Development of sample preparation and chromatographic mass spectrometric techniques for determination of selected organic pollutants in wastewater

Abstract : In the recent past, there has been a great concern on the ever-increasing emergence of organic contaminants in the various environmental compartments, that pose great health concerns to humans and aquatic life. These organic pollutants have been ubiquitous in the environment for decades, however, they were not identifiable until the emergence of new and advanced analytical technologies. Therefore, the main objective of this study was to develop robust and efficient analytical and modelling techniques, for the extraction and analysis of selected multiclass organic contaminants from wastewater samples. This is because their analytical determination is very challenging due to their occurrence in trace levels (ng L-1 to μg L-1) in the environment. The analytical techniques comprise of optimization of both the sample preparation procedures and instrumental analysis for detection and quantification. Solid phase extraction (SPE), dispersive liquid-liquid microextraction (DLLME) and ultrasonic-assisted magnetic solid phase dispersive extraction (UA-MSPDE) were the selected sample preparation techniques used for the extraction and preconcentration of methylparaben, ethylparaben, propylparaben, ethoprofos, parathion methyl, azinphos methyl and chlorpyrifos in water samples. This was followed by instrumental analysis for their detection and quantification using liquid chromatography tandem mass spectrometry (LC-MS/MS). The developed analytical techniques were applied in real environmental samples obtained from different water treatment stages of a local wastewater treatment plant in Gauteng province, South Africa. Experimental factors that had an influence on the analytical response in terms on highest percentage recoveries were optimized using both univariate (one factor a time) and multivariate approach for all the experiments in this study. Multivariate optimization was accomplished using Statistica and Minitab software. The performance characteristics of the LC-MS/MS facilitated the determination of these organic contaminants at trace levels. Multiple reaction monitoring mode (MRM) was used for specific and sensitive targeted analysis, where the quadrupole analyzers were set at multiple ion frequencies for the specific analytes under investigation together with their product fragment ions. MRM is ideally suitable for trace level analysis of complex mixtures. Oasis HLB cartridges were found to be suitable for extraction of parabens giving satisfactory results. Vortex assisted dispersive liquid-liquid microextraction (VA-DLLME) was used for the extraction and enrichment of organophosphorus pesticides in wastewater samples. Selection of the appropriate organic solvent (extractant and disperser solvents) used for this method was of utmost importance and was performed using univariate optimization. v The results revealed chloroform to be the most suitable extractant solvent while acetone was the optimum disperser solvent. This was followed by the chemometric optimization of the independent variables that significantly affect the outcome of the analytical response. The organophosphorus compounds that were extracted in wastewater samples using this technique with satisfactory results were ethoprofos, parathion methyl and azinphos methyl. Also, a novel method was developed for the extraction and preconcentration of multiclass organic compounds (parabens and organophosphorus pesticides) using synthesized pristine carbon nanodots (CNDs) applied as SPE adsorbent. A comparison between the synthesized CNDs and commercial based SPE sorbent was analyzed. Two-level factorial design and response surface methodology based on central composite design were used for multivariate optimization of the experimental variables. Furthermore, the CNDs were also functionalized with magnetite. The magnetic CNDs were applied for the development of magnetic solid phase dispersive extraction method with ultrasonic dispersion for the simultaneous extraction of chlorpyrifos and triclosan in environmental water samples. This method offered a very rapid and simple extraction and preconcentration of these organic contaminants with satisfactory results.Ph.D. (Chemistry