| Capacitively coupled contactless conductivity detector (C4D) has many advantages such as simply designed of detection cell, easy to manufacture electrodes, small cell volume. C4D has found many applications in determination of ions in microchannels such as capillary or microfluidic chip electrophoresise. But the investigation in the response of C4D is needed to perfect. Our expermental results reveal that the equivalent circuit parameters of C4D measured are much less than that predicted. There is space to enhance the sensitivity of C4D. Some of experimental phenomena are not investigated. In this thesis, an impedance analysis method was proposed to measure the equivalent circuit parameters of C4D under different conditions. The response of C4D was analysed based on a model of electronic line distribution. The response signal of C4D was enhanced according to a resonant approach while the noise level of C4D was reduced in an end-to-end differential measurement mode. Hence, the sensitivity of C4D was improved. Recently, food safety has become a very hot topic in China. The determinations of of trace dyes in aquatic flavourings and phthalate esters in powdered milk by liquid chromatography-tandem mass spectrometry method were investigated. The detailed descriptions are listed below.1. Investigation the response and measurement of the equivalent circuit parameters of a contactless conductive detector by an impedance analysis methodAn impedance analysis method was reported to measure the equivalent circuit parameters of C4D. It was shown that only part of the liquid in capillary within the electrode takes part in the conducting of signal current. Under the typical condition of capillary electrophoresis, the wall capacitance (Cw) in C4D is much less than the prediction from a cylinder capacitor model. With increasing electrode length, the value of Cw increase until maximum then decreases slightly, which is different to the predicted linear correlation. Unlike to predicted constant Cw value, the value of Cw increases with increasing solution conductivity in an exponential decay way. With increasing electrode gap size or measuring frequency, the value of Cw is reduced. To explore the difference between the predicted and measured equivalent circuit parameters of C4D, several detectors were designed to simulate the responses of C4D. The change in the parameters was analyzed by an electrical line distribution model. It was shown that the total impedance of a C4D under capillary electrophoresis conditions is composed mainly by the impedance from the wall capacitor. The response of C4D is due to the change in both of Cw and solution resistance. But the change in Cw is the dominating factor.2. Determination of anions using monolithic capillary column ion chromatography with end-to-end differential contactless conductometric detectors under resonance approachAn anion-exchange monolithic capillary column for ion chromatography was prepared by thermally initiated radical polymerization of poly(glycidyl methacrylate) in a fused-silica capillary of320μm i.d. and modified by quaternary ammonium latex surface coating. With15mM potassium hydrogen phthalate used as the eluent, good separation of a mixture of inorganic anions (F-,Cl-, NO2-, NO3-) was achieved. Under the experimental conditions used, the total impedance of a C4D is almost the same as the impedance from the wall capacitor. Because the value of Cw is not sensitive to the change in solution conductivity in this region, the sensitivity of the C4D is low. In an end-to-end differential measurement model with double C4D, an improvement in the sensitivity by a factor of3-4was achieved. The detection limits are1.6,0.28,0.53, and0.47mg/L for F, Cl-, N02-and NO3-, respectively. To further enhance the sensitivity, the capacitive impedance from C4D was neutralized by an inductive impedance from a piezoelectric resonator. The change in signal current from solution resistance were detected. An increase in sensitivity by a factor of7-8was achieved in the resonating C4D in comparison with the conventional C4D. The detection limits of the resonating C4D are0.23,0.041,0.065, and0.059mg/L for F-, Cl-,NO2-and NO3-,respectively. 3. Determination of water-soluble ionic in PM10using capillary electrophoresis with contactless conductometric detectors under resonance approachBy using a resonant C4D in capillary electrophoresis apparatus, signal-to-noise ratio is about5times of that in a conventional C4D of the same dimension. The design of the end-to-end differential C4Ds under resonance approach, signal-to-noise ratio can be improved further. This system was applied in determination of water-soluble ions in PM10by capillary electrophoresis. A pressure-assistant sample injection equipment was designed to improve the precision of determination in capillary electropheresis. The detection limits of most ions are less then10μg/L. The data were in accord with those determinated by ion chromatograph method.4、Contactless conductivity detector as a probe for the disarrangement of capillary wall/solution interfaceExperimental results reveal that different response is obtained in a C4D with the capillary filled heterogeneous electrolyte solution with the same conductivity. If the same response in a C4D is measured, the relative diffencece in conductivity is in the range of1-13%, depending on the kind of electrolyte. When two electrolyte solutions with the same resonse in C4D are injected into the capillary in turn, a positive and negative peaks occur in the response of the C4D. The difference between the peak-to-valley increases with increasing background conductivity level. And it is also related to the kind of electrolyte. The influence of coexistence of surfanctant, inner diameter of capillary and position of C4D was tested. It was presumed that the positive and negative peaks are related to the rearrangement of the interfance on capillary wall/solution interfance. Hence, C4D is a potential tool to probe the change of solid/solution interfance.5. Simultaneous determination of RhodamineB, Gentian Violet and Malachite in aquatic flavourings by solid-phase extraction coupled with ultra performance liquid chromatography-tandem mass spectrometryA method was developed for the simultaneous determination of RhodamineB, Gentian Violet and Malachite in aquatic flavourings based on solid phase extraction-ultra performance liquid chromatography-tandem mass spectrometry. The samples were extracted with acetonitrile-buffer, partitioned against methylene dichloride and then cleaned up by a solid phase extraction cartridge. The target analytes were separated on a BEH C18column with gradient elution using acetonitrile and water (containing0.2%formic acid (v/v)) as mobile phases. The identification and quantification were achieved by using ESI-MS/MS in positive ion mode and with multiple reaction monitoring. The linear ranges were from0.5μg/L to1mg/L with the correlation coefficients (r) for RhodamineB0.999,Gentian Violet0.997,Malachite0.997; The limits of detection were0.2,0.5and0.2μg/kg, respectively; The recoveries of the method were90.7-96.8%,78.4-90.3%,78.6-88.9%,respectively.The relative standard deviations were less than10%.The method is simple,highly sensitive, rapid, broad concentration range and suitable for the simultaneous determination of RhodamineB,Gentian Violet and Malachite in aquatic flavourings.6. Simultaneous determination of sixteen phthalate esters in milk powder by solid-phase extraction coupled with ultra performance liquid chromatography-tandem mass spectrometryA method was developed for the simultaneous determination of sixteen phthalate esters in powdered milk based on solid phase extraction ultra performance liquid chromatography-tandem mass spectrometry. The samples were extracted with methanol saturated by n-hexane in an ultrasonic field, cleaned up by refrigerated centrifugation, and then the supernatant cleaned up by a solid phase extraction cartridge. The target analytes were separated on an HSST3column with gradient elution using methanol and water (containing0.2%formic acid (v/v)) as mobile phases. The identification and quantification were achieved by using ESI-MS/MS in positive ion mode and with multiple reaction monitoring. Limits of detection were from0.02to0.7μg/L; The average recoveries of the method were between82%and108%. The relative standard deviations were less than10%.The method is simple, sensitive, rapid, and suitable for the simultaneous determination of phthalate esters in milk powder. |
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