3.3.1. Selection of extraction solventAqueous acidic solutions were selected as extraction solvent andfurther optimized. The extraction efficiencies of all target analyteswere calculated by means of absolute recovery. Various aqueousacidic extraction solvents were examined under initial conditionsat 80◦C and 1300 psi over 3 extraction cycles (15 min for eachcycle). Once one parameter was established, its value was appliedand the next parameter in the initial conditions was tested. Theextraction efficiencies of all target analytes were related to thepH value of extraction solvent to a great extent. Pure water andtheir combinations in neutral or acidic conditions were investi-gated. With extraction solvent at higher pH value, poorer extractionefficiencies of these six organic acids were observed with lowersensitivity. A water solution containing 5 mmol L−1sulfuric acidwas selected as extraction solvent, and the lowest detection limits,the best responses, and appropriate sensitivities were obtained, asshown in Table 2.3.3.2. Selection of operating parametersIncreasing temperature in ASE operation resulted in enhancedextraction of the organic acids. In this study, the extraction tem-perature was tested at 60, 80, and 100◦C with the pressure of1300 psi, 3 extraction cycles and static period of 15 min in eachcycle. All target analytes recorded the highest recovery values at80◦C. A suitable temperature not only increased the diffusion ofan extraction solvent, as well as the mass-transfer kinetics and sol-ubility of organic acids from the matrix into the solvent, but alsodecreased the viscosity and the surface tension of the solvent, con-sequently facilitating the penetration of solvent into the matrix andaccelerating the extraction efficiency of the organic acids. However,excessive temperature could decompose thermo-labile biologicalanalytes. Therefore, the extraction temperature was set to 80◦C.High pressure was applied during ASE operation in order to keepextraction solvent in liquid state while temperature was held nearthe boiling point. The application of high pressure also facilitated