Electrochemically assisted injection (EAI) is a hydrodynamic injection concept applied for CE/MS that enables an electrochemical analyte conversion at a substrate electrode during the injection process. The electrochemical formation of charged species from neutral ones enables an electrophoretic separation in aqueous and non-aqueous media. In some cases, a significant enhancement of electrospray ionization efficiency in mass spectrometry can be achieved.
The first part of the present thesis covers the development and characterization of novel automated injection arrangements for EAI/CE/MS. The stepwise development started with the fabrication of amperometric capillary probes to investigate the dependency of the capillary-to-substrate electrode distance on the injection efficiency. An electrochemical characterization of the capillary probes was carried out with the help of hydrodynamic voltammetry. Probe approach curves were recorded using a scanning electrochemical microscope for the vertical alignment of capillary-based probes over a substrate electrode. The SECM allowed a vertical fine-positioning with a resolution in the micrometer range. In this context the SECM bipotentiostat was exploited for amperometric detection. Probe approach curves turned out that the current response remained nearly constant for distances between 5 µm and 60 µm and decreased significantly for larger distances.
In a further development step a semiautomated injection cell was constructed considering the required accuracy in capillary positioning. A fast and precise piezo motor was chosen for vertical capillary movement over a substrate electrode. Commercially available screen-printed electrodes served as substrate electrodes. A CE buffer reservoir was incorporated next to the substrate electrode holding compartment. The change between injection mode and separation mode was carried out manually by moving the arrangement to the relevant position. The device was hyphenated to a home-built CE/MS system and characterized regarding precision and reproducibility using ferrocene methanol as a model system.
Based on the EAI/CE/MS results obtained with the semiautomated cell, a fully automated EAI cell was developed to enhance the precision of the whole EAI injection process. In contrast to the semiautomated cell, the latest version enabled a fully automated, microprocessor-controlled injection. Two servo motors were responsible for the vertical capillary alignment and the automated change between injection and separation mode. The motion sequence of the servo motors was programmed and triggered either by computer software or a handheld controller unit. The injection cell was characterized in the same way as the semiautomated cell. The fully automated cell was applied for all further EAI/CE/MS experiments.
In the second part of the present thesis, the investigation of screen-printed electrodes in the context of EAI/CE/MS studies of nitroaromatic compounds is described. Screen-printed electrodes consist of a concentric arrangement of three electrodes on a suitable glass or ceramic substrate. They are easy to prepare and modify and a wide range of electrode materials is commercially available. Additionally, sample volumes in the microliter range are sufficient to cover the electrode structures. In the present studies it is focused on carbon-based SPEs. The materials of choice were carbon, carbon nanofibers and reduced graphene oxide. A protocol for the drop coating modification of uc SPEs using reduced graphene oxide was developed. The electrode materials were compared as they were used for mass voltammetric EAI/CE/MS studies of the electrochemical reduction of 4-nitrotoluene under acidic conditions. It was demonstrated that the reduction products 4-hydroxyl-aminotoluene and 4-aminotoluene, are formed at rather negative potentials at reduced graphene oxide electrodes, compared to carbon and carbon nanofiber electrodes. Moreover, the relative abundance of both species formed varied for different electrode materials.
In further experiments, isotopically labeled 4-nitrotoluene-d4 served as an internal standard to investigate the electrodes’ liability regarding electrode fouling, a well-known problem when working with solid state electrodes. Both mass voltammetry and cyclic voltammetry turned out that neither 4-nitrotoluene nor its reduction products tended to adsorb at the electrode surface.
Finally, 4-Nitrotoluene-d4 was exploited as an internal standard for the quantification of 4-Nitrotoluene in soil and drinking water samples. A liquid-solid extraction protocol was developed for soil samples. In the case of drinking water samples, a solid-phase extraction protocol using a C-18 stationary phase was developed. The samples were spiked with a defined amount of 4-nitrotoluene and the internal standard was added. The ratio of the peak intensities recorded in the mass traces of the reduced amine species allowed for the calculation of method recovery values.
In conclusion EAI/CE/MS is a powerful technique that combines the high sensitivity and selectivity of mass spectrometry with capillary electrophoresis. The key point of this work is the electrochemically assisted injection prior to CE/MS measurements. EAI allows CE/MS measurements of commonly inapplicable analytes due to an electrochemical conversion during the injection process. EAI enables the separation of neutral analytes by an electrochemical generation of charged species and, additionally, enhances the electrospray ionization efficiency. It was demonstrated that EAI/CE/MS can be applied for various quantitative and mechanistic studies that are presented in this Ph.D. thesis
