Abstarct: In the first section of this thesis, a simple, sensitive and rapid microextraction method namely ultrasound-assisted surfactant-enhanced emulsification microextraction baxsed on the solidification of floating organic droplet method coupled with high performance liquid chromatography was developed for the simultaneous preconcentration and determination of nitrazepam and midazolam in real samples. The significant parameters affecting the extraction efficiency were considered using a Plackett-Burman design as a screening method. In order to obtain the optimum conditions with consideration of the selected significant variables, a Box-Behnken design was used. So, the microextraction procedure was performed using 29.1 µL of 1-undecanol, 1.36% (w/v) of NaCl, 10.0 µL of SDS (25.0 µg mL-1) and 1.0 µL of Tween80 (25.0 µg mL-1) as an emulsifier in extraction time of 20.0 min at pH 7.88. In order to investigate the validation of the developed method, some validation parameters including the linear dynamic range, repeatability, limit of detection and recovery were studied under the optimum conditions. The detection limits of the method were 0.017 and 0.086 ng mL-1 for nitrazepam and midazolam, respectively. The extraction recovery for the drugs studied were in the range of 98.0-108.1% with RSD range of 1.0-3.8%. The proposed methodology was successfully applied for the determination of these drugs in a number of human serum, tablet in injectable samples. In the second section of this thesis, a novel coupling of ultrasound-assisted dispersive liquid-liquid microextraction baxsed on solidification of floating organic droplets (UA-DLLME-SFODs) with high performance liquid chromatography (HPLC) was developed for the simultaneous pre-concentration and determination of citalopram (CIT), diazepam (DIZ), and sertraline (SER) in the human serum , urine and tablet samples. In this proposed procedure, 1-undecanol and ethanol were used as the extraction and disperser solvents, respectively. Several parameters affecting the extraction efficiency were investigated by a Plackett-Burman factorial design as a screening design. The variables showing significant effects on the analytical response were pH and volume of the extraction solvent; they were optimized using central composite face centered design (CCFD). For validation of the technique, its accuracy, precision, limit of detection, linearity, and sensitivity were evaluated. limits of quantification (LOQ) of the method were 1.3, 2.7, and 1.2 ng mL-1 for CIT, DIZ, and SER, respectively. Extraction recovery percentages for all the above-mentioned drugs were above 85.0 with acceptable relative standard deviations. The proposed methodology was successfully applied to the determination of these drugs in the human serum, urine and tablet samples. In the third section, a new approach baxsed on ultrasound assisted-reverse phase-dispersive liquid liquid microextraction technique is developed for the extraction and determination of vitamin A and vitamin E from oil matrices before a high performance liquid chromatography analysis. A methodology baxsed on the full factorial design is carried out for choosing the significant parameters. Then the significant factors affecting the extraction efficiency including pH, volume of extraction solvent, and volume of disperser solvent are optimized using the Box-Behnken design. After analyzing the results obtained, the optimum conditions were: pH 4.5, 80-20 µL of the ethanol-water solvent mixture as extraction solvent, 110 µL of 1, 4- dioxane as the disperser solvent, and a sonication time of 10 min. For validation of the developed method, the linear dynamic range, repeatability, limit of detection, and recoveries were obtained under the optimum conditions. The detection limits of the method were 1.6 and 2.3 ng mL-1 for vitamin A and vitamin E, respectively. The extraction recovery percentages for the studied drugs were above 91%, with acceptable RSD. The proposed methodology was successfully applied for the determination of the vitamins in different oil samples. Finally, application of a new free organic solvent-reverse phase-dispersive liquid-liquid microextraction (FOS-RP-DLLME) procedure is developed for the pre-concentration and quantification of vitamin A and vitamin E in various vegetable oil samples. This microextraction method is baxsed upon transferring the analytes from the oil matrix into the surfactant-rich phase, and therefore, organic solvents are not used in this procedure. The variables of interest in the FOS-RP-DLLME method are optimized using the chemometrics approaches. Firstly, in the screening experiments, the full factorial design (FFD) is used for selection of the variables that significantly affect the variables significantly affecting the extraction procedure. Then the selected significant variables are optimized using the response surface methodology (RSM) baxsed on the central composite design (CCD). Under the optimum conditions, the linear ranges were 3.1-3.0 × 103 and 2.3-2.0 × 103 ng mL-1 for vitamin A and vitamin E, respectively. The relative standard deviations (RSDs) were in the range of 0.77–4.71% and 0.61-4.37% for vitamin A and vitamin E, respectively. The relative recoveries of these vitamins from different oil samples were above 98.3%.