Contactless conductivity detector

A portable electrophoretic contactless conductivity detection (C4D) system for analysis on a microfluidic chip houses in one embodiment a fluidic compartment for receiving the microfluidic chip, and four detection electrodes: first and second emitting electrodes, and first and second receiving electrodes. The first emitting electrode and the first receiving electrode are adjacent to a first channel wall of the microfluidic chip, and the second emitting electrode and the second receiving electrode are adjacent to a second channel wall, where the second channel wall is opposite to the first channel wall. In an embodiment, the electrodes are provided as portions of a removable cartridge cell.

FIELD OF THE INVENTION

The present invention relates to a device for analyzing and detecting analytes and more specifically to a new portable microfluidic analytical instrument based on electrophoresis as a separation technique and capacitive coupled contactless conductivity as a detector (C4D). More particularly, it relates to a novel design of the C4D detection cell that allows for low background noise and low limit of detection (LOD). The portable system includes all the electronics and hardware components to do analysis of ionic species. The system is connected to a laptop computer for data acquisition and processing and is powered up by a battery for absolute autonomy.

BACKGROUND AND SUMMARY OF THE INVENTION

Lab-on-a-chip (LOC) based device requirements for analyte detection are sensitivity, universality and portability. To this date, these conditions have not been fully met and detection remains the main challenge in the development of LOC technology. Optical detectors, including fluorescence detection, have demonstrated the highest sensitivity. However, optical detectors are not universal and not easily made portable due to the size of the light sources. The use of electrochemical methods is well-suited for integration into portable systems, but they are less sensitive and prone to interferences. From the group of electrochemical sensors, C4D detectors are the most appealing as they fulfill the requirements of portability, universality for charged analytes and acceptable sensitivity.

The principle of C4D in combination with electrophoresis will now be described with reference toFIG. 1.FIG. 1shows an arrangement of two external metal electrodes100a,100bin close proximity to an electrophoretic separation channel102in a microfluidic chip104. The microfluidic chip104comprises two polymer sheets, namely top sheet104aand bottom sheet104b. The top sheet104aprovides access to reservoirs as will be described below, and the bottom sheet104bprovides the separation channel102that has been hot embossed into the bottom sheet104b. In use, a run buffer reservoir107, a first sample reservoir109and an outlet reservoir111of the microfluidic chip104are filled with electrophoretic run buffer solution, and a second sample reservoir113is filled with target analytes, typically ionic species dissolved in the run buffer solution. A separation voltage is then applied between the second sample reservoir113and the first sample reservoir109. This drives ‘plugs’ of ions114into the separation channel102. Subsequently, the separation voltage is applied between the run buffer reservoir107and the outlet reservoir111with all other reservoirs floating. This causes the plugs of ions114to be driven towards the electrodes100a,100bfor detection.

The two external metal electrodes100a,100band the electrophoretic separation channel102together form the C4D cell or detection cell. When the upstream/emitting electrode100aemits an AC signal through the channel102, it is capacitively captured by the downstream/receiving electrode100b. The electrodes100a,100bare in the same plane and are attached to a top plate that seals the channel102and are typically placed in an anti-parallel configuration with respect to the length of the channel102. The applied AC signal (typically 50-600 kHz) from the emitting electrode100acapacitively couples through the channel102to the receiving electrode100b, resulting in a small current that is amplified by an amplifier106, rectified and offset-corrected using a rectifier108, filtered and that undergoes data acquisition using a data acquisition tool (DAQ)110and finally recorded in a computing device112or other storage device as a DAQ graph.

The C4D cell can be considered as a combination capacitor-resistor-capacitor (CRC) electrical circuit, where the electrodes100a,100band the channel's electric double layer form the capacitors, and the section of the channel102between the electrodes100a,100bforms the resistor. When a plug of ions114is driven through the section of the channel between the electrodes, the measured impedance of the system changes instantaneously because of the change in the resistance due to the different conductivity of the ionic species passing through the electrodes within the background electrolyte. In practical terms, this leads to a sudden change in the zero leveled output voltage or a peak in the DAQ graph. By electrophoresis, separated plugs of ions can be driven through the C4D cell at different times and the corresponding signal recorded, thus obtaining separated peaks according to the times at which the ions cross the C4D cell. Each peak is related by time to a specific ion, and the area under the peaks is related to the concentration of the specific ion. C4D in combination with electrophoresis therefore provides qualitative and quantitative analysis.

The capacitive coupled contactless conductivity detection (C4D) cells reported to date use two electrodes placed externally over the separation channel. An example is illustrated inFIGS. 2(a) and (b), which respectively show a perspective view and plan view of a conventional detection cell. As noted earlier, electrodes100a,100bin conventional detectors are fixed to a top plate200that seals the separation channel102and are typically placed in an anti-parallel configuration with respect to the channel102. In this configuration, the capacitance coupling to the solution in the channel102is inefficient and requires a high frequency and high voltage to couple the signal to detect low concentration samples. High frequencies, however, result in stray capacitance having a more significant effect. Changes in the conductivity of the solution will then only result in a small change over the background signal.

To reduce or eliminate the stray capacitance, different strategies have been employed such as placing a ground plane202between the electrodes100a,100bto shield their direct crosstalk (as shown inFIGS. 2(a) and (b)). However, while these strategies decrease the stray capacitance somewhat, the resulting detection sensitivity remains limited.

One alternative option to improve capacitance is to increase the magnitude of the AC voltage. However, high voltage levels are difficult to produce and are not safe to handle in portable systems. Another option to have increased capacitance is to use relatively large electrodes or detection lengths, but these approaches severely decrease resolution.

Without compromising resolution, one effective way to increase sensitivity in capacitive coupling detection is to reduce the distance between the electrodes at the detection area or the section of the channel between the electrodes (also known as the ‘detection cell volume’). Known arrangements have achieved this by either: (i) scribing off some portion from the chip surface so that electrodes can be disposed nearer to the channel, or (ii) incorporating electrodes within the chip (integrated chip) during the microfabrication so that they are close enough to the channel. These approaches are either inaccurate (for option (i) above) or require complex fabrication processes (for option (ii) above).

To improve the portability of LOC devices, improvements have been made to releasbly attach the system to a laptop computer. The advantage of portable systems is that tests and analysis can be done onsite and when needed, which improves efficiency and decreases costs. In addition, transportation of the samples exposes the samples to conditions where the sample may possibly be contaminated or degraded. The portable system is user friendly and simple to operate. Therefore, it does not need highly trained personnel to operate the lab-on-a-chip analytical instrument. The system is also low cost and rugged to be carried and transported.

In the past decade, there have been efforts on developing portable instrumentation with somewhat limited success. In the area of electrophoresis, currently a few portable systems have been described where C4D is used as detector. One of them was developed for glass capillaries which typically require higher voltages to substantiate a high electric fields as the capillaries have longer length than microfluidic devices. This system is described in Electroanalysis 19, 2007, No. 19-20, pp. 2059-2065 by Kuban, P; Nguyen, H T A; Macka, M; Haddad, P R; Hauser, P C. Another system is from Innovative Sensor Technologies GmbH called TraceDec. This system uses glass microfluidic chips which are not disposable due to cost. The sensing electrodes are place at a far distance form the defection volume which results on low sensitivity. Another version that has been commercialized is described in IEEE SENSORS JOURNAL, VOL. 8, NO. 5, MAY 2008 by Holger Mühlberger, Wonhee Hwang, Andreas E. Guber, Volker Saile, and Werner Hoffmann. This system uses microchips with detection electrodes that are micro patterned on the outer surface of a plastic microfluidic chip. Due to this extra fabrication step, the cost of these types of chips increases drastically. Another C4D system is being commercialized by eDAQ Pty Ltd This system is modular, not fully integrated and involves the powers supply module, the C4D electronics module and the microfluidic C4D detection platform The electrophoresis microfluidic chip sits atop the C4D platform where the pair of electrodes for transmitting and receiving the AC signal are located only at the bottom plane. The electrodes are brought into contact with the chip from one side only and there is no ground plane between them to shield them from direct coupling to eliminate stray capacitance. The sensing electrodes are of fixed dimensions and not exchangeable. The cell is not encased into a housing to shield the whole cell from external interference effects or noises.

With the present invention, we developed a new generation of portable E-dC4D that provides improved sensitivity, low detection limit and at the same time it is low cost which reduces the price per analysis.

The present invention relates to the use of emitting electrodes positioned or positionable adjacent to and on opposite sides of a microfluidic channel, and receiving electrodes adjacent to and positioned or positionable on opposite sides of a microfluidic channel.

In one specific expression, the present invention relates to a contactless conductivity detection cell including: a microfluidic chip having a channel defined by channel walls, first and second emitting electrodes, and first and second receiving electrodes, wherein the first emitting electrode and the first receiving electrode are adjacent a first channel wall, and the second emitting electrode and the second receiving electrode are adjacent a second channel wall, the second channel wall being opposite the first channel wall.

Preferably the emitting electrodes and receiving electrodes are substantially planar and substantially parallel to each other.

Preferably the emitting electrodes are placed one on top and one at the bottom of the chip passing over the channel, and are configured to act as electrostatic image of each other to concentrate and focus signals from each other into a detection cell volume of the detection cell. Similarly, the receiving electrodes are preferably placed one on top and one at the bottom of the chip covering channel, and are configured to act as electrostatic images of each other to extract coupled signal from a detection cell volume of the detection cell.

Preferably the electrodes are each positioned at a distance of between 1 μm and 1000 μm from the channel and preferably the microfluidic chip has a thickness in the range of 30 μm to 1 mm.

Preferably the detection cell further comprises a first ground plane between the emitting electrodes and the receiving electrodes, and a grounded metal housing containing the emitting electrodes, the receiving electrodes and the first ground plane.

Preferably the detection cell further comprises a second ground plane configured to shield the emitting electrodes and the receiving electrodes from interferences from electronic components housed in the grounded metal housing while keeping a very close distance between the receiving electrodes and the receiving amplifier encased in a second shielded housing.

Preferably at least part of the channel between the emitting electrodes and the receiving electrodes has a restricted submicron-sized or nano-sized width/cross-section.

In one form, the first emitting electrode and the first receiving electrode are preferably arranged on or in a top plate of the microfluidic chip, and where the second emitting electrode and the second receiving electrode are adjacent a base of the channel. In another form, the first emitting electrode and the first receiving electrode are preferably arranged adjacent one side of the channel, and wherein the second emitting electrode and the second receiving electrode are arranged adjacent an opposite side of the channel.

Preferably the detection cell further comprises multiple parallel channels, each channel having a pair of emitting electrodes and a pair of receiving electrodes, wherein all of the emitting electrode pairs are connected to a single input.

In another expression, the present invention relates to a portable electrophoretic micro fluidic system and a contactless conductivity detection system comprising: a platform having an opening configured to receive a microfluidic chip having a channel defined by channel walls, a cover configured to close at least part of the opening, first and second emitting electrodes, and first and second receiving electrodes, wherein the first emitting electrode and the first receiving electrode are configured to be positioned adjacent a first channel wall, and the second emitting electrode and the second receiving electrode are configured to be positioned adjacent a second channel wall, the second channel wall being opposite the first channel wall.

Preferably the system is battery powered, which allows for greater portability.

Preferably the cover is configured to secure at least part of a microfluidic chip between the cover and the base of the opening.

Preferably the second emitting electrode and the second receiving electrode are positioned on or adjacent the base of the opening.

Preferably the first emitting electrode and the first receiving electrode are positioned on or adjacent an internal surface of the cover. Preferably the cover includes a holder on the internal surface, and wherein the first emitting electrode and the first receiving electrode are positioned on the holder.

Preferably the holder is resiliently coupled to the cover and is configured to press the first emitting electrode and the first receiving electrode against a microfluidic chip.

Preferably the detection system further comprises one or more slots to allow a microfluidic chip to be inserted into the opening.

Preferably the cover is selected from a group consisting of: a pivotable cover and a detachable cover.

Preferably the emitting electrodes and/or the receiving electrodes are movable along the channel.

Preferably the opening is configured to allow a microfluidic chip to be movable within the opening.

Preferably the detection system further comprises a current-to-voltage converter adjacent and connected to the receiving electrodes, and a rectifier, low-pass filter, and offset circuit connected to the current-to-voltage converter. Preferably the detection system further comprises an alternating current function generator adjacent and connected to the emitting electrodes, and a miniaturized high voltage power supply. Preferably the detection system further comprises detection electronics arranged on a circuit that comprises a top layer and a bottom layer, the top layer being isolated from the bottom layer.

In yet another specific expression, the present invention relates to a capacitive coupled contactless conductivity detection cell comprising: a microfluidic chip having a channel, and detection electrodes placed in a top-bottom geometry and in close proximity to the channel.

In still another specific embodiment, the present invention relates to a capacitive coupled contactless conductivity detection cell including: detection electrodes placed in a top-bottom geometry in a housing, a detection area located within the housing, a Faraday shield, and a grounded metal housing, wherein the electrodes are shielded from direct cross talk or external noise by the Faraday shield and the grounded metal housing.

Preferably the detection electrodes comprise two emitting electrodes and two receiving electrodes.

Preferably the emitting electrodes are placed one on top and one at the bottom of the channel, and are configured to act as electrostatic images of each other to concentrate and focus the signals from each other into a detection cell volume of the detection cell.

Preferably the receiving electrodes are placed one on top and one at the bottom of the channel, and are configured to act as electrostatic images of each other to extract coupled signals from a detection cell volume of the detection cell.

Preferably the detection electrodes comprise two emitting electrodes and two receiving electrodes separated by the Faraday shield and located in the grounded metal housing.

Preferably the housing holds a microfluidic chip inserted into a holder within the emitting and receiving electrodes.

Preferably the emitting and receiving electrodes are placed close to the separation channel, and are movable by a cover to adjust a detection cell volume of the detection cell.

Preferably the detection area is adjustable by moving the microfluidic chip within the emitting and receiving electrodes.

In one form of the invention the electrodes are provided within a replaceable cartridge cell. In use the cartridge cell is intended to sandwich the detection cell with the microfluidic channel. The cell may be releasably attached to the rest of the detecting system by a mechanism which electrically connects the emitting electrodes and the receiving electrodes to corresponding electrical contacts of the housing. A variety of replaceable cartridges may be provided, suitable respectively for different applications in terms of detection sensitivity and resolution. For example, they may differ in their electrode dimensions and gap distance. Thus, in contrast to embodiments in which these parameters are variable, this form of the invention is robust, and less complex, which reduces cost. A user can adapt the measurement which is made very easily by replacing one cartridge with another.

The present invention provides a C4D cell with improved sensitivity and detection limit compared to detection cells of the state of the art. A benefit of improving sensitivity is that the present invention can be implemented using lower power inputs than previously employed. The gap distance between the detection electrodes is made adjustable in accordance with certain embodiments of the invention, hence the detection cell length and thus the limit of detection (LOD, which determines sensitivity) and/or peak separation (which determines resolution) can be fine-tuned depending on demands of the specific application Where a shielded housing is provided containing all the necessary electronics and the C4D cell, an enhanced signal-to-noise ratio (S/N) is able to be obtained, which results in a highly sensitive portable electrophoretic analyzer. Embodiments of the present invention also provide a C4D detection device that has low power requirements. These and other related advantages will be readily apparent to skilled persons from the description below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Set of Embodiments

Referring toFIG. 3, the present invention in one preferred embodiment comprises a capacitively-coupled contactless conductivity detector comprising a microfluidic electrophoretic chip (not shown) having a separation channel102thereon, first and second emitting electrodes100a,101a, and first and second receiving electrodes100b,101bon the chip and adjacent the separation channel102. Between the emitting electrodes100a,101aand the receiving electrodes100b,101bis a detection area generally indicated with arrow300. As will be described in detail below, the first and second emitting electrodes100a,101aare configured to concentrate signals emitted from one another to the detection area300, and the first and second receiving electrodes100b,101bare configured to extract signals from the detection area300.

FIG. 3shows that the separation channel102is defined by channel walls, with the first emitting electrode100aand the first receiving electrode100bbeing located adjacent a first channel wall302. The first channel wall302forms part of the top plate of the microfluidic chip. The second emitting electrode101aand the second receiving electrode101bare located adjacent a second channel wall304, which is opposite the first channel wall302. This allows for maximum concentration and extraction of signals within the detection area300. The receiving electrodes100b,101bare displaced along the length of the channel102from the emitting electrodes100a,101a, with the area in the channel102between the emitting and receiving electrodes defining the detection area300. The second channel wall304forms part of the base of the channel102in the preferred embodiment. For ease of reference, this arrangement or geometry of electrodes will herein be referred to as a top-bottom geometry. Accordingly, the first emitting electrode100ais a top emitting electrode, the second emitting electrode101ais a bottom emitting electrode, the first receiving electrode100bis a top receiving electrode and the second receiving electrode101bis the bottom receiving electrode. Also for ease of reference, the emitting and receiving electrodes will collectively be referred to as detection electrodes.

All of the detection electrodes100a,100b,101a,101bare substantially planar, parallel to each other, and are arranged in close proximity to the detection area300to enhance the coupling of the excitation signals into the channel102and to prevent loss of signals to be extracted. The distance between the detection electrodes and the channel is preferably between 75 μm-150 μm. This is, however, a non-limiting range as the distance may be set anywhere from 1 μm-1000 μm depending on a variety of factors (e.g. chip thickness, method of electrode placement, application desired etc). To avoid direct coupling or crosstalk between the emitting and receiving electrodes, a ground plane202is provided.

The separation channel102of the preferred embodiment is fabricated in a thin plastic (e.g. 30 μm-1 mm, a non-limiting example being a 125 μm polymeric) or glass film with the detection electrodes being disposed (e.g. by sputtering) on hot embossed sealed channels on the film that are arranged to allow positioning of the detection electrodes near the detection cell volume. The use of thin microfluidic chips allows the placing of the detection electrodes in close proximity to the channel so as to achieve a large capacitance, and thus, a larger signal output. It is, however, not essential that the detection electrodes be printed on the microfluidic chip itself. An alternative placement of the detection electrodes will be described in further detail with reference to the portable detector embodiment inFIG. 13.

As will be appreciated fromFIGS. 4 and 5, the electrode arrangement above improves the coupling and extraction of the signals through the detection cell volume, resulting in a more precise and sensitive analysis capability. Specifically, the figures show a comparison of the output signals captured by an oscillator between the cell geometry of the preferred embodiment (FIG. 4) and that of a conventional detection cell (FIG. 5). Given an input signal of 300 kHz, 50 Vpp, the amplitude of the output signal of the preferred embodiment is at 600 kHz, 0.62 Vpp. A similar input to the conventional electrode arrangement (with the same electrode dimensions and the same detection volume) also produces an output of 600 kHz but at only 0.46 Vpp.

The voltage amplitude increase in the output signal in the present invention is the result of a better coupling between the electrodes placed in the configuration of the present invention. One reason for this is that both the top and bottom emitting electrodes100a,101aare connected to one source of excitation signal, which causes the emitted signal from the top electrode100ato be repelled and concentrated into the detection area of the channel102by the bottom electrode101aand vice versa. In other words, the electrodes100a,101form a quasi closed conductor (acting as electrostatic images of each other) reflecting the electrical signals into the detection cell volume, thus concentrating and increasing the electric field lines in the detection cell volume. Also, at the receiving electrodes100b,101b, the chances of losing signals are reduced as both electrodes100band101bare there to collect the incoming signals. As a result of the increased number of electric field lines in the detection cell volume and the reduced loss of signals to be extracted, improved signal interaction and signal extraction are made possible from every charge passing through the detection cell volume.

It should be noted that the detection cell length determines the signal output. Increasing the detection length by increasing the displacement or separation gap between the electrodes will increase the signal intensity. However, at the same time, it will also reduce resolution or the ability to discern between two separated plugs, which is important in order to identify and quantify analytes of interest. Depending on the particular application, these two parameters (signal intensity vs. resolution) will need to be optimized simultaneously and balanced. In the preferred embodiment, therefore, the displacement between the emitting and receiving electrodes is adjustable by having at least one pair movable along the separation channel. This allows adjustment of the detection limit (efficiency) and/or peak separation (resolution) depending on the demands of the specific application.

A series of experimental analysis were carried out to demonstrate the improvement in the output signal comparing conventional cell geometry and the cell geometry of the present invention. During the experiments, other conditions were kept exactly the same: input signal (sine wave 300 kHz, 50Vpp), detection length (2 mm), electrode width (1 mm), buffer solutions and samples' concentration. A sample containing four cations at 0.05 mM concentration was analyzed by electrophoresis and the resulting signals were recorded in an electropherogram using a DAQ system. The following were the electrophoretic separation conditions: separation buffer, 10 mM His/tris buffer solution; L injection protocol by applying potential of 4 kV between the first and second sample reservoirs for 2 s, computer controlled switching to run buffer reservoir and the grounded outlet reservoir using 4 kV for separation; sample 0.05 mM cations (NH4+, K+, Na+, Li+).

FIG. 6shows the comparison between the electropherogram obtained from the top-bottom configuration of the preferred form (line600) and the conventional configuration (line602). The experimental results indicate an average of 20% increase in the peak height or approximately 0.2V increase for every volt of the output signal. For example, for the peak height of K+at 50 μm concentration, the signal is increased from 20 mV to 28 mV. This means that the ratio concentration/signal has been decreased from 2.5 Km/mV to 1.7 Km/mV. This accordingly improves the detection limit (i.e. a lower concentration of a sample is now sufficient to produce a discernable output).

In addition to providing a top-bottom configuration, embodiments of the present invention also provide additional improvements to C4D cell geometries.

Referring toFIGS. 7(a) and (b), a chip104is shown with a separation channel102, a plug of ions114and pairs of detection electrodes100a,101aand100b,101b. The detection electrodes are shown adjacent the detection cell volume of the separation channel102, with at least part of the detection cell volume being scaled down to submicron and/or nanosize with a restrictive or narrowed-down channel102across-section/width at or about the detection area. The restrictive channel102anot only functions to increase resistance but also functions to enlarge the change of resistance from background to analyte. A higher change of resistance over the background signal translates to a higher output signal level i.e. the S/N ratio. The restrictive channel102amay be fabricated using the molds shown inFIG. 7(c) or (d), which respectively provide two peripheral restrictions (i.e. around an obstruction700in the centre of the channel102) and a central restriction to the channel102.

In another embodiment, as shown inFIG. 8, the detection cell of the present invention comprises emitting electrodes100a,101a, ground shields201a,201band receiving electrodes100b,101bthat have been integrated into the chip104using thermal bonding. The ground shields201a,201bin addition to the plane ground shield202help to reduce further the stray capacitance due to coupling through the bulk polymer material between the emitting electrodes100a,101a, and receiving electrodes100b,101b. Each pair of electrodes is positioned laterally in parallel (i.e. the pairs are parallel in the same plane), in close proximity to the channel102and are bonded at the same time the channel and top cover of the chip are bonded. In another approach, the electrodes can be bonded between the plastic sheets (top sheet104aand bottom sheet104b) that form the chip104before the assembly is aligned and bonded to another piece of polymer sheet. A lateral hot embossed groove may be provided to facilitate the alignment before bonding. In this embodiment, the top electrodes are essentially on one side of the channel, and the bottom electrodes are on an opposite side.

FIG. 9shows a modification to the lateral electrode embodiment. In this modification, each lateral pair is in the form of enclosed lateral channels which are in close proximity to the separation channel102and which are filled with conductive material (although only one pair is shown inFIG. 9). In particular, the electrodes begin as channels hot embossed into the chip at the same time as when the cross channel is hot embossed, and then sealed. The design and the gap to the channel are tailored using the mold, hot embossing process and bonding conditions. Once the channels are formed and sealed, the channels are connected to reservoirs and are filled via pumps with a conductive material (e.g. silver paint) which, after drying, become conductive electrodes. The closest gap between the electrodes and the channel achieved in this configuration is 5 μm. By optimizing the hot embossing and boding conditions, the gap can be scaled down to even smaller sizes (e.g. 1 μm).

In addition to a cell electrode geometry, the present invention also comprises a shielded housing for the C4D detector of the electrophoretic system. A schematic of one shielded housing of the detection cell is shown inFIG. 10. The housing1000acts as a Faraday shield to shield only the detection cell within the separation channel102, leaving the rest of the microfluidic chip104accessible for input of fluids and application of high voltage for electrophoresis separation. The housing1000substantially eliminates interferences from external noise sources, from the DC power applied at the terminus of the channels, and from handling operations, such as sample and reagent injection etc. Specifically, a first ground plane202is provided to eliminate direct coupling between the detection electrodes, and a second ground plane203is provided to isolate the detection electronics from the C4D electrodes. In the embodiment illustrated, the first and second ground planes202,203are in an anti-parallel (i.e. perpendicular) configuration with respect to each other.

The shielded housing1000, which is made of metal and which preferably extends perpendicularly to the length of the chip104, is grounded and includes a cavity1004in which the detection electronics and circuit is positioned. The detection electronics are arranged on a circuit that comprises two layers, i.e. top and bottom printed circuit board (PCB) layers. The top layer is isolated from the bottom layer and comprises an operational amplifier and exchangeable feedback resistor to convert current to voltage. A very short coaxial cable transfers the signal from the receiving electrodes through a hole to the top layer. Then, the converted voltage signal is transferred to the bottom layer for rectification, low pass filtering, and offset/gain baseline suppression to bring the level of the output signal to zero.

An alternative shielded housing1000is shown inFIG. 11A. As before, the housing1000includes top electrodes100a,100band bottom electrodes101aand101btogether with a ground plane202. A Faraday shield1100is provided to shield the detection electrodes and the detection area between the electrodes. The shielded housing ofFIG. 11Afurther includes a pivotable cover1102to allow access to the detection cell. In one embodiment, the cover1102includes a retractable electrode mechanism1104, which is configured to sandwich a microfluidic chip in the housing1000and to press the detection electrodes against the chip tightly when the cover1102is closed so as to achieve closest proximity and to avoid air gaps between the detection electrodes and the separation channel.

The retractable electrode mechanism1104of the preferred embodiment includes a holder1106that is resiliently coupled to the internal surface1102aof the cover1102and that secures the top emitting and receiving electrodes100a,100b. The resilient coupling is by way of springs1108as shown inFIG. 11B.FIG. 11Balso shows the placement of a microfluidic chip1110having a channel102in the opening between the cover1102and the base1112of the opening of the device in or on which the bottom electrodes101a,101bare located. The cover1102, chip1110and base1112are shown inFIG. 11Bhaving gaps therebetween for clarity. In use, gaps in the arrangement are minimized by having the cover1102clamp down so as to press and secure the chip1110on the base1112.

Another alternative embodiment of the shielded housing1000is shown inFIG. 11C. The housing1000in this embodiment is similar to that ofFIG. 10but includes a detachable cover1102. The cover1102is arranged to press against a resilient spacer1114, which in turn presses the top electrodes100a,100bagainst the microfluidic chip1110. This in turn presses against the bottom electrodes101a,101b, thus optimizing the proximity of the detection electrodes to the detection area. In this configuration, the chip1110is slid into position via slots1116in the housing1000. The top and bottom electrodes100a,100b,101a,101bare positioned such that when the chip1110is slid into position, the top electrodes100a,100bare automatically positioned on one side of the chip1110, while the bottom electrodes101a,101bare automatically positioned on the opposite side of the chip1110.

In the arrangement of the housing1000ofFIG. 11C, the chip1110is placed as close as possible to the electronics in the housing1000. The housing therefore includes a cavity or compartment1004to contain the electronic components of the device, like the embodiment ofFIG. 10. A first ground shield (not shown) is arranged between the emitting and receiving electrodes while a second ground shield203is arranged to separate the detection cell from the electronic components. This arrangement allows minimal signal losses and interference from noise signals from outside the detection cell.

FIGS. 12(a) and (b) respectively show a comparison of the noise level without and with the shielded housing. Without using the shielded housing, the level of noise for a signal of 300 kHz, 20Vppis between 3 to 4 mV, while with the shielded housing, this value is reduced to 1 to 2 mV.

In the exemplary embodiment ofFIG. 13, the invention provides a portable electrophoretic system1300that is configured to sandwich a chip (not shown) that is located in an opening1304(herein chip slot1304) on a platform1301. The chip slot1304is configured to allow the chip to be moved along the slot1304to allow a repositioning of the detection area of the chip.

The portable system1300includes a metal housing1302for shielding the detection cell of the chip. The metal housing1302is provided in two halves: one half extending from the cover1102and enclosing the top electrodes100a,100b, and the other half located in the base of the chip slot1304. Once the cover1102is closed, the two halves make up a complete housing. Much like the embodiment ofFIG. 11A, a retractable electrode mechanism is provided in the cover1102to retract the top electrodes100a,100bso that they make perfect contact with the chip when the cover clamps and secures the chip on a base of the slot1304. The portable system1300also includes bottom electrodes101a,101bon or adjacent the base of the chip slot1304. The arrangement of the detection electrodes on the cover and the base are such that when a chip is placed in the chip slot1304and the cover1102is in its closed position, the detection electrodes are in an operative arrangement described earlier with reference toFIG. 3.

Next to the housing1302are encased detector electronics which are shielded from the detection cell and from external noise to provide minimal noise and/or signal loss and easy transport. The portable device1300also includes a switching mechanism for alternating electrodes in operation or toggling the voltage in the reservoirs between sample injection and ion separation using high-voltage DC electrodes1306for electrophoresis. Also provided in the portable system1300are: a display1308, an optional ground electrode acting as an electric ground to shield the detection electrodes from direct coupling, and an electronic signal processing unit comprising a signal generator and amplifier (including a potentiometer1310for baseline suppression and a resonator1312for AC generation), and a miniaturized high voltage power supply system. Specifically, for the receiving side, a battery-powered current-to-voltage converter is provided adjacent and connected to the receiving electrodes, and a rectifier, low-pass filter, and offset circuit are connected to the current-to-voltage converter. For the emitting side, a battery-powered alternating current function generator is provided adjacent and connected to the emitting electrodes, together with a miniaturized high voltage power supply. Connections1314are provided to interface the portable device1300with a computing device. By providing portability, detection or analysis can be carried out at the point of use, which obviates the need to transport samples back to a laboratory.

In one embodiment, the portable device1300further comprises wireless capabilities. Specifically, the device1300may be configured to send signals wirelessly to a personal digital assistant (PDA), a smart phone, a portable meter or the like. This may be implemented using electronics having a Bluetooth or WiFi module configured to send and/or receive signals wirelessly. The device1300may also operate wirelessly such that sample injections and switching on and off of the device may be controlled wirelessly.

In other embodiments, such as that shown inFIG. 14, the portable device is provided with multiple detection capabilities using multiple pairs of top-bottom electrodes where all the emitting pairs A, B and C are connected together to a single input signal1400and all the receiving pairs D, E and F are connected to individual receiving amplifiers and processing electronics1402, and using a single analog-to-digital converter (ADC)1404with multiple inputs and a DQA system1406with multiple electropherograms, each corresponding to one sensor and analysis. This may be carried out by providing a microchip having multiple enclosed parallel cross channels all in one chip, which is inserted into the chip slot of the portable device to be operatively coupled in parallel to the multiple emitting and receiving electrodes in the device. A miniaturized high voltage power supply system1408and relays1410are used in this embodiment to selectively activate the separation electophoresis in the multiple enclosed parallel cross channels.

Another alternative embodiment of the C4D detection cell according to the present invention is an application of the top-bottom configuration in an on-line conductivity monitoring device, e.g. for monitoring total dissolved ionized solids in water samples. This device is shown schematically as1500inFIG. 15. The device1500includes the top and bottom emitting electrodes100a,101a, and the top and bottom receiving electrodes100b,101b. A ground plane202is also provided together with a shielded housing1000. Fluid for monitoring will enter the device at an inlet1502, flow down the microfluidic channel1504to the outlet1506. As will be appreciated by skilled persons, the on-line monitoring device has applications in, for example, on-line water quality control, on-line monitoring of dialysis water treatment system, on-line conductivity monitoring of haemodialysis process, chemical concentration control, etc.

Experiments

The experiments for the present invention began with the fabrication of microfluidic chips. Specifically, thin microfluidic chips were fabricated by hot embossing on polycarbonate (PC) or polymethyl methacrylate (PMMA) sheets with a thickness of only 125 μm. To emboss PC and PMMA, a hard stamp carrying the microfluidic channel features was used. The stamp and polymer were heated together on a hot plate to a temperature slightly above the Tg. After the stamp polymer sandwich reached the embossing temperature, a uniform pressure of 3.2-4.8 kN was applied to the chip area for 10 minutes. With the force still applied, the system was cooled below the Tg. On reaching this temperature, the force was released and the embossed substrate was de-molded. Sealing of the embossed channels was performed by bonding a second substrate where inlet reservoirs have been drilled in. Bonding was preformed by applying a uniform pressure, exerting 1.6 kN to the chip area at 140° C. for 20 minutes.

Electrophoresis is performed on the plastic chips. Initially the channels were preconditioned and then filled through one of the reservoirs with separation buffer: 10 to 30 mM MES-His (2-N-morpholinoethanesulfonic acid/histidine), 2 mM 18-Crown-6, and pH 6.4. Reservoirs107,109and111(seeFIG. 10) were filled with the electrophoretic run buffer solution, while reservoir113was filled with the sample mixture (target ions dissolved in the separation buffer).

After an initial loading, the sample was injected into the separation channel by applying potentials of +1000 V (cations) or −1000 V (anions) for 1 s to 5 s between reservoirs109and113. This voltage drove the ions electrokinetically through the channels intersection, which were subsequently separated by applying a separation voltage of 2,000 to 10,000V between the reservoirs107and111.

The C4D detector was formed using a pair of emitting and receiving electrodes of 1-2 mm in width displaced by 0.3-2 mm in distance. Typical AC actuation signals for the thin plastic chips and top-bottom electrode design are 10-100Vppat 100 to 300 kHz.

Experiments were then carried out to detect sample inorganic cations and anions. For the experiment to detect cations, the concentration of each ion was 0.1 mM (6 mg/L). as shown inFIG. 16. The experiment conditions were: injection voltage, 3000V; injection time 2 s; separation voltage 4000V; running buffer 30 mM MES-His pH 6; sinus input waveform with a frequency of 300 kHz and 50Vpp; electrode distance 0.5 mm; and electrode width 1 mm. For the experiment to detect anions, the concentration of each ion was 1 mM. The experiment conditions were: microfluidic chip 130/128 mm total/effective length; electrolyte solution 30 mM MES/His, 2 mM 18-crown-6, pH 6; injection voltage, 1 kV for 1 s; separation voltage, 7 kV; C4D detector: sine waveform of 300 kHz and 15 Vpp; electrode distance, 1 mm; electrode width, 1 mm.

The resulting electropherogram of cations at concentration of 0.1 mM (6 mg/L) is shown inFIG. 16(x-axis in seconds and y-axis in volts). All analyte peaks of interest were baseline resolved. The electropherogram of anions at concentration of 1 mM is shown inFIG. 17(x-axis in seconds and y-axis in milivolts). Both figures show accurate determinations of the target ions.

Referring toFIG. 18, the electrophoretic analysis and conductometric detection were carried out for inorganic cations in a standard mixture containing 1-50 μM of each ion. The experiment operating conditions were: injection voltage 4000 V; injection time 1-3 s; separation voltage 10 kV; running buffer 10 mM MES-His pH 6; sinus input waveform with a frequency of 300 kHz and 20Vpp; electrode distance 0.5 mm; electrode width 1 mm. The graph depicted shows the detected spectra for the following concentrations: 50 μM—line1800, 10 μM—line1802, 5 μM—line1804, 2.5 μM—line1806and 1 μM—line1808(x-axis in seconds and y-axis in volts).FIG. 18also shows the detection of cations at the limit of detection (LOD) of the experimental set up, i.e. LOD of 0.001 mM, 1 μM. The inset ofFIG. 18shows a magnified spectrum of the 1 μM sample (line1808) compared to that of the 50 μM sample (line1800).

Advantages arising from the present invention will be apparent from the foregoing description. For example, it will be appreciated that the top-bottom cell geometry allows electric field lines from each emitting electrode to be confined or focused in the detection cell volume, and signals from the detection cell volume to be optimally extracted by the receiving electrodes. This provides the present invention with improved sensitivity (i.e. the ability to detect smaller amounts of samples) over existing microfluidic based electrophoretic C4D systems. It is important to be able to provide good sensitivity at the same time as portability and be able to extend the analytical method to a bigger range of analytes including those minor elements like heavy metals.

Sensitivity is also improved by embodiments of the invention that use thin microfluidic chips, as they allow the detection electrodes to be in very close proximity to the channel to improve capacitive coupling between the AC voltage and the solution in the detection cell. This can be contrasted with known systems that use thick microfluidic chips (i.e. >1 mm thickness). When the microchips are made of thick polymer sheets, the electrodes are placed at larger distance from the channel, which causes the capacitance to drop. To address this, conventional systems provide a higher frequency input AC signal to achieve sufficient coupling between the electrodes. In general, this leads to higher stray capacitance and overall loss in sensitivity. Another conventional alternative is to increase the magnitude of the AC signal from typical values of 20-50V to 300Vpp. Such a voltage, however, is difficult to handle from the point of view of safety and is impractical in portable systems.

The portable C4D-LOC analytical system with optimized sensitivity requires lower inputs of voltage than previously employed to achieve low detection limits. That is to say, by increasing the electrodes' capacitance by placing the detection electrodes in a top-bottom configuration and in close proximity to the detection cell volume, and optionally improving the signal-to-noise ratio by using a ground plane in a shielded housing to eliminate crosstalk and external electrical noise, the present invention is feasibly operable on a lower power. This is a particularly important advantage for portable systems since a high AC voltage is dangerous to handle and the instrumentation required to produce such high signals can be prohibitively bulky.

Where a shielded housing is implemented, further improvements in sensitivity and thus LOD can be obtained since the C4D detection cell is further isolated from the high voltage, environmental noise and artifacts. This can be contrasted with known C4D detectors that do not implement a shielded housing and that accordingly suffer from much higher LODs.

Where the electrode arrangement of the invention is implemented using external electrodes (i.e. not integrated with the chip) with adjustable distance integrated into a shielded housing, a robust and low-cost process is provided for the fabrication of the detection cell. This avoids the cost and complexity of micro-fabrication processes.

The foregoing describes a first set of preferred embodiments, which, as will be understood by those skilled in the art, may be subject to variations or modifications in design, construction or operation without departing from the scope of the claims. For example, while most embodiments have been described with reference to electrophoresis, this is not essential. As described with reference toFIG. 15, the detection cell or detection system may be used for non-electrophoresis purposes (e.g. liquid chromatography, stand alone conductivity sensor). Also, in terms of electrode arrangement, while the figures show electrodes aligned to the channel, this is not essential. All that is required is for the electrodes to cover and be adjacent the channel so that signal coupling is achieved as described earlier. The position of the electrodes with respect the channel is not critical and signal coupling can be accomplished when the electrodes cover totally or partially the channel width. It will also be appreciated that while the term top-bottom has been used to describe the electrode geometry, the invention is not limited to the top and bottom areas of the channel; an opposing-sides placement as shown inFIG. 8is also encompassed. Also, the electrodes need not be positioned flat on the microfluidic chip but may, where necessary or desired, be positioned vertically (i.e. so the edges of the electrodes are adjacent the channel). The above variations, for instance, are intended to be covered by the scope of the claims.

Additional Preferred Embodiment

Another aspect of the present invention is a compact portable lab-on-a-chip analytical instrument based on electrophoresis integrated with a dC4D detector. The portable instrument is shown schematically inFIG. 19. The instrument comprises two sections: main body housing and a fluidic compartment. The housing accommodates the instrument electronics including the signal input/output devices2000(the output devices generate a signal to send to a computer or PDA) and signal processing electronics2100. The front top part of the instrument housing has a lid, and opens up to a fluidic compartment where the fluid chip is placed in the chip stage2300and high voltage (HV) is applied by means of HV electrodes2200(to supply voltage to a microfluidic chip on the chip stage2300when the lid closes), and electrophoresis and C4D detection is performed on the chip by a removable detection cell cartridge2400.

Main Body Instrument Electronics

The instrument is lodged on a housing containing the analytical hardware and processing electronics in the instrument. The hardware and electronic circuit boards include the input signal generator and amplifier2500, signal processing electronic circuit2100, relays switching, and miniaturized HV power supplies. The details of the components are shown schematically inFIG. 20. This main body housing is shielded to prevent from interference and for noise reduction. The device and components are designed to be powered by ±15 Volts or ±5 Volts batteries. Two sealed lead acid rechargeable batteries less than 0.5 kg in weigh and life time more than 20 hrs are used one to feed the electronics and the other for the HVs electrophoresis power supplies. The electronics section consists of pickup amplifier that collects the signals from the pickup electrode and converts them to voltage while amplified and then transfer them to rest of the electronics for further processing. This analytical signal from the cell is a very weak current in the order of nA. Compared to commercial systems available, we carried out novel engineering to pickup and collect this current very carefully. A combination of improved self latching connectors plus shielded gold plated electrodes connected to self-latching pins with minimal capacitive effects but perfect and tight contacts provide the perfect low noise signal pickup, low loss, and safe signal transfer into the rest of the system. The current is transferred to the pickup OPA through coaxial cables to reduce any Interference from outside. The pickup OPA electronics are placed in very close proximity to the pick up electrode for minimal signal loss. The contacts are made to the shielded dC4D cell by means of short coaxial cables and low capacitive surface mount self latching plugs and jacks. The self latching mating and unmating feature provides perfect electrical contact against vibration, shock, pull, and facilitates miniaturization and operation is small areas.

The stability of the output signal is dependent on the stability of the supply voltage of the electronic components (±Vin). One approach is to put capacitors from the positive and the negative power supply rail to ground to keep the supply voltage stable when the IC chips change their current consumption (the supply paths on the PCBs have a small resistance so to get a low voltage drop when current flows). Otherwise one might see oscillations and other artifacts One typically puts 10 μF (tantalum) on each printed circuit board and 100 nanoF (foil) for each IC, right next to the supply pins. Here a new generation of tantalum capacitors NeoCapacitor® (NECTOKIN) that employ a conductive polymer, has excellent noise-absorbing characteristics were added to all the circuits next to the ±supply voltage of every IC components. Every section of the device in general is sealed within the grounded housing and the only signal transfer is through low loss coaxial cables connected to surface mount miniaturized plugs and jacks. The output signal of the pickup amplifier is transferred to signal processing circuit for further rectification, low pass filtering and suppression to zero level. All the electronics are low noise and miniaturized surface mount components with 4 layers of grounding and double sided boards for eliminating signal interference between circuits or capacitive effects which are source of noises of these type of circuits with frequencies as high as 100-300 kHz typically used for C4D sensors. The input signals are 10 to 20 Vppand can be amplified to 250 Vppusing HV amplifier.

The double sided PCB electronic circuit is constructed to have high frequency sensitive components on both sides of the PCB which has dual grounding in between. This provides a high degree of shielding and isolation within different components at the same time that reduces the space needed to house the circuits. The connections are made through the board which allows a very short distance connection improving shielding. This is especially important for the pickup amplifier electronic circuit where the feedback resistor converts the pickup current to voltage and is completely shielded from the OPA itself therefore, ensuring minimal signal interference.

FIG. 20is a sectional view of the E-dC4D device with hardware and electrophoresis components. The C4D hardware includes: a dual C4D cell cartridge2400, pickup signal electronics2450, signal generator2500, signal processing electronic circuit2100, and USB connector2150to a computer DAQ card for signal transfer and auto control of the system using a laptop. The electrophoresis hardware includes a plastic chip104, high voltage power supplies2050a, high voltage relays electronics2050b, and electrophoresis high voltage electrodes2200.

The analytical fluidic operations are accessed by opening a top lid2200on the instrument. In this compartment, the microfluidic chip104is placed on the chip stage2300. High voltage to perform electrophoresis is applied from four miniaturized power supplies by the means of four pins2600,2700,2800, and2900positioned on the top lid to coincide with the microchip liquid reservoirs, as shown inFIG. 21. The corresponding reservoirs are sample (2650), buffer (2750), sample waste (2850), and buffer waste (label2950indicates the general position of this reservoir, but the buffer waste is not visible in the figure as the cover is closed; note, however, that the corresponding component111of the first embodiment is clearly visible inFIG. 1).

The application of High voltage (HV) is computer controlled and allows performing the operations of sample injection and separation. During these operations the lid is kept closed for the HV pins to establish electrical connection with the solution which at the same time isolates and prevents the user from contact with the High voltage (HV) avoid and any electric shock hazard. Ultra miniaturized high voltage power supplies with dual polarity of ±0.5 kV up to 5 kV are used. The potentials at all four electrodes can be controlled independently. The output of the power supplies is proportional to the input voltage range from 0.7V up to 5V and can be controlled directly via computer DAQ card. The Injection and separations are computer controlled using a code written in LabVIEW. The software allows controlling separation and injection voltage level and time. It also allows to create different sample injection protocols, such as, cross-injection, L-injection (two power supplies), or pinch-injection (using four power supplies).

As safeguard mechanism an external feedback circuit loop is employed to prevent excessive current passing through the DAQ card. To provide a perfect electrical ground for the injection and separation voltage during electrophoresis, one of the four power supplies connected to the ground reservoir is set to slightly less than 0V, i.e. negative voltage to sink the currents from the other HV supplies toward the ground reservoir and prevent any reverse flow toward other reservoirs.

The microchips used in this system are of 250 μm in thickness or less for effective signal coupling and detection. The thin microchips improve the capacitive coupling between the AC voltage and the solution found in the cell. This lowers the requirements for high voltage or high frequency. The detection is performed on the second half of the chip by a dual-C4D detection cell.

C4D Detection Cell Cartridge

FIG. 22(a)-(c) show replaceable dC4D cartridges with top-bottom excitation and pickup electrodes with a ground plane in between. These components are positioned precisely within a small Faraday cage with an external surface isolated by a polymer. A Faraday cage or shield is an enclosure formed by conducting material or by a conductive mesh. The enclosure blocks out external non-static electric fields.

The cell design is based on the embodiments described earlier, which are also described in co-pending U.S. Ser. No. 13/057,922, which is the dual top-bottom cell configuration. The cell box is attached close pack to integrated pickup electronic circuits. The cartridge can be plugged in and out using self-latching contact system, which makes perfect Ohmic contact with the cartridge cells as illustrated inFIGS. 23 and 24described below.

The dC4D detection cell is based on the concept of replaceable exchangeable cartridge. This is a unique feature in the dC4D detection cell that allows selecting the appropriate cell electrode dimension to the analytical problem to adjust resolution and sensitivity. The C4D cell of the present invention is connected to a battery-powered alternating current function generator, which allows for portability of the lab on a chip analyzer.FIG. 22(a)-(c) shows images of the cartridge cells. Sensitivity and resolution depend highly on the cell design and importantly on electrode dimensions and gap distance. These dimensions affect these two parameters in an opposite manner. Hence, the different cartridges are designed with specific dimension to provide with higher sensitivity or high resolution depending on the application requirements.

The cartridge encloses the detection cell and consists of a pair of top-bottom excitation electrodes and a pair of top-bottom pickup electrodes encased in a rectangular metal box (of typical dimensions: 1×1.5×3.5 cm3). The excitation and pickup top-bottom electrodes are well aligned and the bottom ones are connected through pins underneath in the housing to their respective sources by a self-latching mechanism, as shown inFIG. 24, and in more detail inFIG. 25(b). Initially the cartridge cell is put into the position shown inFIG. 23. Then, it is moved downward to plug it in: first to the position shown inFIG. 24(a), and then the position shown inFIG. 24(b) in which it is locked in position by the self-latching mechanism. The cartridge cell with three pins that is to contact to input signal, pickup signal, and ground is plugged in from top. The schematic of dC4D cell construction is shown inFIGS. 25(a) and (b). The top lead contains the set of top excitation electrode, pickup electrode and groundings all are aligned to face exactly to bottom pair and contact the fluidic chip upon closing. To eliminate any loose contacts or capacitive gaps which are source of noises, a spring load in the box lead is constructed to push the top lead against the bottom providing very close contact to the fluidic chip. The detection cell is fabricated by electroplating to achieve precise geometrical dimensions of the electrodes and shield plane. The parts are then embedded on a polymer matrix for isolation. The precise microfabrication of the electrodes and ground plane eliminates the electric field non-uniformity, noise, and direct coupling or stray capacitance which is highly important for improving sensitivity and enhancing the detection limit of the system. The ground plane isolates the entire space between the electrodes like a wall such that it separates the cell metal box into two absolutely isolated sections for the emission electrodes and pickup electrodes (FIG. 24). These are connected to the electronic circuit through gold plated pins. The pins allow the cartridge to be plugged in and out with minimal signal distortion or any loss. The precise and closed pack cell with a ground plane separating the electrodes or excitation and pick up sections of the cell, eliminates the stray capacitance between the electrodes.

The cartridge is designed to sandwich the chip placed in the chip holder and to press the electrodes against the surface of the chip to provide maximum proximity and avoid air gaps. The cell allows for maximum signal coupling with minimal noise of less than 0.5 mV. To protect the pickup signal and to eliminate signal loss the pickup amplifier were housed just underneath the detection cell connected to the pin in contact with pickup electrodes. The double sided high density circuit boards are used to place some sensitive components on opposite sides of the printed circuit boards with grounding. The signal transfer was performed by shielded coaxial cable that was solder-contact to eliminate any external capacitive noise. The dC4D detection cell is isolated from electronic noises and induces the confinement of field lines with in the detection cell. The ground plane eliminates direct coupling (i.e, stray capacitances) that completely divide the cell into two compartments the excitation and pickup compartments. Therefore, the distance for the signal to be transported between different components is minimal and signal losses are reduced.

FIG. 25(a) is a diagram of the one side of the dC4D cartridge showing the details of the electrodes and ground plane connections. InFIG. 25(a), the cartridge cell3000has three sockets to receive pins which electrically transmit an input signal, a ground plane between the electrodes, and pickup signal. The cartridge cell3000is plugged in from the top. The cartridge cell has an outer wall3050which is electroplated metal and is grounded in use. The top lead contains the set of: a top excitation electrode3200, a ground plate3300between the electrodes, and a pickup electrode3400. Each of these element has a corresponding pin contact (e.g. the top excitation electrode3200has the pin contact3100), which is at the top of a respective socket. All the sockets are aligned to face in use towards a corresponding pin (plug) of the housing, and contact is made in each socket upon inserting the cartridge cell3000into the housing. The plugs and sockets provide a self-latching mechanism, which locks the pins to the cartridge cell. InFIG. 25(a) the ground plane isolates entirely and divides the cell to two parts. One is for the excitation electrode and pin connection, and the other is for pickup signal entirely by the pick up electrode and pin connection.

FIG. 25(b) is a diagram of the pin self-latching mechanism to contact the cell to input output signals and ground. Contacts pads of the pins are brought into contact with the fix pads of the housing pins and the locking latches fix them. A spring load pulls the locks inward and disengages the latches for pull out. The dimension of the pins in the cell is cylinders of 2 mm diameter×15 mm length. Specifically,FIG. 25(b) shows the mechanism of one of the plugs (i) approaching the respective socket, (ii) engaging the latches and electric contact pads, (iii) locking into the respective socket, and (iv) disengaging by pulling out the spring loads.

As shown inFIG. 25(b), the self-latching contacts fix the cell with perfect contact tightly free from vibration, loose contact or misalignment. The self-latching contacts or connectors provide perfect Ohmic contacts and is tight and vibration free.

EXAMPLES

The analytical capability of the portable electrophoretic E-dC4D system was investigated in three fields of application: water quality control (i.e., analysis of cations in water); food analysis: analysis of organic acids and preservatives in fruit drinks; and bio analysis (i.e., DNA PCR fragment analysis).

Analysis of Cations in Water

FIG. 26displays the electropherogram of NH4+, K+, Ca2+, Na+, Mg2+and Li+cations at concentrations of 0.1 mM using a 30 mM MES/His buffer. The full width half maximum-values for the analytes peaks at 0.1 mM are 0.8, 0.8, 0.9, 0.8, 0.9, to 0.9 s and for 0.5 mM are 1.05, 1.06, 1.27, 1.49, 1.36, 1.43 s respectively, which is in a well defined range for a high resolution analytical system. Calibration curves for these six inorganic cations were determined to be linear for concentrations range of 0.01 to 1 mM.

FIG. 27shows the electrophoretic analysis and conductometric detection of NH4+, K+, Na+, and Li+cations in a standard mixture containing 5 μM of each ion. Operating conditions the same as above.

Food Analysis: Analysis of Organic Acids and Preservatives in Fruit Drinks

Ten commonly found organic and inorganic anions in beverages namely chloride, sulfate, oxalate, fumarate, malate, citrate, succinate, acetate, lactate and phosphate were examined. Experimental conditions were carefully optimized as a large number of analytes was examined. The best background electrolyte for good sensitivity and separation was achieved by 10 mM His and 7 mM glutamic acid at its natural pH. Electropherograms of the separation of the 10 common organic and inorganic anions in a standard solution and in an apple and aloe vera fruit juice are depicted inFIG. 28(a) and (b) respectively. The electrophoretic conditions were microchip 85 mm total length, background electrolyte 10 mM His, 7 mM glutamic acid (pH 5.53); separation voltage −4 kV, injection voltage −4 kV for 1 s.

FIG. 28(b) are electropherograms of apple and aloe vera fruit juice. The upper line is apple and aloe fruit juice at a dilution of 1:50, and the lower line is the same apple and aloe vera fruit juice spiked with fumarate to confirm the identity of the fumarate peak.

Malate, citrate and lactate anions are expected in the apple juice as major anions as stated on the beverage packaging. Chloride and sulfate were also previously identified as trace ions in fruit juice samples. The upper trace (inFIG. 28(b)) shows the separation of the anions in the diluted apple juice and chloride, sulfate, malate, citrate and lactate ions were identified together with a small peak corresponding to fumarate. Apple juices are known to contain various concentrations of fumaric acid naturally and due to processing. Peak confirmation was carried out by spiking with a 30 mg/L of fumarate solution the diluted fruit juice. From the lower trace, it can be confirmed that the unknown peak is that of fumarate.

Analytical parameters such as the RSD (%) of migration time, peak heights and peak areas as well as the r2values of the ten organic and inorganic anions are summarized in Table 1. The RSD (%) values are based on six consecutive injections of 0.01 mg/ml of Cl−, SO42−and oxalate, 0.03 mg/ml of fumarate, malate, citrate and succinate, 0.05 mg/ml of acetate and lactate, and 0.08 mg/ml of phosphate.

The RSD values for migration time are all well below 3% and thus, good reproducibility is shown. The RSD values for peak height and peak area are all lower than 5%.

Values are in mg/L and each value is a mean value of six measurements, RSD (n=6), ND, not detected. Determined using peak area for electrokinetic injection at −4 kV for 1 s.

Bio Analysis: DNA PCR Fragment Analysis

The size analysis of DNA fragments has been also carried out in the system and it is shown inFIG. 29. The dC4D detector captured the signal corresponding to the fragments; hence the detection of DNA was performed directly without the need for markers or labeling.

FIG. 29is a graph of the electrophoresis of 11 base pairs Hae DNAs 72, 118, 194, 234, 271, 281, 310, 603, 872, 1078, and 1353 bp respectively with concentration of 25.6 ug/ml in water in CHESTris buffer 2% PVP by E-dC4D. The experimental conditions were an excitation frequency at 300 KHz and amplitude of 74.1 Vpp. Injection between sample reservoir and last reservoir as ground was carried out using −3.5 kV for 1 second and the separation between buffer reservoir and last reservoir as Ground with −3.5 kV.

The C4D detection cell of the present invention allows for low background noise and low limit of detection (LOD). The portable lab-on-a-chip analytical instrument based on electrophoresis as a separation technique allows for simultaneous detection of ionic compounds, including trace elements. The present invention due to its improved capacitive coupling requires a much lower signal amplitude for the input excitation AC signal than previous version. This is an advantage due to safety concerns because an AC voltage of 300 Vpp is dangerous to handle and the instrumentation required to produce this signal is bulky. Another advantage of the present invention is that the C4D detection cell is isolated from the internal high voltage, environmental electrical noise and artifacts, which contributes to the lower detection limit of the system. As configured, the C4D detection cell of the present invention only requires external electrodes with adjustable distance integrated into shielded housing, therefore, it is user friendly and the cost is lowered.

It is understood that the foregoing description is given by way of illustration and that many variations may be made therein without departing from the embodiments described herein.