Capacitively coupled contactless conductivity detection and sequential injection analysis in capillary electrophoresis and capillary electro-chromatography

This thesis focuses on the applications of capacitively coupled contactless conductivity detection (C4D) in capillary electrophoresis (CE) hybridized with high-performance liquid chromatography (HPLC), i.e. in capillary electrochromatography and pressure-assisted capillary electrophoresis, as well as on the development and applications of an extension of CE-C4D with sequential injection analysis (SIA).
At first, the in-house built C4D was used for electro-chromatographic determinations of different classes of non-UV absorbing cations using monolithic octadecylsilica capillaries. Combined mechanisms of differential interactions and electrophoresis were employed by the application of a high-voltage through a monolithic octadecylsilica structure in order to achieve high separation efficiencies, which otherwise are difficult to obtain with either CE or HPLC alone. CEC conditions were optimized for both home-made and commercial monolithic columns for baseline separations of inorganic cations, amines and amino acids. Detection limits were found to be comparable with those obtained from CE with conductivity detection.
C4D was then utilized for sensitive detection in pressure-assisted capillary electrophoresis with separation columns as slender as 10 μm. UV-radiation detection is impossible in this case due to very limited optical pathlengths. The introduction of a hydrodynamic flow at a specific time during electrophoresis can be employed to achieve high separation efficiencies or short analysis time for improved sample throughput, or to compensate for the electro-osmotic flow (EOF) for electrophoretic separation of anions at high pH without addition of an EOF modifier. The application of pressure also allows converting a monotonous electrophoretic run into a separation scheme with flow gradient akin to HPLC with gradient elution.
For monitoring applications, an automated system was designed and constructed based on an extension of conventional CE-C4D with a sequential injection analysis (SIA) manifold via an interface. The developed system allows automated sampling, separation, detection and data acquisition. Both cations and anions can be analyzed successively by automatic switching of the high-voltage polarity. The system was applied successfully to monitor the variations of concentrations of major ions in a lake in Switzerland during a rainy period. Crosschecking with discrete samples analysed with ion-chromatography proved that the system is suitable for unattended long-term monitoring tasks.
To overcome the inherent problem of sensitivity limitation in CE due to small injection volumes, a new configuration of SIA-CE-C4D was designed and developed in order to implement an on-line solid-phase-extraction (SPE) preconcentration procedure prior to automated CE separation. The system was designed as an industrial prototype. Some drug residues in water, including ibuprofen, diclofenac, bezafibrate and naproxen, were selected as exemplary analytes to demonstrate the functionality of the system. With preconcentration prior to CE-C4D determination, enrichment factors of several hundreds can be obtained and the concentrations of drug residues in water can be monitored down to the nM scale, which is impossible with normal CE setups.