One of the major drawbacks of electrophoresis in both capillary and microchip is the unsatisfactory sensitivity. Online sample preconcentration techniques can be regarded as the most common and powerful approaches commonly applied to enhance overall detection sensitivity. While the advances of various online preconcentration strategies in capillary and microchip employing aqueous background electrolytes are well-reviewed, there has been limited discussion of the feasible preconcentration techniques specifically developed for capillary and microchip using nonaqueous background electrolytes. This review provides the first consolidated overview of various online preconcentration techniques in nonaqueous capillary and microchip electrophoresis, covering the period of the last two decades. It covers developments in the field of sample stacking, isotachophoresis, and micellar-based stacking. Attention is also given to multi-stacking strategies that have been used for nonaqueous electrophoresis.
Recent advances in microfluidic systems, particularly in the Micro Total Analysis System (μTAS) or Lab On a Chip (LOC), drive the current analysis tools and equipment towards miniaturization, rapid at-line testing and mobility. The state-of-the-art microfluidic technology targets a wider range but smaller volumes of analytes, making the analytical procedure relatively easier and faster. This trend together with faster electronics and modern instrumentation systems will make real-time and in situ analysis a definite possibility. This review focuses on microchip capillary electrophoresis with amperometric detection (MCE-AD) for the detection of DNA and other electroactive analytes. The problems associated with the microchip design, in particular the choice of materials and the configuration of electrodes are discussed thoroughly and solutions are proposed. Significant developments in the related areas are also covered and reviewed critically.
A portable microchip electrophoresis (MCE) coupled with on-chip contactless conductivity detection (C(4)D) system was evaluated for the determination of vancomycin in human plasma. In order to enhance the detection sensitivity, a new online multi-stacking preconcentration technique based on field-enhanced sample injection (FESI) and micelle-to-solvent stacking (MSS) was developed and implemented in MCE-C(4)D system equipped with a commercially available double T-junction glass chip. The cationic analytes from the two sample reservoirs were injected under FESI conditions and subsequently focused by MSS within the sample-loading channel. The proposed multi-stacking strategy was verified under a fluorescence microscope using Rhodamine 6G as the model analyte and a sensitivity enhancement factor (SEF) of up to 217 was achieved. The developed approach was subsequently implemented in the aqueous-based MCE, coupled to C(4)D in order to monitor the targeted antibiotic (in this case, vancomycin) present in human plasma samples. The multi-stacking and analysis time for vancomycin were 50s and 250s respectively, with SEF of approximately 83 when compared to typical gated injection. The detection limit of the method for vancomycin was 1.2μg/mL, with intraday and interday repeatability RSDs of 2.6% and 4.3%, respectively. Recoveries in spiked human plasma were 99.0%-99.2%.
A new multi-stacking pre-concentration procedure based on field-enhanced sample injection (FESI), field-amplified sample stacking, and transient isotachophoresis was developed and implemented in a compact microchip electrophoresis (MCE) with a double T-junction glass chip, coupled with an on-chip capacitively coupled contactless conductivity detection (C4 D) system. A mixture of the cationic target analyte and the terminating electrolyte (TE) from the two sample reservoirs was injected under FESI conditions within the two sample-loading channels. At the double T-junction, the stacked analyte zones were further concentrated under field-amplified stacking conditions and then subsequently focused by transient-isotachophoresis and separated along the separation channels. The proposed multi-stacking strategy was verified under a Universal Serial Bus (USB) fluorescence microscope employing Rhodamine 6G as the model analyte. This developed approach was subsequently used to monitor the target quinine present in human plasma samples. The total analysis time for quinine was approximately 200 s with a sensitivity enhancement factor of approximately 61 when compared to the typical gated injection. The detection and quantification limits of the developed approach for quinine were 3.0 μg/mL and 10 μg/mL, respectively, with intraday and interday repeatability (%RSDs, n = 5) of 3.6 and 4.4%. Recoveries in spiked human plasma were 98.1-99.8%.
Miniaturisation of microchip capillary electrophoresis (MCE) is becoming an increasingly important research topic, particularly in areas related to micro total analysis systems or lab on a chip. One of the important features associated with the miniaturised MCE system is the portable power supply unit. In this work, a very low electric field MCE utilising an amperometric detection scheme was designed for use in DNA separation. The device was fabricated from a glass/polydimethylsiloxane hybrid engraved microchannel with platinum electrodes sputtered onto a glass substrate. Measurement was based on a three-electrode arrangement, and separation was achieved using a very low electric field of 12 V/cm and sample volume of 1.5 µl. The device was tested using two commercial DNA markers of different base pair sizes. The results are in agreement with conventional electrophoresis, but with improved resolution. The sensitivity consistently higher than 100 nA, and the separation time approximately 45 min, making this microchip an ideal tool for DNA analysis.