Chapter Six Conclusions The following conclusions were made regarding the optimization of microwave-assisted extraction techniques for carbamate pesticides, and the application of liqui
Trang 16 Chapter Six
Conclusions
The following conclusions were made regarding the optimization of
microwave-assisted extraction techniques for carbamate pesticides, and the application of liquid
chromatography-mass spectrometry for identification and determination of carbamate pesticides and their related compounds in environmental and biological samples
Microwave-assisted extraction was applied to study the thermal degradation of five carbamates (propoxur, thiuram, propham, methiocarb, and chlorpropham) after heating for 6 min at 95 °C with different extractants Evidence was presented that significant thermal degradation of the five targets occurred during extraction and the breakdown percentage ranged from 10% to 100%, depending on polarities of both analytes and extractants The recoveries of carbamates from soil were also dependent on the polarity
of the extractant, and the protection afforded by the soil to the analytes was apparent when a less polar solvent was used as extractant Hydrolysis of carbamates occurred in water under the applied MAE conditions and led to low recoveries when soil moisture was present In a comparison of extraction behaviour, it was observed that recoveries of the pollutants from soil in the extractants considered (methanol, hexane-acetone (1:1), dichloromethane) were dependent on the relative polarities of the class of analytes: recoveries increase in the order of increasing analyte polarities (triazines > PCBs > PAHs)
Trang 2Orthogonal array design was employed to optimize both microwave-assisted extraction and supercritical fluid extraction conditions for extracting carbamates (propoxur, propham, methiocarb, and chlorpropham) from spiked soil An OA16 (44) matrix derived from OA16 (215) was employed to optimize the MAE conditions The average recovery of the carbamates was above 94% with good reproducibility under optimum MAE conditions SFE was optimized by a two-level OAD procedure Recoveries of the analytes from spiked soil by SFE ranged from 87% to 98%, except for propham (66%), which probably underwent thermal degradation under the extraction conditions The results indicate that although carbamates tested in this study are thermally labile, there was little observable degradation under the optimum MAE and SFE conditions, except for propham From the comparison of both methods, it was found that although higher recoveries were obtained by MAE, recoveries by SFE were good (for propoxur, methiocarb and chlorpropham) under their respective optimum conditions Therefore, both MAE and SFE are suitable techniques for extracting carbamate pesticides from soil However, when extraction is from time-aged soil, MAE exhibited better recoveries than SFE
Liquid chromatography- atmospheric pressure ionization- ion trap mass spectrometry was used to investigate the chemical degradation behavior of N-methylcarbamate in environmental water samples Propoxur was selected as a model compound to study the degradation behavior in ultrapure water, drinking water, rain water, river water and seawater under various pH and irradiation sources (sunlight, darkness, indoor incandescent lighting and artificial UV lighting) Hydrolysis was found to be very much
Trang 3pH-dependent irrespective of the type of water matrix Light intensity could prompt degradation significantly under mild alkaline conditions, while its influence was minor at higher pH (pH > 9) Furthermore, half-lives and degradation rates were obtained in various matrix and irradiation sources Based on kinetic studies, propoxur was found to degrade more rapidly at higher light exposure and pH
Furthermore, liquid chromatography- atmospheric pressure chemical ionization- ion trap mass spectrometry, coupled to microwave-assisted extraction technique, was also been applied to analyze the residues of N-methylcarbamate and related compounds (propoxur and its metabolisms as model compounds) in biological materials (goldfish, tortoises and sea lettuce) Orthogonal array design with an OA8 (27) matrix was employed to optimize MAE conditions It is found that optimized conditions were different in different biological matrices The average recoveries of tested compounds in goldfish, tortoises and algal samples under the respective optimum MAE conditions ranged from 85% to 87% with good reproducibility The conjugate of 2-isopropoxyphenol occurred in all tested biological samples under the applied MAE conditions and led to low recoveries The stability of propoxur under microwave irradiation was maintained as neither evaporation losses nor propoxur decomposition was observed during the process The extracts of goldfish, tortoises and sea lettuce were stable and a 3-days delay prior to analysis was possible