time-based loading and dispensing of samples of variable-volume in a microfluidic chip. A novel push-pull effect is produced during the loading and dispensing processes by the application of an appropriate control voltage distribution. The experimental and numerical results show that this push-pull loading technique produces compact sample plugs and hence improves the detection resolution of the microfluidic device. The injection system is integrated with a microflow switch, and a suitable voltage control scheme is proposed to guide the sample to the desired outlet port such that the microfluidic device can function as a microdispenser. The time-based variable-volume T-form injection method presented in this study is performed using a compact geometry and a simple control scheme and can be readily integrated with other microfluidic devices to form a microfluidic system capable of continuous monitoring and analysis of bioreactions in the life science and biochemistry fields.

Capillary isotachophoresis from the student point of view – images and the reality

A review of some fundamental aspects of ITP from the student point of view, imaginations of some basic facts and laws, use of ITP, and the recent trends are presented. The results of theoretical computations of ITP separation processes are added for comparison of imaginations with the exact mathematical description.

Optimal configuration of capillary electrophoresis microchip with expansion chamber in separation channel

This study develops a novel capillary electrophoresis (CE) microfluidic device featuring a conventional cross-form injection system and an expansion chamber located at the inlet of the separation channel. The combined injection system/expansion chamber arrangement is designed to deliver a high-quality sample band into the separation channel such that the detection performance of the device is enhanced. Numerical simulations are performed to investigate the electrokinetic transport processes in the microfluidic device and to establish the optimal configuration of the expansion chamber. The results indicate that an expansion chamber with an expansion ratio of 2.5 and an expansion length of 500 microm delivers a sample plug with the correct shape and orientation. With this particular configuration, the peak intensities of the sample are sharp and clearly distinguishable in the detection region of the separation channel. Therefore, this configuration is well suited for capillary electrophoresis applications which require a highly sensitive resolution of the sample plug. The novel CE microfluidic device developed in this study has an exciting potential for use in high-performance, high-throughput chemical analysis applications and in many other applications throughout the field of micro-total-analysis-systems.

Rapid chloride analysis using miniaturised isotachophoresis

A new design of miniaturised separation device for performing isotachophoresis (ITP) has been produced. The device contains a simple arrangement of channels comprising a single separation channel with a 'double T' injection geometry. The device was produced in poly(methyl methacrylate) and incorporates an on-column conductivity detector. A new electrolyte system was developed to enable the rapid determination of chloride to be made. This electrolyte system uses a leading ion of 3.5 mM nitrate at pH 3.0 with 0.5 mM indium(III) added as a complexing agent. Use of this electrolyte system with the new separation device allowed chloride samples to be analysed in under 100 s, with a limit of detection (LOD) calculated to be 2.2 mg l(-1).

Miniaturised isotachophoresis analysis

The application of miniaturized total analysis systems (microTAS) has seen rapid development over the past few years. Isotachophoresis (ITP) has been transferred into microchip format for both electrophoretic separation and pretreatment purposes, due to its advantageous features including separation parameters controlled by electrolyte composition and high sample load capacity. The primary focus of this concise review is to summarize the basic features of microchip based ITP and its applications to the analysis and pretreatment of ionic compounds and biomolecules that have arisen since 1998.

Negative pressure pinched sample injection for microchip-based electrophoresis

A simple method for injecting well-defined non-biased sample plugs into the separation channel of a microfluidic chip-based capillary electrophoresis system was developed by a combination of flows generated by negative pressure, electrokinetic and hydrostatic forces. This was achieved by using only a single syringe pump and a single voltage supply at constant voltage. In the loading step, a partial vacuum in the headspace of a sealed sample waste reservoir was produced using a syringe pump equipped with a 3-way valve. Almost instantaneously, sample was drawn from the sample reservoir across the injection intersection to the sample waste reservoir by negative pressure. Simultaneously, buffer flow from the remaining two buffer reservoirs pinched the sample flow to form a well-defined sample plug at the channel intersection. In the subsequent separation stage, the vacuum in headspace of the sample waste reservoir was released to terminate all flows generated by negative pressure, and the sample plug at the channel intersection was electrokinetically injected into the separation channel under the potential applied along the separation channel. The liquid levels of the four reservoirs were optimized to prevent sample leakage during the separation stage. The approach considerably simplified the operations and equipment for pinched injection in chip-based CE, and improved the throughput. Migration time precisions of 3.3 and 1.5% RSD for rhodamine123 (Rh123) and fluorescein sodium (Flu) in the separation of a mixture of Flu and Rh123 were obtained for 56 consecutive determinations with peak height precisions of 6.2% and 4.4% RSD for Rh123 and Flu, respectively.

Novel variable volume injector for performing sample introduction in a miniaturised isotachophoresis device

A microdevice design furnished with a novel sample injector, capable of delivering variable volume samples, for miniaturised isotachophoretic separations is presented. Micromachining by direct milling was used to realise two flow channel network designs on poly(methyl methacrylate) chips. Both designs comprised a wide bore sample channel interfaced, via a short connection channel, to a narrow bore separation channel. Superior injection performance was observed with a connection channel angled at 45 degrees to the separation channel compared to a device using a channel angled at 90 degrees. Automated delivery of electrolytes to the microdevice was demonstrated with both hydrostatic pumping and syringe pumps; both gave reproducible sample injection. A range of different sampling strategies were investigated. Isotachophoretic separations of model analytes (metal ions and an anionic dye) demonstrated the potential of the device. Separations of ten metal cations were achieved in under 475 s.