Analysis Of Several Key Factors And Corresponding Improvement On Capacitively Coupled Contactless Conductivity Detection

Capacitively coupled contactless conductivity detection（C⁴D）is an important non-contact conductivity measurement technique.Since its electrodes are isolated from the solution and it is instrumentally simple,it can be used either independently or,as the detector of capillary electrophoresis（CE）,high performance liquid chromatography（HPLC）and ion chromatography（IC）.CE-C⁴D has been extensively used in fields of medicine,biology,food,environmental science and even criminal investigation.However,because C⁴D is based on capacitive coupling,it’s operation can be easily influenced by the wall capacitance and the stray capacitance between the electrodes.Thus,the frequency response of C⁴D can be quite complex in different solution concentrations,and in certain cases,the signal response may not adequately reflect the conductivity of the solution.Focusing on these issues,this thesis is dedicated to the optimization and improvement of the performance of C⁴D.By analyzing several key factors affecting the C⁴D performance,especially that of electronic circuits and detection cell design,through combination of simulation and experimental validation,it is expected to clarify the direction for further improvement of C⁴D.The main work and conclusions of this thesis are as follows.1.The history of C⁴D and the factors which affect the performance of the detector are briefly summarized.The development of commercial C⁴D detectors and their miniaturization are also introduced together with typical applications of CE-C⁴D.2.In this chapter,the influencing factors of the circuit designing of the detection module of C⁴D are discussed.Approaches for improving its performance and the selection of the corresponding components are determined.Based on these,we also analyzed the key factors of electronic circuit that may affect the signal and noise,which provide a theoretical basis for the selection and improvement of the C⁴D circuits.3.Furthermore,we designed a C⁴D based on printed circuit board（PCB）,and evaluated the influence of several factors.Depend on its equivalent circuit,we clarified the effects of wall capacitance,stray capacitance and cell resistance on the signal response with simulation.The key factors affecting the linear response to the solution concentration of C⁴D detection were found and verified experimentally.Under the selected condition,the limit of detection（LOD）for MES-His（pH=6.0）could be as low as 7.2 nmol/L and the linear range is 2-20μmol/L.4.Finally,we designed a dual-channel differential PCB-based C⁴D to counteract the influence of stray capacitance and extend the frequency range for the detection.The test conditions for the lock-in amplifier and the factors influencing the dual-channel consistency were optimized,and the frequency response of C⁴D in the differential mode was investigated.Compared to the single-channel detector,C⁴D in differential mode extended the linear range to 1-100μmol/L for MES-His（pH=6.0）,and LODs of 0.09μmol/L was achieved.The detector used with CE technique enables the separation and detection of the common cations in real samples.The LODs of four cations,including K⁺,Ca²⁺,Na⁺and Mg²⁺,were within range of 0.28-0.86μmol/L in dual-capillary mode,better than that of 0.10-19μmol/L for single-capillary mode system tested under similar condition.