Doctoral Degrees (Chemistry and Polymer Science)

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Browsing Doctoral Degrees (Chemistry and Polymer Science) by browse.metadata.advisor "Crouch, A. M."

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    Application of modern analytical techniques and chemometric methods to the chemical characterisation of South African wines : determination of non-volatiles
    (Stellenbosch : Stellenbosch University, 2004-12) De Villiers, A. J. (Andre Joubert); Sandra, P. J. F.; Crouch, A. M.; Stellenbosch University. Faculty of Science . Dept. of Chemistry & Polymer Science.
    ENGLISH ABSTRACT: The present study deals in the first instance with the improvement of current analytical techniques for the analysis of the non-volatile content of wines. An improved sample preparation method, using solid phase extraction (SPE), was initially developed for the analysis of organic acids, sugars and phenolic compounds. Consequently, modem analytical methodologies were assessed to obtain optimal techniques for the separation of various non-volatile compounds. A capillary electrophoresis (CE) method, demonstrably more reliable than currently used high performance liquid chromatography (HPLC) and CE methods, is proposed for the analysis of organic acids. HPLC with refractive index (RI) or evaporative light scattering detection (ELSD) proved more suitable than CE for the analysis of sugars in dry wines. Liquid-chromatography-mass spectroscopy (LC-MS) offered superior sensitivity and resolution compared to the relatively new technique of CE-MS for the analysis of wine phenolics. LC-MS was further applied for the efficient and sensitive analysis of non-coloured phenolics and anthocyanins in wine. Negative- and positive electrospray ionisation, respectively, were used in conjunction with an ion-trap mass analyzer, for the identification of 34 phenolics and 31 anthocyanins in red wine samples. Complementary CE and LC methods were developed to allow the identification of artificial dyes in red wines, added illegally to improve their colour. Also, the application of stir bar sorptive extraction (SBSE) with liquid desorption and micellar electrokinetic chromatography (MEKC) for the analysis of bitter acids in beer is reported. In the second part of the thesis, the analytical results obtained for South African red and white wines were evaluated. Following comparison of the results with literature reports, several pattern recognition techniques were employed. A classification function obtained by linear discriminant analysis (LDA) was used to classify both red and white wines according to variety, based on their chemical composition. This classification is achieved independent of the factors of wine age or geographical origin, making it useful for authenticity evaluation.
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    Application of modern chromatographic technologies for the analysis of volatile compounds in South African wines
    (2009-12) Weldegergis, Berhane Tekle; Crouch, A. M.; De Villiers, A.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The present study was initiated by the wine industry of South Africa to overcome the lack of available information on the flavor and aroma of South African wines. The aim was to develop new analytical methods and improve existing ones for the analysis of volatile compounds in the South African wines. Initially a new analytical method based on stir bar sorptive extraction (SBSE) in the headspace mode for the analysis of 37 pre-selected volatile compounds was developed and validated. Consequently, the method was improved by making important modifications and increasing the number of compounds analyzed to 39. This method was successfully applied to a large number of Pinotage wines of vintages 2005 and 2006. The quantitative data of these wines were subjected to chemometric analysis in order to investigate possible co- /variances. A clear distinction was observed between the two vintages, where the 2005 wines were more characterized by wood-related compounds and the 2006 wines by the fermentation compounds. The developed method was further applied to other cultivars of vintage 2005, including two white (Sauvignon Blanc and Chardonnay) and three red (Shiraz, Cabernet Sauvignon and Merlot) cultivars. In a similar fashion, the quantitative data of the six cultivars of vintage of 2005 were analysed by chemometric methods. Significant differences were observed between the two white cultivars and among the four red cultivars. It was shown that among these cultivars, the major role-players were the wood and fermentation related volatiles. A striking observation was the confirmation of the unique character of the Pinotage wines compared to the other red cultivars, mainly influenced by the high level of isoamyl acetate and low level of isoamyl alcohol, the former being categorized as a varietal compound for Pinotage expressed by a fruity (banana) odor. In addition, advanced chromatographic technology in the form of comprehensive twodimensional gas chromatography (GC × GC) coupled to time-of-flight mass spectrometry (TOFMS) was investigated for the detailed analysis of volatile compounds in young South African wines. This work focused primarily on Pinotage wines. In the first instance, solid phase micro extraction (SPME) in the headspace mode in combination with GC × GC-TOFMS was used. Due to the high resolution and large peak capacity of GC × GC, more than 200 compounds previously reported as wine components were identified. These compounds were dominated by the highly volatile and less polar compounds, mainly due to the characteristics of SPME. In an attempt to further extend these results, another selective extraction method, solid phase extraction (SPE) was used in combination with GC × GC-TOFMS analysis. Using this technique, more than 275 compounds, most of them unidentified using the previous method, were detected. These groups of compounds include volatile phenols, lactones as well as mostly aromatic esters and norisoprenoids, which can potentially influence the aroma and flavor of wine. The techniques developed as part of this study have extended our knowledge of the volatile composition of South African wines.
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    Environmental electrochemistry of organic compounds at metal oxide electrodes
    (Stellenbosch : Stellenbosch University, 2004-12) Makgae, Mosidi Elizabeth; Crouch, A. M.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Sciences.
    ENGLISH ABSTRACT: This study investigates the electrochemical oxidation of phenol. Phenol is a major toxin and water pollutant. In addition, during water treatment it reacts with chlorine to produce carcinogenic chlorophenols. lts treatment down to trace levels is therefore of increasing concern. For this purpose, dynamically stable anodes for the breakdown of phenols to carbon dioxide or other less harmful substances were developed and characterized. The anodes were prepared from mixed oxides of tin (Sn) and the precious metals ruthenium (Ru), tantalum (Ta) and iridium (Ir), which in tum were prepared using sol-gel techniques. This involved dip-coating the aqueous salts of the respective metals onto titanium substrates and heating to temperatures of several hundreds of degree Celsius. The properties of these mixed oxide thin films were investigated and characterized using thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), atomic force microscopy (AFM), elemental dispersive energy X-ray analysis (EDX), X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), particle induced X-ray emission (PIXE) and electrochemical measurements. A variety of different electrode materials including Til Sn02-Ru02-Ir02, Ti/Ta20s-Ir02 and Ti/RhOx-Ir02 were developed and tested for their potential as oxidation catalysts for organic pollutants in wastewaters. Depending on the anode type, phenol was found to be electrochemically degraded, to different extents, on these surfaces during electrolysis. It was however found that the oxidation rate not only depended on the chemical composition but also on the oxide morphology revealed, resulting from the preparation procedure. The Ti/SnOz-Ru02-Ir02 film was found to be the most efficient surface for the electrolytic breakdown of phenol. This film oxidized phenol at a potential of 200 mV vs Ag/AgC!. The activity of the catalytic systems was evaluated both on the basis of phenol removal efficiency as well as the kinetics of these reactions. Phenol removal efficiency was more than 90% for all the film surfaces prepared and the rate of the reaction followed first order kinetics. A pathway for the electrochemical degradation of phenol was derived using techniques such as HPLC to identify the breakdown products. These pathway products included the formation of benzoquinone and the further oxidation of benzoquinone to the carboxylic acids malic, malonic and oxalic.
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    Separation and speciation of biodegradable complexes using capillary zone electrophersis
    (Stellenbosch : Stellenbosch University, 2004-12) Khotseng, Lindiwe Eudora; Crouch, A. M.; Sandra, P.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science.
    ENGLISH ABSTRACT: Separation of the metal ions Cu2+, Pb2+, Cd2+ Mn2+, Cr3+, Fe3+ and Zn2+ with poly ethylenediamine tetra-acetic acid (EDTA), ethylenediamine disuccinic acid ([S,S']- EDDS) and ethylenediamine dimalonic acid (EDDM) was performed by Capillary Electrophoresis. The electropherograms obtained by Capillary Electrophoresis were validated with a speciation model Joint Expert Speciation System (JESS). Excellent agreement was observed for the speciation diagrams for these species with the pH curves determined by capillary electrophoresis with those determined by speciation modelling using JESS. The ligands EDDS and EDDM are readily biodegradable. They have been proposed as substitute ligands for EDTA. Detection of the above species was performed using both an electrochemical and a UV detector. The obtained electropherograms were used for the determination of the detection limits of these species. The electrochemical detector has a lower detection limit for these species than the UV detector in conformity with reported literature for these detectors.
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    Separation and speciation of biodegradable complexes using capillary zone electrophoresis
    (Stellenbosch : Stellenbosch University, 2004-12) Khotseng, Lindiwe Eudora; Crouch, A. M.; Sandra, P.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Separation of the metal ions Cu2+, Pb2+, Cd2+ Mn2+, Cr3+, Fe3+ and Zn2+ with poly ethylenediamine tetra-acetic acid (EDTA), ethylenediamine disuccinic acid ([S,S']- EDDS) and ethylenediamine dimalonic acid (EDDM) was performed by Capillary Electrophoresis. The electropherograms obtained by Capillary Electrophoresis were validated with a speciation model Joint Expert Speciation System (JESS). Excellent agreement was observed for the speciation diagrams for these species with the pH curves determined by capillary electrophoresis with those determined by speciation modelling using JESS. The ligands EDDS and EDDM are readily biodegradable. They have been proposed as substitute ligands for EDTA. Detection of the above species was performed using both an electrochemical and a UV detector. The obtained electropherograms were used for the determination of the detection limits of these species. The electrochemical detector has a lower detection limit for these species than the UV detector in conformity with reported literature for these detectors.
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    Sol-gel preparation, characterisation and electrochemistry of mixed metal tin oxide electrodes
    (Stellenbosch : Stellenbosch University, 2004-12) Baker, Priscilla G. L.; Crouch, A. M.; Sanderson, R. D.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science.
    ENGLISH ABSTRACT: Please see fulltext for abstract
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    Stir bar sorptive extraction and gas chromatography : mass spectrometry for the analysis of biological matrices
    (Stellenbosch : University of Stellenbosch, 2007-12) Stopforth, A.; Sandra, P. J. F.; Crouch, A. M.; Burger, B. V.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
    This study describes the development of simplified analytical methods for the analysis of trace quantities of selected naturally occurring target compounds in complex biological matrices by stir bar sorptive extraction (SBSE) and gas chromatography/mass spectrometry (GC/MS). SBSE facilitates the direct extraction of organic compounds from aqueous samples by allowing the solutes to partition between the aqueous phase and a glass stir bar that is coated with a layer of polydimethylsiloxane (PDMS). The partitioning of polar compounds into the PDMS coating was enhanced by using different derivatization techniques in combination with SBSE. The derivatization of polar functional groups was performed with ethyl chloroformate, acetic acid anhydride, and O-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine directly in the aqueous samples. Headspace derivatization of compounds containing a secondary alcohol group was performed directly on the stir bar coating in the presence of acetic acid anhydride vapors. The derivatized compounds were thermally desorbed (TD) and analyzed on-line by GC/MS. A number of experimental parameters, including salt addition, temperature and time were optimized to improve the recovery of the derivatized compounds by SBSE. The optimized methods were validated in terms of linearity, precision, and detection and quantitation limits prior to performing the quantification. Trace levels of tuberculostearic acid, a marker of tuberculosis, was detected in sputum samples that were decontaminated and concentrated before being analyzed by SBSE-TD-GC/MS. The method is sufficiently sensitive to detect the marker without the need to culture the organisms, namely M. Tuberculosis. The analysis of 4-hydroxynonenal has also been demonstrated by detecting trace levels of this oxidative stress marker in urine samples obtained from healthy volunteers. Furthermore, abnormally low testosterone/epitestosterone ratios were detected in a group of HIV positive patients by means of SBSE-TD-GC/MS. Further research is required to determine the clinical significance of this finding in the context of HIV infection. Finally, the excessive urinary excretion of estrone and 17β-estradiol following the administration of a high dose of the conjugated equine estrogens to a female volunteer has also been demonstrated.
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    Stir bar sorptive extraction for the analysis of beverages and foodstuffs
    (Stellenbosch : University of Stellenbosch, 2008-03) Tredoux, A. G. J.; Sandra, P. J. F.; Crouch, A. M.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
    The main goal of this study was the development of new technologies based on modern analytical techniques for analysis of volatiles in wines. Due to the exponential growth of the wine industry and consumer demands for an enjoyable, safe-to-consume, and high quality product, the need for arose for methodologies aiding the understanding of wine better arose. Chemical analysis is a valuable way of studying the composition of wine in depth. Very sophisticated instrumentation is available nowadays but almost always the sample needs to be cleaned up or concentrated before such analysis. This study investigates the use of stir bar sorptive extraction (SBSE) as such a technique. It is shown that SBSE combined with gas chromatography-mass spectrometry (GC-MS) is extremely suited for a wide number of analyses and during the course of the study the technique was applied for troublesome analytical challenges in various beverages and foodstuffs. The study focuses on the development of a screening technique for volatiles in wine using SBSE and the application of the data to various chemometrical techniques for classification purposes. A second part of the study shows the applicability of SBSE for extraction of pesticides, contaminants and preservatives from wine, water, lemon flavoured beverages and yoghurt. The method is also elaborated upon by development of faster analysis methods for wine and beer and the investigation of using SBSE for headspace sampling of wine. In all the applications, SBSE technology was shown to be sensitive, repeatable, robust and very simple to use.