The Response Fundamental Research Of Low Impedance Contactless Conductometric Detector

Conductivity detection is a simple and universal detection technique for inorganic and in some cases even organic ions in ionic chromatographic and electrophoretic systems, which are often not readily detected by other techniques. For conductivity detection, the metal electrodes can be in direct contact with liquid (galvanic detection) or separated from liquid with an insulating film (contactless detection). The contact conductivity detector has encountered the difficulty in the application of capillary electrophoresis. To design of the micro-conductivity cell in capillary, drills on the capillarity wall and implant electrodes need a high technical process. In addition, the detection electrodes are easily contaminated and inconverental for cleaning and replacement. The separation voltage affects the performance of the detector.Capacitively coupled contactless conductivity detection (C4D) is a new detection technique, which has been developed in recent years and used mainly in capillary and microchip electrophoresis. The C4D offers the advantage in simplicity in detection cell design, smaller volume, and it completely overcame the disadvantages of electrodes fouling and interference of separation voltage on measurement signal. Compared with the application range of the C4D, the related C4D fundamental research report is few. Although some of the equivalent circuit model is proposed to analyze the response characteristics, they mainly make some qualitative description to experiment rules. The computational method is still a weak link in the fundamental research at present. Under the capillary electrophoresis conditions, the high impedance between the contact electrodes and the liquid is one of the factors for the low sensitivity of C4D detector. This thesis discusses the various parameters under different conditions, and simulates the response characteristics of C4D, and on this basis, a low impedance capacitively coupled contactless conductivity detector (LIC4D) is proposed. The main contents of this thesis are listed below:1,Equivalent circuit simulation and analysis for contactless conductivity detectorBased on the equivalent circuit model of the C4D, simulated detectors including contact parallel plate electrodes, contact double parallel plate electrodes, contact parallel column electrodes, contactless parallel plate electrodes, and contact parallel column electrodes were designed to analyze the electric behavior of the C4D. The impedance data were measured and analyzed. The changes in the equilivalent circuit parameters were discussed. This analysis is helpful to simulate the response of the C4D and the optimization in detector parameters.2,Fundamental research and application of low impedance contactless conductometric detectorTo reduce the C4D detection cell volume, a C4D with two opposite contactless electrode was designed. Howerver, the sensitivity of this detector to the changes of solution conductivity is poor due to the fact that the absolute value of imaginary part. In order to improve the response of the sensitivity of this C4D, we adopt the principle of impedance annihilation. A quartz crystal resonator was added in series combination of the C4D. Under the resonance frequency of the combination, the imaginary of the impedance is close to zero. The influence of the wall capacitor impedance on the sensitivity of the C4D is reduced effectively. It was shown that sensitivity of the low impedance C4D is closes to that of a contact conductivity detector.3,Piezoelectric quartz crystal liquid density sensor based on longitudinal wave responseQuartz crystal microbalance is a sensor with high sensitivity to the change in surface mass change. It has found wide applications in monitoring the mass changes in insurface processes. When a piezoelectric quartz resonantor is operated in liquid phase in thickness shear model, a longitudinal wave is usually occurred. The longitudinal wave is a potential error source for the measurement of thickness shear model. Under some extreme cases, the renant peak of the resonator is split to a group of small peaks. In this chapter we studied the resonance peak cleavage by an impedance analysis method. The interaction between the thickness shear wave and longitudinal wave was investigated. The influence of liquid density was discuused. By the comdination of a longtudianl wave reflector and longtudianl wave eliminator, the changes in surface mass, liuid density, and liquid viscosity were imultaneously determined by a piezoelectric sensor in impedance analysis method.