Application Of Dispersive Liquid-liquid Microextraction In Sample Preparation

Although great improvement has been achieved in recent years, the complexity of the sample matrix makes it necessary to carry out proper sample pretreatment step prior to instrumental analysis.Sample preparation usually includes extraction, enrichment and cleanup steps. Sample preparation is an important step in the analytical process.The traditional liquid-liquid extraction not only requires a large amount of organic solvent and sample, but also has low enrichment factor. In this study, dispersive liquid-liquid microextraction?DLLME? was used to extract anthraquinones from cassia occidentalis seed tea for the first time, and DLLME was also combined with the use of magnetic nanoparticles for the extraction of phenolic acids in vegetable oils.This thesis is divided into three parts and the main conclusions and original aspects are listed as follows :Chapter 1: DLLME was described in detail. The classification and application as well as the development trend of DLLME were summarized. In addition, the main thought and experimental design were also presented.Chapter 2: DLLME was combined with high performance liquid chromatography to determine the content of five anthraquinones in cassia occidentalis seed tea, and the relationship between the content of anthraquinones in cassia occidentalis seed tea and soaking time was also studied. For DLLME, acetone and chloroform were used as dispersive and extraction solvents, respectively. Some important parameters, such as p H of the aqueous phase, the type and volume of extraction and dispersive solvents, salt effect and extraction time were carefully investigated and optimized to achieve the best extraction efficiency of aloe-emodin, rhein, emodin, chrysophanol and physcion. Under the optimal conditions, the recoveries of the five anthraquinones were higher than 81.1%, with RSD?9.97%, and the detection and quantification limits were in the range of 7.3-12.9 ng/m L and 24-43 ng/m L, respectively. Compared with the most widely used method in the analysis of anthraquinones in cassia occidentalis seed tea, the proposed method provides similar or higher extraction efficiency. The proposed method is simple, economic and environmental- friendly.Chapter 3: A novel dispersive liquid–liquid microextraction method based on amine-functionalized Fe₃O₄ magnetic nanoparticles was developed for the determination of six phenolic acids in vegetable oils by high-performance liquid chromatography. Amine-functionalized Fe₃O₄ was synthesized by a one-pot solvothermal reaction and characterized by transmission electron microscopy and Fourier transform infrared spectrophotometry. A trace amount of phosphate buffer solution?extractant? was adsorbed on bare Fe₃O₄-NH₂ nanoparticles by hydrophilic interaction to form the “magnetic extractant”. Rapid extraction could be achieved while the “magnetic extractant” on amine-functionalized Fe₃O₄ nanoparticles was dispersed in the sample solution by vortexing. On one hand, the buffer solution adsorbed on Fe₃O₄-NH₂ nanoparticles could extract the phenolic acids, on the other hand, an electrostatic interaction exists between negatively charged carboxyl groups on phenolic acid compounds and positively charged Fe₃O₄-NH₂ by adjusting p H of the solution. After extraction, the “magnetic extractant” was collected by application of an external magnet and the analytes were eluted. Some important parameters, such as p H and volume of extraction and desorption solvents, the extraction and desorption time needed were carefully investigated and optimized to achieve the best extraction efficiency. Under the optimal conditions, satisfactory extraction recoveries were obtained for the six phenolic acids in the range of 84.2–106.3%.. Relative standard deviations for intra- and inter-day precisions were less than 6.3 and 10.0%, respectively. The proposed method could achieve rapid extraction of the target analytes by the “magnetic extractant” dispersive liquid-lquid microextraction. The “magnetic extractant” could be retrieved by application of an external magnet, avoiding time-consuming centrifugation step. The economical phosphate buffer solution served both as the extractant and desorption solvent in this method, completely avoiding the use of organic solvent. Furthermore, the desorption solution was directly supplied for instrumental analysis without tedious drying and reconstituting steps. The whole pretreatment process was accomplished within 10 min, which is more time-saving than most of the reported methods. Finally, the established method was successfully applied for the determination of six phenolic acids in eight kinds of vegetable oils.