Frederic Laugere

Capillary electrophoresis with on‐chip four‐electrode capacitively coupled conductivity detection for application in bioanalysis

Microchip capillary electrophoresis (CE) with integrated four‐electrode capacitively coupled conductivity detection is presented. Conductivity detection is a universal detection technique that is relatively independent on the detection pathlength and, especially important for chip‐based analysis, is compatible with miniaturization and on‐chip integration. The glass microchip structure consists of a 6 cm etched channel (20 νm×70 νm cross section) with silicon nitride covered walls. In the channel, a 30 nm thick silicon carbide layer covers the electrodes to enable capacitive coupling with the liquid inside the channel as well as to prevent interference of the applied separation field. The detector response was found to be linear over the concentration range from 20 νM up to 2 mM. Detection limits were at the low νM level. Separation of two short peptides with a pI of respectively 5.38 and 4.87 at the 1 mM level demonstrates the applicability for biochemical analysis. At a relatively low separation field strength (50 V/cm) plate numbers in the order of 3500 were achieved. Results obtained with the microdevice compared well with those obtained in a bench scale CE instrument using UV detection under similar conditions.

Fabrication of a glass-implemented microcapillary electrophoresis device with integrated contactless conductivity detection

Glass microdevices for capillary electrophoresis (CE) gained a lot of interest in the development of micrototal analysis systems (νTAS). The fabrication of a νTAS requires integration of sampling, chemical separation and detection systems into a microdevice. The integration of a detection system into a microchannel, however, is hampered by the lack of suitable microfabrication technology. Here, a microfabrication method for integration of insulated microelectrodes inside a leakage‐free microchannel in glass is presented. A combination of newly developed technological approaches, such as low‐temperature glass‐to‐glass anodic bonding, channel etching, fabrication of buried metal interconnects, and deposition of thin plasma‐enhanced chemical vapour deposition (PECVD) silicon carbide layers, enables the fabrication of a CE microdevice with an integrated contactless conductivity detector. The fabrication method of this CE microdevice with integrated contactless conductivity detector is described in detail. Standard CE separations of three inorganic cations in concentrations down to 5 νM show the viability of the new νCE system.

Considerations on contactless conductivity detection in capillary electrophoresis

Nearly all analyses by capillary electrophoresis (CE) are performed using optical detection, utilizing either absorbance or (laser‐induced) fluorescence. Though adequate for many analytical problems, in a large number of cases, e.g., involving non‐UV‐absorbing compounds, these optical detection methods fall short. Indirect optical detection can then still provide an acceptable means of detection, however, with a strongly reduced sensitivity. During the past few years, contactless conductivity detection (CCD) has been presented as a valuable extension to optical detection techniques. It has been demonstrated that with CCD detection limits comparable, or even superior, to (indirect) optical detection can be obtained. Additionally, construction of the CCD around the CE capillary is straightforward and robust operation is easily obtained. Unfortunately, in the literature a large variety of designs and operating conditions for CCD were described. In this contribution, several important parameters of CCD are identified and their influence on, e.g., detectability and peak shape is described. An optimized setup based on a well‐defined detection cell with three detection electrodes is presented. Additionally, simple and commercially available read‐out electronics are described. The performance of the CCD‐CE system was demonstrated for the analysis of peptides. Detection limits at the νM level were obtained in combination with good peak shapes and an overall good performance and stability.

Electronic baseline-suppression for liquid conductivity detection in a capillary electrophoresis microchip

Liquid conductivity detection for application in chip-based capillary electrophoresis (CE) has received attention because the sensor electrodes can be integrated in the chip and there is no loss of sensitivity when downscaling the detector. A well known disadvantage of conductivity detection is the high level of the baseline (due to the conductivity of the carrier electrolyte) that has a high influence on the measured values and deteriorates the detection limit. By using a lockin amplifier, making use of its two differential inputs and of its phase-locked sinewave output, we can generate a signal of controlled amplitude and phase, related to the level and phase of the baseline signal. By subtracting this signal from the AC output signal of the detector the baseline is significantly suppressed. Ideally, only the changes of the background signal are then measured. When applying the baseline-suppression method to real measurements, the level of the background signal was decreased 50 times. The detection limit (which is related to the signal-to-noise ratio) was found to be 10 times lower. Reproducible separations with concentrations from 1 mM down to 10 /spl mu/M of potassium, sodium, and lithium ions were obtained. For our separation and detection system, the concentration of 10 /spl mu/M could not be detected without the baseline-suppression technique described here.

Miniaturized analytical assays in biotechnology

Biotechnology today is a well-established paradigm in many areas of human endeavor, such as the pharmaceutical industry, agriculture, management of the environment and many others. Meanwhile, biology is undergoing a spectacular transition: whereas systematic biology was replaced gradually by molecular biology, the latter is rapidly being transformed into a new systematic era in which entire genomes are being charted by ever more sophisticated analytical techniques. In the wake of this onslaught of data, new fields are germinating, such as bioinformatics in an attempt to find answers to fundamental questions, answers that may be hidden in the massive amounts of data already available today.

Fabrication of nanochannels using glass to glass anodic bonding

In this work, we present a technology for fabrication of nanochannels created in glass with which bio-analysis can be performed in combination with fluorescence microscopy. The technology is based on a glass-to-glass anodic bonding process. In the bonding process, an intermediate layer (thin insulating film) is deposited on one of the two glass wafers. The channel is then defined, with one photo-patterning step, in the intermediate layer. In our approach, a 33 nm thick amorphous silicon layer (deposited by LPCVD) was used as an intermediate layer. The depth of the channel is defined during the etching of this layer.

Separation and detection of organic acids in a CE microchip with contactless four-electrode conductivity detection

In this paper, we report the separation of 6 biologically relevant organic acids in a capillary electrophoresis microchip. Electrophoretic separation was done in a fully-glass microchip into which four contactless electrodes were integrated for conductivity detection. The microchip design, the electronic interface, and the injection setup are described. Concentrations down to 50 μM (each) of the mixture of organic acids could be detected.

Measurement System for Biochemical Analysis Based on Capillary Electrophoresis and Microscale Conductivity Detection

The development of miniaturized integrated physical chemosensors and biosensors is an emerging field [1]. Physical chemosensors determine (bio)chemical properties of a fluid using physical properties or phenomena in the fluid instead of using a chemical interface. Capillary electrophoresis (CE) combined with liquid conductivity detection is a good illustration. The integration of this measurement system in a microchip aims to achieve fast (less than a minute) and reliable analysis with very low sample consumption (few nanoliters).

Electronic protection of the conductivity detector in a micro capillary electrophoresis channel

In this paper we present a method to prevent breakthrough of the thin insulating film on the contactless conductivity detector in a micro CE device, caused by the DC voltage present in the separation channel. We use a control loop to adjust the DC level of the detector to the potential present in the channel. The voltage in the channel is monitored by means of an integrated probe. The probe is in galvanic contact with the liquid. This method allows optimisation of the detector for better coupling to the liquid, yielding higher sensitivity and a lower detection limit.