Optimization of solvent composition for dispersive liquid-liquid microextraction and high-performance liquid chromatography based on a fluorescence protocol using fluorescein as a fluorescent tracer

Finding a proper solvent composition in a typical extraction procedure or during the chromatographic runs is of great importance because with an optimized composition in a solvent mixture higher extraction efficiencies and better chromatographic separations in a reasonable time can be achieved [1]. Types of solvents which are typically employed in routine extraction or chromatographic experiments are as follows: (1) Nonpolar solvents: these solvents have low dielectric constants ( 5) and are not good solvents for charged species such as anions; (2) Polar aprotic solvents: these solvents have moderately higher dielectric constants than the nonpolar solvents (between 5 and 20). They are “aprotic” because they lack O-H or N-H bonds; (3) Polar aprotic solvents: these solvents all have large dielectric constants ( 20) and large dipole moments, but they do not participate in hydrogen bonding (no O-H or N-H bonds). Their high polarity allows them to dissolve charged species such as various anions used as nucleophiles (e.g. CN-, HO-, etc.). Polar protic solvents tend to have high dielectric constants and high dipole moments. Furthermore, since they possess O-H or N-H bonds, they can also participate in hydrogen bonding. These solvents can also serve as acids (sources of protons) and weak nucleophiles (forming bonds with strong electrophiles). There are two common ways of measuring the polarity in a mixture of solvents. One is through measuring the dielectric constant or permittivity. The greater the dielectric constant, the greater the polarity (water = high, gasoline = low). A second comes from directly measuring the dipole moment. In this work, a statistical mixture-design technique followed by a fluorescence approach was used to measure the polarity and the subsequent composition in a mixture of solvents(acetonitrile, methanol, water). Firstly, a fluorescent dye (fluorescein) with a fixed concentration was prepared in different solvents and their mixtures as well. Then the fluorescence intensity of each sample was recorded and plotted against diverse solvent compositions. By using statistical approaches and from this plot with a given fluorescence intensity an unknown solvent composition can be extrapolated.