Within this master’s thesis, I developed a method for determining furanic compounds in synthetic ester-based insulating liquids used in power transformers. The aim of this work was to quantify furanic compounds in the synthetic ester MIDEL 7131. Initially, I reviewed relevant literature to identify suitable HPLC (high performance liquid chromatography) methods for the determination of the target analytes. Based on the literature, I selected a reversed-phase C18 column as the stationary phase and a methanol–water mixture (60:40, v/v) as the mobile phase in gradient mode at a constant flow rate of 1 mL/min. The injection volume was 25 µL, and the detector wavelengths were set at 220 nm and 278 nm. Prior to HPLC analysis, extraction of the analytes from the synthetic ester was required. According to established methods for mineral oils, which predominate in the literature, this step typically involves solid-phase extraction (SPE). However, this approach proved unsuitable for the synthetic ester-based insulating liquid MIDEL 7131, as the extraction efficiency for four out of five analytes was below 10 %. Consequently, during method optimisation, I evaluated various elution solvents for the SPE cartridges as well as different mobile phase solvents for HPLC, of which I opted for mixture of water and acetonitrile (ACN). In addition, I investigated the possibility of using liquid–liquid extraction with different solvent mixtures that form a two-phase system with the synthetic ester. I found that water acts as an interfering factor when it interacts with the ester. Using HPLC and nuclear magnetic resonance (NMR) spectroscopy, I confirmed that no chemical reactions occur between analytes and water or products of ester hydrolysis. Regarding the mixture of water and ester, the issue arises from the mutual (in)solubility of the two phases, leading to the formation of an emulsion. As this is undesirable for sample preparation, I subsequently assessed liquid–liquid extraction using dimethyl sulfoxide (DMSO), which proved to be a suitable solvent. By optimising the ester-to-DMSO ratio, mixing time, and chromatographic conditions, I achieved satisfactory extraction efficiencies while maintaining a sufficiently low limit of quantification (LOQ). Three furanic compounds (2-furfural (2-FAL), 2-acetylfuran (2-ACF), and 5-methylfurfural (5-MEF)) were quantified by HPLC analysis after extraction from samples using liquid–liquid extraction with DMSO. In the later stages of the research, I focused only on 2-FAL, as it is considered the most stable analyte and is therefore present at the highest concentrations. For this analyte, I established two separate, overlapping calibration curves to cover both the lower (0 – 0.3 µg/g) and higher (0.2 – 3 µg/g) ends of the concentration range. Both curves showed exceptionally good linearity and produced repeatable results. Notably, they consistently yielded comparable results for a standard in the overlapping concentration region. Moreover, the method exhibits a low limit of quantification (LOQ), which I estimated to be 0.03 µg/g. This is especially important, as lower amounts of analytes are present in synthetic ester samples compared to mineral oils. Therefore, I showed that the method can be used to successfully determine the 2-FAL content in synthetic ester-based transformer liquids.
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