Sensitivity Enhancement And Equivalent Circuit Parameters Measurement By A Resonant Approach

Over the last decade there are several reasons for the increasing popularity ofcapacitively coupled contactless conductivity detector（C⁴D）. The metrical electrodesdo not contact with the pending solution，the simply designed of electrode structure，the advantages of convenient maintenance and small volume of detection cell. Thismeasuring approach avoids many problems as they do not contact with the solutionand completely solved the electrode pollution and the gas produced in the electrolysisand the freedom of any potential interference from the electrophoretic separationvoltage. Hence, C⁴D have a wide range of applications in capillary and chipelectrophoresis.Because the measuring electrode and the test solution was segregated by theinsulating capillary wall, the response of C⁴D have significantly different with contactconductivity method. The equivalent circuit model of C⁴D can characteristic theresponse of C⁴D only qualitatively. The theoretical simulation values and theexperimentally measured values are much different. The equivalent circuit modelitself is reasonable from the analysis of AC current conduction path of C⁴D. But therelationships between the equivalent circuit parameters and the conductivity of thesolution, the dimension of detection cells are not clear. In this thesis, we proposed aresonant method to measure equivalent circuit parameters of C⁴D and to improve thesensitivity of the detector.1. Application of a low impedance contactless conductometric detector for thedetermination of inorganic cations in water samples in capillary monolithiccolumn chromatographyPoly（glycidyl methacrylate） cation exchange monolithic column was prepared infused-silica capillaries of 320μm by thermally initiated radical polymerization andutilized in capillary ion chromatography. By optimizing the condition of the detector, with 15mM methanesulfonic acid as the mobile phase, the mixture of inorganiccations （Li⁺, Na⁺, NH₄⁺, K⁺） was well separated in the prepared monolithic column..But the impedance between capillary monolithic column ion chromatography ofseparation conditions and detection of C⁴D are mismatched, the sensitivity of C⁴Dwas low. By analysising the equivalent circuit and experimental determination of C⁴D,three approaches were tested to improve the sensitivity of the C⁴D for monolithiccolumn detection. By optimizing the design of the detector, the series combination ofa C⁴D and a quartz crystal resonator works as a low impedance capacitively coupledcontactless conductivity detector（LIC⁴D）, where the capacitor impedance fromcapillary wall was offset by the inductance impedance from the quartz crystalresonator. The LIC⁴D has the similar sensitivity as a contact conductivity detector .On the other hand, the LIC⁴D offers the advantage in simplicity in detection celldesign, much smaller volume and on-column detection without extra peak broadening.Under the chromatographic conditions used, the mixture of inorganic cations （Li⁺,Na⁺, NH₄⁺, K⁺） was well separated in the prepared monolithic column. The detectionlimits of Li⁺, Na⁺, NH4⁺and K⁺were 0.031, 0.075, 0.10 and 0.16 mg/L, respectively.2.The determination of impedance parameters of the contactless conductivitydetector by resonance methodThis chapter presents a measurement method based on resonant approach. In theseries combination of a piezoelectric quartz crystal （PQC） resonator with a C⁴D, thecapacitor impedance from capillary wall was offset by the inductance impedance fromthe quartz crystal resonator. The real part and imaginary part of impedance of C⁴Dwere measured. At the frequency of 100 kHz, the values of wall capacitance andsolution resistance of C⁴D were estimated. The influences of the conductivity ofsolution, electrode gap, electrode length and inner diameter of capillary wereinvestigated. Under the conditions of capillary electrophoresis, the experimentallymeasured data were significantly lower than the simulated ones according to theformula in the literature. The response signal of C⁴D is due to the changes in the wallcapacitance under different conductivity of the solution rather than the change inresistance of solution directly. And we proposed an electronic line distribution model to qualitatively explain the impact of various parameters to the response ofC⁴D. The resonant method with simple equipment can provide a new method tomeasure and understand of the response of C⁴D. The series combination of PQC andC⁴D under resonance lowers the impedance of measurement system can greatlyimprove its sensitivity.