Study Of Contactless Conductivity Detection On Capillary Electrophoresis Chip

Contactless conductivity detection for capillary electrophoresis completely overcame the disadvantages of electrodes fouling and interference of separation voltage because the detector electrodes are not direct contact with the solution. It has been widely recognized as a powerful tool for chemical and biochemical analyses. How to increase the detection sensitivity on contactless conductivity detection is mail problem. In this paper, according to the structure of detector, a novel extended model for the detection cell consisting of a network of resistors and capacitors was proposed on contactless conductivity detection. The parameters of detector were optimized. The stray capacitance between two electrodes was decreased using the boundary element method. A contactless conductivity detection microchip was designed using PMMA material. The separation and detection of inorganic cation was achieved in capillary electrophoresis system with homemade contactless conductivity detector.Based on the basic theory of capillary electrophoresis and physical structure of the cell on contactless conductivity detector, a new equivalent circuit model of the cell with the extensive spatial RC networks is established. The transfer function of equivalent circuit was deduced to predicte the output of the conductivity cell. The current flow through each segment of the cell can be simulated. Using the complex model the effect of the detector geometry parameter on the sensitivity of contactless conductivity is studied. By simulation, the optimal parameters of the detector were obtained.The metal mould fabricated with ultra-precision machine manufacture technique instead of the traditional Si or quartz templates solved the problem of frangibility in imprinting procedures. Using the powder hot embossing machine, the effects of the bonding temperature, pressure and time on the deformation of microchannel dimensions is studied systematically. The bonding temperature is mainly influence factor on the deformation of microchannel determined by the method of orthogonal experiment. According to experimental curve, the optimal experimental model (83℃, 0.4Mpa and 7 min) is suggested. Under the process parameters the deformation of crossing section dimension before and after thermal bonding is 40 %. The chip with microchannel is successfully sealed with a thin foil PMMA material about 30μm thickness for contactless conductivity detection.The boundary element method was used to simulate the stay capacitance of contactless conductivity detection detector for different electrode structure. A moveable electrode structure based on sandwich is designed to decrease the direct capacitive coupling between the electrodes. Electrodes were separated with CE chip which are fabricated on the PMMA pedestal. The effective length of the electrode can be very short and decrease the stray capacitance. The experimental and emulational results indicated that the stray capacitance can be decreased about one order of magnitude proposed electrode structure comparing with conventional electrode structure. The stray capacitance is 1.26×10^-15 F.The hardware platform for contactless conductivity detection based on ARM was implemented. The synchronous rectification was carried out the signal detection with a four quadrant multiplier (MTL04), which allows phase-sensitive detection. Programmable amplifier was used to amplify the signal in order to assure the precision of A/D converter. Four high voltage power supplies are driven by independent regulators, which are in turn controlled by ARM microprocessor. The high-precision 16 bit ADC was used in the data acquisition. For data processing, archived electropherograms can be downloaded onto a PC via two approachs, USB or RS232 serial interface. In the Microsoft Visual C++ software platform, data acquisition and analysis package of contactless conductivity detecting system was developed. The software can accomplish the function of CE signal acquisition, display, storage and repeat. According to the characters of CE signal, the mathematic model of CE signal was established. Two denoising methods were described for capillary electrophoresis signal denoising, improved wavelet thresholding function with translation invariant and the optimal fuzzy thresholding with translation invariant. Comparing with hard threshold denoise method, S/N ratio increase 3.5dB and the the error of peak shape is 0.29% using the methods proposed in the paper. Pseudo-Gibbs phenomena can be suppressed comparing with conventional denoising methods for CE signal.Using the optimal parameter of the cell, the two peaks corresponding to K⁺ and Mg²⁺ are clearly resolved with complete separation at the homemade detecting system. The separation time is less than 40s when the separation voltage is 220V/cm. The single-peak repeatability results are better for detecting K⁺ cation. The peak heigh of K⁺ has relation to the injection time. Detection limits of K⁺ was found to be in the order of 10^-6-10^-7 mol/L. The separation of inorganic cation can be monitored at different location of microchannel. The experimental indicated that inorganic cation can be effectivelyseparated and detected using this system. This paper has guidance to improve the sensitivity of contactless conductivity detector.